1Key Laboratory of Transportation Tunnel Engineering of Ministry of Education School of Civil EngineeringSouthwest Jiaotong University Chengdu 610031 China2China Railway First Survey and Design Institute Group Co Ltd Bridge and Tunnel Design Office Xirsquoan 710043 China3Rail Transit Construction Corp Ltd of the 21st China Railway Construction Corporation Ltd Jinan 250021 China
Correspondence should be addressed to Xiaojun Zhou zhouxjyu69163com
Received 22 August 2019 Revised 22 December 2019 Accepted 23 December 2019 Published 27 January 2020
Academic Editor Rafael J Bergillos
Copyright copy 2020 Lu Zhang et al -is is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
In order to study the design of incomplete well point for groundwater extraction in open cut foundation pit in which the inner andouter aquifers are not completely isolated and the change mechanism of ground subsidence due to dewatering in the foundationpit an open cut foundation pit for a subway station on ChengduMetro Line 6 is taken as an example meanwhile the typical sandypebble soil strata are also considered as a research object in this paper Firstly a newmethod for designing groundwater extractionin the open cut foundation pit is presented and applied to the practical project -e dewatering funnel curve is derived based onDupuitrsquos assumption and the ground subsidence around the foundation pit due to groundwater extraction is calculated using thestratification summation method as well as considering the effect of seepage force -e finite difference software FLAC3D isemployed to simulate the groundwater extraction process in the foundation pit and the simulation of groundwater extraction bysingle well point and group well points is also carried out and the unapparent effect of group well points is obtained -ecomparison among on-site monitoring theoretical calculation and numerical simulation shows that these values have the sametrend in indicating ground subsidence and the conventional stratification summation method is conservative and the algorithmconsidering the effect of seepage force is more accurate-erefore the ground subsidence curve caused by groundwater extractionin the foundation pit is presented-e above research methods and results can be applicable for practical engineering and be usedto guide the design and construction of groundwater extraction in the foundation pit by using the open cut method in sandypebble soil strata
1 Introduction
With the rapid development of economy the construction ofcities is remarkably increasing and the urbanization processis being apparently accelerated so more and more urbanunderground spaces are being exploited in cities around theworld -us urban high-rise buildings and undergroundmunicipal facilities are being increasingly developed indensely packed urban areas
Since most city subway stations are being built inbustling area due to the influence of the narrower
construction site and heavier traffic flow the foundationpit for building a subway station can only be excavatedunder the condition without ground water At suchnarrower and bustling urban area there is no possibilityof performing groundwater extraction outside the opencut foundation pit -erefore groundwater extractioninside the foundation pit is usually employed in thedesign and excavation of foundation pits for buildingunderground subway stations in addition a largenumber of bored waterproof curtains are adopted inconsideration of such factors as construction difficulty
HindawiAdvances in Civil EngineeringVolume 2020 Article ID 1046937 25 pageshttpsdoiorg10115520201046937
and cost -e waterproof curtains refer to the ones thatdo not penetrate the entire aquifer but enter a certaindepth in the aquifer and combine the design ofgroundwater extraction in the foundation pit to form agroundwater treatment method for internal water low-ering and external water stopping When groundwaterextraction inside a foundation pit is carried out thegroundwater outside the pit will bypass the bottom of thewaterproof curtains and pass through the aquifer into thefoundation pit Compared with the dewatering outsidethe foundation pit it not only increases the seepage pathof the foundation pit but also reduces the head lossoutside the foundation pit -e influence of dewateringinside the foundation pit on the surrounding environ-ment is less than the one caused by the dewateringoutside the pit If it is a fully enclosed foundation pit tobe specific the enclosure structure or the diaphragmwalls can then be extended to the bottom of the aquiferand be inserted into impermeable stratum underneaththe bottom the groundwater outside the foundation pitwill be completely isolated from that one inside the pitCurrently groundwater extraction in the foundation pitbasically has no effect on ground surface outside thefoundation pit If it is a semiclosed foundation pit that isthe waterproof structure or the diaphragm wall isinserted into the middle and lower parts of the aquiferthe groundwater inside and outside the upper foundationpit will be discontinuous and the bottom aquifer willbecome continuous -erefore the groundwater insidethe foundation pit can be replenished by the aquiferoutside the pit At this time the groundwater extractionin the foundation pit will lead to a series of problemssuch as ground subsidence deformation of supportstructure and uplift of the bottom of the foundation pitAmong them the ground subsidence outside the pit ismore likely to occur so this paper focuses on addressingthe problem
Yihdego [1] studied the relationship between the re-duction in flow and cutoff of hydraulic barriers in a period oftime and found the effect of barriers begins to be significantafter cutoff exceeds 60 But as for this project the inserteddepth of enclosure structures is far smaller than the distancebetween the bottom of the foundation pit and the top of theimpermeable layer so the inserted depth is not consideredand the enclosure structures have no effect on groundwaterflow below the foundation pit ideally And the design schemeof groundwater extraction in the open cut foundation pit isillustrated in Figure 1
In Figure 1 H indicates the thickness of the phreaticaquifer eg original water table in the foundation pit m Sdenotes the maximal depth of dewatering outside thefoundation pit m Sw denotes the depth of dewatering in thewell point m hprime indicates the water head at the bottom of thecentral axis of the enclosure structure m and h denotes thewater level after dewatering in the foundation pit m
Many scholars have studied the dewatering in thefoundation pit Zhang et al [2] proposed an analyticalcalculation method for predicting tunnel deformation in-duced by upside excavation and also discussed the role of
dewatering in the deformation mechanism Wang et al [3]established a conceptual and mathematical model thatconsidered hydrogeological conditions curtain depth andpumping well screens and performed numerical simulationsbased on themodel Xu et al [4] investigated the engineeringgeology and hydrogeology related to foundation dewateringand discussed the current state of foundation dewateringworks resulting in land subsidence in Shanghai Wang et al[5] introduced a transparent soil model test to address thelimitation of the existing experimental method and nu-merical simulations in modeling the coupling mechanismbetween the cutoff wall and the pumping wells and proposedthe optimal depth of the pumping wells and the optimalhorizontal distance between the cutoff wall and the pumpingwells In order to analyze the influence of layering themechanical parameters and the relationship betweenground settlements and drawdown Pujades et al [6]adopted a radially symmetric conceptual model and con-ducted several hydromechanical simulations by varying theboundary conditions the size of the modeled domain andthe presence or absence of an overlying layer Based on alarge deep excavation of the buildings in Oriental Fisher-manrsquos Wharf Wang et al [7] performed single-well andgroup-well field pumping tests and carried out a numericalsimulation by using the 3D finite difference method (FDM)Taking Qianjiang Century City Station foundation pit as anexample Wang et al [8] performed field experiments toobserve the coupling non-Darcy flow in round gravelestablished a generalized conceptual model to study thecoupling effect under different combinations of curtain andpumping wells and carried out numerical simulations of thecoupling non-Darcy flow in foundation pit dewatering basedon Forchheimerrsquos equation Based on a deep excavationproject in Tianjin Wu et al [9] conducted field measure-ments of the groundwater head and the building settlementduring excavation and analyzed the influence range ofdewatering and the relationship between the drawdownhead and the settlement To predict the behavior of landsubsidence due to groundwater extraction Zhang et al [10]established a three-dimensional numerical model consid-ering the confined aquifer and soft deposit and then ana-lyzed and compared between the calculated result andmeasured value -is paper mainly takes the dewateringproject of an open cut foundation pit of a metro station onChengdu Metro Line 6 as an example -e results of groundsubsidence around the foundation pit calculated by usingtheoretical formulae and numerical simulation FLAC3D arerespectively compared with the on-site monitoring data-e design scheme of the foundation pit dewatering is alsoproposed and the ground subsidence curve caused bydewatering is also compared -us the results proposed inthis paper can be used as a reference and guidance for similarprojects under similar geological conditions
2 Design and Calculation of Dewatering inFoundation Pit
21 Calculation of Dewatering in Foundation Pit in Single SoilLayer under Waterproofing Enclosure Structure It is seen
2 Advances in Civil Engineering
from references [11ndash13] that if the boundary of the seepagefield is impervious the flow line in the flow net is parallel tothe boundary and while the seepage field is equal to thewater head boundary the flow line is orthogonal to theseepage boundary -erefore the seepage field around thefoundation pit under the geological conditions of single-layered soil is shown in Figure 2
Both the enclosure structure and the bottom im-pervious layer can be regarded as impervious boundariesand the horizontal seepage velocity of the groundwater atthe bottom of the enclosure structure is far greater thanits vertical one so the flow of groundwater at differentdepths below the bottom of the center axis of the en-closure structure is approximately regarded as horizontalflow that is laminar flow -erefore the water head lineat the bottom of the central axis of the enclosurestructure is vertical -us according to these vertical flowlines the seepage field around the foundation pit is di-vided into two seepage fields respectively one is insidethe foundation pit and the other is outside the foundationpit Water inflow from the two seepage fields can then besolved separately It is known that the groundwateroutside the pit provides water inflow for groundwaterinside the pit thus the water inflow Q1 inside the pit isequal to the water inflow Q2 outside the pit namely
Q1 Q2 (1)
-e radius of influence is defined as the maximumdistance at which the drawdowns can be detected with theusual measure devices in the field [14] -e most commonway to find the radius of influence is the use of empiricalformulae [15ndash17] such as Sichardtrsquos formula as well asKusakinrsquos formula Furthermore related influence factorslike time t and radius of the pit re are also taken into accountin the formulae by some scholars [15ndash17] In this project thedesign is based on the Chinese Code According to theChinese Technical Specification for Retaining and Protec-tion of Building Foundation Excavations (JGJ 120-2012)[18] the radius of influence for phreatic aquifers in the
foundation pit can be calculated according to the followingequation
R 2Sw
Hk
radic (2)
where R is the radius of influence m Sw denotes the depth ofdewatering in the well point m H indicates the thickness ofthe phreatic aquifer eg original water table in the foun-dation pit m and k refers to the permeability coefficient ofthe ground md
In order to analyze the water inflow inside and outsidethe foundation pit two conditions are taken into account asfollows
① If considering the enclosure structure as the wall of awell point the entire foundation pit can then beconsidered as a submersible incomplete well and thewater inflow outside the foundation pit away fromthe boundary can be calculated approximately byusing the normative formula presented in theTechnical Specification JGJ 120-2012 [18] As for acircular or a rectangular pit with length-width ratioless than 20 the water inflow Q2 is calculatedaccording to the following equation [19]
H
h
S
Phreatic aquifer
Dewatering funnel curveEnclosure structure
Well pointGround
Impermeable stratum
Groundwater level
S whprime
Figure 1 Schematic diagram of dewatering design in the open cut foundation pit
Water level in the pit
Impermeablestratum
Phreatic aquifer
Groundwater level
Enclosure structure
Ground
Flow line
S w
H
hprime
Figure 2 Seepage field distribution around a single-layered soilfoundation pit
Advances in Civil Engineering 3
Q2 1366k H2 minus h2
m1113872 1113873
lg 1 + Rr0( 1113857( 1113857 + hm minus l( 1113857l( 1113857lg 1 + 02 hmr0( 1113857( 1113857
(3)
hm H + hprime
2 (4)
where r0 stands for the equivalent radius of thefoundation pit m it is calculated according tor0 0565
A0
1113968A0 denotes the foundation pit area m2
hprime indicates the water head at the bottom of the centralaxis of the enclosure structure m and l means thelength of the water inlet part of the dewatering well m
② -e enclosure structure and the bottom boundary areboth impervious layers According to Darcyrsquos seepageexperimental conditions the seepage field distribu-tion in Figure 3 is simplified to be a one-dimensionalflow field distribution as shown in Figure 4
-at is in the assumption that the groundwater in the pitone-dimensionally flows in a circular glass tube and itsatisfies Darcyrsquos flow law water inflow Q1 into the pit isderived theoretically as follows
Q1 kA hprime minus h( 1113857
L (5)
A V
L (6)
V πr20 l2 + l3( 1113857 (7)
L 2l2 + l3 + r0( 1113857
2 (8)
H l1 + l2 + l3 (9)
h l2 + l3 (10)
Sw H minus hprime (11)
where h denotes water head height in the foundation pit afterdewatering m l1 denotes water table drawdown in thefoundation pit m l2 refers to the distance from the watertable to the bottom of the enclosure structure after dew-atering in the foundation pit m l3 indicates the distancefrom the bottom of the enclosure structure to the imperviouslayer m and A V and L refers to the cross-sectional area ofthe seepage field m2 the total volume of seepage m3 andaverage seepage path m respectively
Simultaneous solution is obtained from equations (1) to(11) and then the following equation is obtained
4πr20 l2 + l3( 1113857 hprime minus h( 1113857
2l2 + l3 + r0( 11138572
03415 4H2 minus H + hprime( 11138572
1113960 1113961
lg 1 + 2 H minus hprime( 1113857Hk
radic( 1113857r0( 11138571113858 1113859 + H + hprime minus 2l( 11138572l( 1113857lg 1 + H + hprime( 111385710r0( 1113857( 1113857
(12)
l 2 (l)
l 3l 1
r0
Initial groundwater level
Water levelin the pit
Enclosure structure
H
h
S whprime
Flow line
Figure 3 Seepage field distribution in the pit
r 0l 3
l 2
L
h
hprime
l 2 (l)
Figure 4 Simplification of the seepage path in the pit
4 Advances in Civil Engineering
As for a real open cut foundation pit it is seen fromequation (12) that there is only one unknown variable inequation (12) that is hprime the water head at the bottom of thecentral axis of the enclosure structure inside the pit -iswater head at the bottom of the central axis of the enclosurestructure inside the pit can be iteratively obtained so that theradius of influence of dewatering in the foundation pit andthe water inflow in the foundation pit can also be obtained
22 Calculation of Dewatering in Foundation Pit in Multi-layered Soil under Waterproofing Enclosure Structure Forthe calculation of water inflow in the foundation pit con-sidering the waterproofing effect of the enclosure structureunder the geological conditions of the multilayered soil thestratification calculation method is adopted to calculate thewater inflow of each layer of soil separately and algebraiccalculation is performed to obtain the total water inflow inthe pit Generally speaking there are many soil layers inactual foundation pits It is very cumbersome and time-consuming to use this method -erefore the geologicalconditions of the multilayered soils are simplified to be asingle formation and the permeability coefficient is averagedfor calculation -ree soil layers are used to illustrate thismethod as shown in Figure 5
-e permeability coefficient is calculated as follows
k k1h1 + k2h2 + k3h3
h1 + h2 + h3 (13)
where h1 h2 and h3 denote the thicknesses of the three soillayers respectively m and k1 k2 and k3 stand for thepermeability coefficients respectively corresponding to thethree soil layers md
23 eoretical Design of Dewatering In the actual projectthe average permeability coefficient of the multilayered soilsis firstly obtained according to equation (13) and then byusing equation (12) engineering parameters are substitutedand simplified to obtain the transcendental equation abouthprime -is equation can only be solved by means of a computerso it is solved with Matlab using the dichotomy By inputtingthe program in Matlab the water head hprime at the bottom ofthe central axis of the enclosure structure can be obtainedand then both the depth Sw of lowering water level in the wellpoint and radius of influence R of the phreatic aquifer infoundation pit can then be obtained
From equations (8) to (11) the equation for calculatingthe water inflow Q1 in the pit is derived as follows
Q1 kA hprime minus h( 1113857
L4kπr20 l2 + l3( 1113857 hprime minus h( 1113857
2l2 + l3 + r0( 11138572 (14)
-e water inflow of a single well is calculated as follows[18]
q0 120πrslk
3radic
(15)
where q0 represents the water inflow capacity of a single wellm3d rs denotes the filter radius m l stands for the length of
the inlet part of the filter m and k denotes the permeabilitycoefficient of the aquifer md
-e number of dewatering wells is calculated as followsaccording to the Chinese Technical Code for GroundwaterControl in Building and Municipal Engineering (JGJT111-2016) [19] if the safety level of the foundation pit isassessed to be in Grade I and the complexity of thefoundation pit is evaluated to be complicated and then thecalculation coefficient ε in equation (16) gets the value of12 -e number of well points is obtained from the fol-lowing equation
n εQ1
q0 (16)
D L
n (17)
where D denotes the space between well points m L rep-resents the circumference of the foundation pit m and nmeans the number of well points
-erefore the layout of the dewatering well points in theactual foundation pit can be obtained from the precedingequations
3 Case Study of Dewatering in Foundation Pit
31 Engineering Background -is paper depends on adeep foundation pit of a subway station on ChengduMetro Line 6 -e station is an underground three-storied island platform station its east side is closelyadjacent to a street-facing commercial store that has 2-3stories of brick-concrete structure In addition a 220 kVpower cable tunnel constructed with conventionalmining method is buried on its east side -e power cabletunnel is 14 m away from the sidewall of the station -ewest side of the subway station closely approaches privatehouses and public shops on the ground surface and theouter edge of a shop on the west side lies within aminimum distance of 17 m away from the foundationpit -e station is a 130 m wide island platform stationwith a standard cross-sectional width of 225 m and itstotal length is 2429 m long on the right side and 2221 mlong on the left side -e depth of the soil on its roof isabout 398 m and the depth of the bottom is about2664 m -e station and its surrounding environmentare shown in Figure 6 According to the hydrogeologicalconditions for this project there are two types ofgroundwater in the site one is the perched water in thebackfill layer above the clay layer and the other is thepore water in the quaternary sand and pebble layer -eprimary geotechnical investigation showed that stablewater table measured in the site was 500sim640 m inOctober 2015 and the detailed investigation showed thatit was 540sim670 m in October 2016 Obviously there isminor difference in the two results of water tables so thewater level in the site is based on the results of the de-tailed geotechnical investigation -e site geotechnicalproperties and its distribution are also shown in Table 1
Advances in Civil Engineering 5
32 Design of Dewatering Well Points Since the subwaystation is closely located to the shops and buildings on its twosides and the underground pipelines are densely packed inpower cable tunnel and municipal sewages there are noother spare places to install dewatering wells outside thefoundation pit -erefore to avoid the impact of dewateringon the surrounding environment well points are used to thelower groundwater level inside the foundation pit prior to itsexcavation
-e groundwater of the construction site belongs to theQuaternary existing in the sandy pebble pore phreaticaquifer -e thickness of the phreatic aquifer is less than30m and the bottom floor of the station is located in thecompacted pebble layer -e purpose of dewatering is tolower the water table in the foundation pit to 1m below itsbottom so that normal construction of the subway stationcan be fulfilled without groundwater -e open cut foun-dation pit of the station is 225m long and 24m wide with a
Family wingsEntrance F
Entrance GEntrance 2Shops
Entrance DEntrance E
Foundation pit
N
Seco
nd ri
ng
Figure 6 Station location and its surrounding environment
⑧ Medium dense pebblelayer 1sim214 1233 255times10minus 2 32
⑨ Compacted pebblelayer Not drilled Not drilled 255times10minus 2 43
h 1h 2
h 3
k1
k2
k3
Ground
Groundwater level
Soil layer 1
Soil layer 2
Soil layer 3
Phreatic aquifer
Impermeable stratum
Figure 5 Geological conditions of multilayered soil
6 Advances in Civil Engineering
length-width ratio of 9375lt 20-e depth of the foundationpit is about 2664m For the convenience of calculation it isset to 27m -e diameter of the dewatering well is 600mmand the well bottom is 35m lower than the one of thestation-e length of the filter pipe is 2m-e distance fromthe bottom of the filter pipe to the impervious layer is 2mand the stable water table is considered to be 6m
From the theoretical design and calculation of Section 2 itis known that 17 well points are to be laid around the foun-dation pit Referring to the Chinese Technical Code forGroundwater Control in Building and Municipal Engineering(JGJT111-2016) [19] and considering an idealized situation inwhichwells can be easily bored around the open cut foundationpit the dewatering wells are arranged at the same distancealong the foundation pit After a well is installed at the center ofthe foundation pit the remaining 16 well points are installedevenly at the inner edge of the pit Since it is a long and narrowopen cut foundation pit the wells can be placed at the inneredge of the long side of the foundation pit and the spacebetween them is about 25m -e specific layout of well pointsinside the open cut foundation pit is shown in Figure 7
33 Comparison between Calculations with and withoutconsidering the Waterproof Effect of Enclosure StructureAccording to the Chinese Technical Code for groundwatercontrol in building and municipal engineering [18] if groupwell points are simplified to be large one the total waterinflow from the incomplete well points in the phreaticaquifer which is calculated by using equations (3) and (4) isreplaced with the following equation
hm H + h
2 (18)
-e parameters in equation (18) are the same as that inthe formulae as stated above If the waterproofing effect ofthe enclosure structure is not considered then the depth ofwater level lowered by well points is expressed below
Sw H minus h (19)
Substituting aforementioned engineering data intoequations (3) and (16)ndash(18) respectively it is seen that thewater inflow from incomplete well point Q3 in the foun-dation pit is
Q3 Q2 1209532m3 (20)
the radius of influence of the foundation pit Rprime is
Rprime 2Sw
Hk
radicasymp 105233m (21)
and the number of well points nprime is certainly obtainedaccording to
nprime 12Q2
q0asymp 23 (22)
After a well point is installed at the center of the pit theremaining 22 well points can then be set evenly at the edge ofthe foundation pit Since it is a long and narrow foundationpit the well points are evenly installed at the edge of the long
side L of the foundation pit and the space Dprime between themis
Dprime L
nasymp 2045m (23)
Compared with the waterproofing effect of the enclosurestructure the water inflow in the foundation pit is893651209532 asymp 074 times of the conventional algorithmproposed in the Chinese Specification and the dewateringradius of influence is 54099105233 asymp 051 times of theconventional algorithm in the Specification -e number ofwell points is 1723 asymp 074 times of the conventional algo-rithm in the Specification and the space between them is252045 asymp 122 times of the conventional algorithm in theSpecification -rough comparison if the enclosure struc-ture of the foundation pit is used as a waterproof curtainthen the waterproofing effect of the enclosure structureshould not be ignored when calculating water inflow insidethe open cut foundation pit
34 Arrangement of Points Monitoring Ground SubsidenceIn order to make further analysis on the ground subsidencecaused by dewatering in the foundation pit the groundsubsidence of the typical positions around the foundation pitis monitored -e ground subsidence monitoring points areset up according to the actual condition of the open cutfoundation pit In the actual project the ground subsidencemonitoring points are arranged around the foundation pitFor the consideration of symmetry and the convenience ofmeasuring six points at the midline of the long side of thefoundation pit are selected and they are 16-6 16-5 16-4 16-3 16-2 and 16-1 respectively the distance between the wellpoints and pit wall is set within 8m 12m 16m 20m 24mand 28m respectively -e monitoring points are shown inFigure 8 which are located at the center axis of the edge lineof the pit Step-by-step dewatering is carried out in the actualproject and the depth of dewatering for every step is set to6m 5m 5m and 6m respectively and the total stepamounts to 22m
4 Theoretical Calculation of GroundSubsidence Caused by Dewatering inFoundation Pit
41 Normative Calculation of Ground Subsidence Caused byDewatering in Foundation Pit
411 Dupuitrsquos Assumption and Derivation of DewateringFunnel Curve Dewatering in the foundation pit will defi-nitely produce a falling funnel curve around the pit and thegroundwater may flow into the dewatering well inside thefoundation pit French scholar Dupuit first studied thesteady well flow put forward Dupuitrsquos assumption andderived the dewatering funnel curve -e hypothesis con-siders a cylindrical homogeneous phreatic aquifer withisotropic and horizontally waterproofing bottom floor afixed water head outside the aquifer a complete pumpingwell in the center no vertical infiltration recharge and
Advances in Civil Engineering 7
evaporation and a steady seepage subject to the linear law[20] -e dewatering well point can develop a dewateringfunnel curve around it Groundwater flows to the well after acertain time and the dewatering curve can reach a steadystate Assuming that the well point dewatering is a stablephreatic well flow without the group well effect the center ofthe well bottom is set to be the origin and the abscissa is setto be the positive x-axis as shown in Figure 9
According to Dupuitrsquos assumption the water flowequation of the stable phreatic well is obtained below [20]
Q kIA 2πrhkdh
dr (24)
If we separate the variable in equation (24) and take anyone point on the dewatering funnel curve then followingequations are obtained by integrating the equation from thepoint to its boundary
r x h z
r R h H
⎧⎪⎨
⎪⎩
⎫⎪⎬
⎪⎭⟶ 1113946
R
x
Q
2πk
1rdr 1113946
H
zh dh (25)
and thenQ
2πkln
R
x12
H2
minus z2
1113872 1113873 (26)
If the boundary condition meets
x r0
z h0 + l(27)
and substituting equation (27) into equation (26) then thefollowing equation is obtained
Q
2πkln
R
r012
H2
minus h0 + l( 11138572
1113960 1113961 (28)
-e solution of simultaneous equations (26) and (28) isused to obtain the following dewatering funnel curveequation of the well point
z2
H2
minus H2
minus h0 + l( 11138572
1113960 1113961ln(Rx)
ln Rr0( 1113857 (29)
where r0 denotes the radius of wells m h0 represents thelength of the inlet part of the filter pipe m R denotes theradius of influence m H represents the height from thebottom of the well to the initial groundwater level m k is thepermeability coefficient of the aquifer md A is the side areaof the dewatering well m2 I denotes the groundwaterseepage field hydraulic gradient Q stands for the boreholewater yield m3d and l means the distance between thebottom of well and the impermeable layer m-e remainingsymbols are shown in Figure 9
412 Ground Subsidence at Any Point outside the Foun-dation Pit At present the calculation of ground subsi-dence around the foundation pit after dewatering isgenerally carried out using the stratification summationmethod given in the Technical Specification for Retainingand Protection of Building Foundation Excavations (JGJ120-2012) [18] -e ground subsidence can be calculated bysummarizing the compression of each layer of soil Firstlywe can calculate the additional effective stress caused bydewatering and then calculate the ground subsidence usingstress calculation
(1) When the soil layer numbered as i is above the initialgroundwater level the effective stress is
Δσziprime 0 (30)
(2) If the soil layer numbered as i is located between thepostdewatering groundwater level and the initialgroundwater level then the effective stress is
Δσziprime cwz (31)
Foundation pit
Pit wall 16-616-516-416-316-216-1
Figure 8 Schematic diagram of the ground subsidence monitoring points around the foundation pit
Well point
Foundation pit wall
Figure 7 Schematic diagram of the layout of the dewatering well
8 Advances in Civil Engineering
(3) When the soil layer numbered as i is below thegroundwater level after dewatering then the effectivestress is
Δσziprime λicwsi (32)
-e soil compression caused by dewatering is as follows
s ψw 1113944Δσziprime Δhi
Esi
(33)
where cw means the bulk density of water kNm3 z denotesthe vertical distance from the midpoint of the soil layer i tothe initial groundwater level m and λi represents the cal-culation coefficient it should be based on the analysis ofgroundwater seepage If the analysis data are not availablethen its value should be based on local engineering expe-rience si refers to the depth of lowering groundwater levelcorresponding to the calculation profile m s denotes theground compression in the calculation profile m and ψw
means the empirical coefficient of subsidence calculationbased on local engineering experience If no experience isavailable then the value is set to be 1 Δσzi
prime denotes the meanadditional stress of the soil layer i under the ground surfacecaused by dewatering kPa Δhi means the thickness of thesoil layer i m and Esi
denotes the compressive modulus ofthe soil layer i kPa
42 Calculation of Ground Subsidence Caused by Dewateringin Foundation Pit under Seepage Force -e pumping anddrainage of the dewatering well will cause the change ofthe underground seepage field which will generate a newseepage field and lead to the variation of the stress fieldaround the well -erefore the seepage force is the maincause of soil consolidation and settlement -e seepage ofgroundwater causes the dissipation of pore water pres-sure resulting in an increase in effective stress -at isadditional stress is generated in the soil the direction ofwhich is vertically downward in addition it produces a
horizontal component Deformation can be caused by theimpact of seepage force -e additional stress namely thevertical component of the seepage force will cause theground subsidence [21] -e horizontal component of theseepage force will cause the lateral deformation of the soilAccording to reference [22] the seepage direction of anywater head at any point is tangent to the phreatic surfaceat that point pointing to the well axis as shown inFigure 10
Wu and Zhu [22] performed related research andproposed a new algorithm of ground subsidence caused byseepage force Yang and Zhao [23] also used this method tocalculate ground subsidence -is section draws lessonsfrom their research to discuss the dewatering in open cutfoundation pit of a subway station on Chengdu Metro Line6
-e stratum is divided into three parts dry soil zonedewatered zone and saturated zone -e dry soil zone isalways above the groundwater level before and afterdewatering which does not contain groundwater andseepage force during dewatering -us additional stressdoes not appear in this zone and the subsidence valuecaused by dewatering is 0 -e other two layers are S1 andS2 respectively as shown in Figure 10 -e soil layer S1 inthe dewatering process is drained -e zone S2 is alwaysbelow the groundwater level and is saturated In thissection the soil subsidence in the dewatered and saturatedzones is calculated separately -e seepage forces in thesetwo areas possess horizontal components as shown inFigure 10
Assuming that the water head that keeps a distance of x0from itself to the well axis is z0 as shown in Figure 10according to the dewatering funnel curve equation (29) inSection 41 the height of the falling funnel curve is obtained-e direction of the seepage force is actually in the straightline vector on the curve which goes through the point(x0 z0) and is tangent to the falling funnel curve and pointsto the well In order to find out the slope of the straight linethe following equation is obtained by performing the de-rivative of equation (29) at point x x0
R
H
h 0
x
z
r0 l
Ground
Impermeable stratum
Well point
Groundwater level
Figure 9 Dewatering funnel curve for the phreatic incomplete well point
-e equation of the line that passes through the point(x0 z0) and is tangential to the curve after the dewateringbecomes stable is assumed to be
y x tan α + b (35)
-e slope of the equation is
tan α zprime x0( 1113857 (36)
According to the trigonometric function conversion
sin α tan α
1 + tan2 α
radic (37)
-en the following equation is obtained
sin α zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (38)
-erefore the vertical component of additional pressureof the surrounding stratum caused by dewatering in thefoundation pit is expressed as
ΔPy ΔP sin α ΔPzprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (39)
where ΔPx denotes the horizontal component of the seepageforce ΔPy denotes the vertical component of the seepageforce and ΔP is the seepage force
-e angle α in equation (39) denotes the one existsbetween the horizontal component of the additional pres-sure and the additional pressure caused by dewatering in thefoundation pit
Based on equations (30) to (33) the additional stresses inthe dewatering zone and the saturation zone are calculatedseparately -e rewriting is carried out on the basis ofequation (34) which derives the ground subsidence causedby dewatering in the foundation pit considering the action ofseepage force -e formula after rewriting is as follows
s ψw 1113944Δσziprime Δhi
Esi
zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (40)
Substituting equation (34) into equation (40) yields thefollowing equation
Figure 10 Simplified calculation model considering seepage force
10 Advances in Civil Engineering
If the ground subsidence S1 and S2 corresponding to thedewatered and saturated zones are separately figured outthen the total ground subsidence is obtained and illustratedin the following equation
S S1 + S2 (42)
where the symbols are illustrated in Section 41
5 Numerical Simulation
With fast development of urban construction various in-frastructures such as subway station high-speed railwaytunnel underground parking lot and basement are beingbuilt especially the construction of deep foundation pits-e geological condition varies in the foundation pit and thesurrounding environment is becoming more and morecomplex after construction -eoretical analysis and em-pirical calculation methods are no longer suitable forpractical projects Nowadays actual projects are generallybuilt ahead of theoretical research and calculation methodsof numerical simulation provide an effective approach forthe design and construction of practical foundation pitAlthough numerical simulation method has not been pro-posed for very long time it has become the most commonmethod used in structural analysis and calculation so farMany scholars [24ndash31] have also used FLAC3D to conductfluid-solid coupling analysis of dewatering in the foundationpit Finite difference software FLAC3D is also employed inthis paper to carry out three-dimensional numerical sim-ulation of the whole process of the environment changecaused by dewatering in the open cut foundation pit of ametro station on Chengdu Metro Line 6 -e calculationresults are compared with the one obtained from theoreticalcalculation and on-site monitoring which are used to makefurther study on the surrounding ground subsidence due todewatering in the open cut foundation pit
51 Physical and Mechanical Parameters According to thegeological properties of the actual project the calculationparameters are obtained and shown in Table 2
52 Establishment of 3D Model and Initial Stress BalanceBased on the engineering geological and hydrologicalconditions a three-dimensional stratigraphic model isestablished From the geological section of the site (Fig-ures 11 and 12) it can be seen that each of soil layers is nearlyhorizontal and almost parallel to each other So in order toestablish the model and conduct the calculation conve-niently [26] the geological layers in the model are simplified-e foundation pit and wells are set up in the stratigraphicmodel according to dewatering scheme According to theprinciple of Saint Venant in elastic mechanics in order toeliminate the boundary influence of the foundation pit oncalculation results the calculation model is extended to 3sim5times of the depth of the pit from the periphery of thefoundation pit on the plane -e depth is set to the im-pervious interface and the entire calculation domain is inthe volume of 420mtimes 198mtimes 72m Referring to
geotechnical mapping and geological properties the modelis divided into 9 strata and the generalized permeabilitycoefficient and stratum thickness are shown in Table 1
According to stratum distribution and initial conditionsthe calculation domain is divided into grids by consideringthe monitoring points the dewatering wells and the dia-phragm wall considering the geological survey In themeshing the grid of the calculation domain is locally refinedin addition the grids around the foundation pit are alsorefined but grids far away from the pit are sparsely meshedconsidering Saint Venantrsquos principle [26] So the entiremodel is then divided into 42 layers 78 rows 300 columnsand a total of 982800 elements and the established three-dimensional model is shown in Figure 13
After the 3D model is set up the initial stress balance isneeded to be applied firstly and the corresponding seepageand displacement boundary conditions are also applied -enormal displacement and the bottom displacement of thefour sidewalls and bottom of the model are restricted andthe horizontal displacement of the wall of the dewateringwell is also constrained -e bottom filter pipe of thedewatering well belongs to the seepage boundary which isachieved by applying the pore water pressure After nu-merical simulation the pore water pressure of initialequilibrium is shown in Figure 14 -e pore water pressurein the initial state is evenly applied on the strata
53 Numerical Simulation of Fluid-Solid Coupling of GroundSubsidence Caused by Dewatering in Foundation PitAfter the initial equilibrium of the numerical model theeffect of single well and group wells were firstly carried outseparately -e stepwise dewatering was conducted underthe conditions of single well and group wells separatelywithout considering excavation -e calculated isograms ofthe dewatering at all levels and the subsidence of themonitoring points are analyzed
531 Numerical Simulation Analysis of Single WellDewatering Considering the symmetry of the well pointthe well point at the center of the calculation domain isselected when carrying out single well simulation -e po-sition of the single well on the model is shown in Figure 15-e isograms showing subsidence and pore water pressure atvarious drawdowns are also shown in Figures 16ndash23
It is seen from Figures 16ndash23 that the influence range ofdewatering well increases with the increase of drawdownAfter the well is dewatered the pore water pressure formsthe dewatering funnel surface -e bigger the drawdown isthe deeper the surface becomes Moreover due to the wa-terproofing effect of the diaphragm wall the stratum insidethe pit bulges but the stratum outside the pit subsides -isis because the groundwater flows through the bottom of thediaphragm wall into the pit -e increase of the pore waterpressure in the pit causes the additional stress to increasewhich leads to the upheaval of the soil layer in the pit -edecrease of the pore water pressure outside the pit causes theeffective stress of the outer stratum to decrease resulting inthe formation of downward additional stress in the outer
Advances in Civil Engineering 11
stratum of the pit which eventually leads to the consoli-dation and settlement of the outer stratum
532 Numerical Simulation Analysis of Group WellsDewatering When conducting the effect of group wells onground subsidence the calculation is based on the well pointlayout diagram designed in Section 3 -e layout of thegroup wells in the model is shown in Figure 24-e isogramsshowing ground subsidence and pore water pressure cal-culated using FLAC3D are shown in Figures 25ndash32
It is seen from the isograms shown in Figures 25ndash32 thatthe deeper the groundwater drawdown the greater theimpact on the surrounding ground is and the lower thefunnel surface formed by the pore water pressure Comparedwith the isogram under the effect of single well in Section 52the influence of dewatering by group wells is much largerGroup wells dewatering has a great impact on the pore waterpressure and strata deformation inside the foundation pit
that is the influence of group wells on ground subsidenceinside the pit is obvious and should not be ignored -esubsidence and pore water pressure isogram under differentdrawdowns are symmetrically distributed As shown in thesingle well effect the strata inside the foundation pit bulgeduring the dewatering of group wells but the strata outsidethe foundation pit subside-is is mainly due to the result ofthe movement of groundwater outside the pit to the dew-atering well inside the pit
533 Effect of Group Wells According to the results ob-tained from 3D simulation the subsidence of the sixmonitoring points on the sides of the foundation pit due todifferent drawdowns caused by the single well and the groupwells are shown in Figure 33
Grade I drawdown including Grade II Grade III andGrade IV drawdown means that the dewatering depth ofgroundwater level is in 6m 11m 16m and 22m respectively
Table 2 Physical and mechanical parameters of strata
Name of stratum Modulus of deformation E (MPa) Poissonrsquos ratio Density (gcm3) Permeability coefficient (md)Miscellaneous fill 2 035 18 125Silty clay 4 029 196 005Clayey silt 4 030 194 015Fine sand 4 028 185 3Medium sand 40 026 19 10Loose pebble layer 18 025 2 22Slightly condensed pebble layer 20 023 21 22Medium condensed pebble layer 28 020 22 22Compacted pebble layer 38 017 23 22
Miscellaneous fill Silty clayClayey siltFine sandMedium sand
12345
Figure 11 Geological section of the territory
12 Advances in Civil Engineering
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
and cost -e waterproof curtains refer to the ones thatdo not penetrate the entire aquifer but enter a certaindepth in the aquifer and combine the design ofgroundwater extraction in the foundation pit to form agroundwater treatment method for internal water low-ering and external water stopping When groundwaterextraction inside a foundation pit is carried out thegroundwater outside the pit will bypass the bottom of thewaterproof curtains and pass through the aquifer into thefoundation pit Compared with the dewatering outsidethe foundation pit it not only increases the seepage pathof the foundation pit but also reduces the head lossoutside the foundation pit -e influence of dewateringinside the foundation pit on the surrounding environ-ment is less than the one caused by the dewateringoutside the pit If it is a fully enclosed foundation pit tobe specific the enclosure structure or the diaphragmwalls can then be extended to the bottom of the aquiferand be inserted into impermeable stratum underneaththe bottom the groundwater outside the foundation pitwill be completely isolated from that one inside the pitCurrently groundwater extraction in the foundation pitbasically has no effect on ground surface outside thefoundation pit If it is a semiclosed foundation pit that isthe waterproof structure or the diaphragm wall isinserted into the middle and lower parts of the aquiferthe groundwater inside and outside the upper foundationpit will be discontinuous and the bottom aquifer willbecome continuous -erefore the groundwater insidethe foundation pit can be replenished by the aquiferoutside the pit At this time the groundwater extractionin the foundation pit will lead to a series of problemssuch as ground subsidence deformation of supportstructure and uplift of the bottom of the foundation pitAmong them the ground subsidence outside the pit ismore likely to occur so this paper focuses on addressingthe problem
Yihdego [1] studied the relationship between the re-duction in flow and cutoff of hydraulic barriers in a period oftime and found the effect of barriers begins to be significantafter cutoff exceeds 60 But as for this project the inserteddepth of enclosure structures is far smaller than the distancebetween the bottom of the foundation pit and the top of theimpermeable layer so the inserted depth is not consideredand the enclosure structures have no effect on groundwaterflow below the foundation pit ideally And the design schemeof groundwater extraction in the open cut foundation pit isillustrated in Figure 1
In Figure 1 H indicates the thickness of the phreaticaquifer eg original water table in the foundation pit m Sdenotes the maximal depth of dewatering outside thefoundation pit m Sw denotes the depth of dewatering in thewell point m hprime indicates the water head at the bottom of thecentral axis of the enclosure structure m and h denotes thewater level after dewatering in the foundation pit m
Many scholars have studied the dewatering in thefoundation pit Zhang et al [2] proposed an analyticalcalculation method for predicting tunnel deformation in-duced by upside excavation and also discussed the role of
dewatering in the deformation mechanism Wang et al [3]established a conceptual and mathematical model thatconsidered hydrogeological conditions curtain depth andpumping well screens and performed numerical simulationsbased on themodel Xu et al [4] investigated the engineeringgeology and hydrogeology related to foundation dewateringand discussed the current state of foundation dewateringworks resulting in land subsidence in Shanghai Wang et al[5] introduced a transparent soil model test to address thelimitation of the existing experimental method and nu-merical simulations in modeling the coupling mechanismbetween the cutoff wall and the pumping wells and proposedthe optimal depth of the pumping wells and the optimalhorizontal distance between the cutoff wall and the pumpingwells In order to analyze the influence of layering themechanical parameters and the relationship betweenground settlements and drawdown Pujades et al [6]adopted a radially symmetric conceptual model and con-ducted several hydromechanical simulations by varying theboundary conditions the size of the modeled domain andthe presence or absence of an overlying layer Based on alarge deep excavation of the buildings in Oriental Fisher-manrsquos Wharf Wang et al [7] performed single-well andgroup-well field pumping tests and carried out a numericalsimulation by using the 3D finite difference method (FDM)Taking Qianjiang Century City Station foundation pit as anexample Wang et al [8] performed field experiments toobserve the coupling non-Darcy flow in round gravelestablished a generalized conceptual model to study thecoupling effect under different combinations of curtain andpumping wells and carried out numerical simulations of thecoupling non-Darcy flow in foundation pit dewatering basedon Forchheimerrsquos equation Based on a deep excavationproject in Tianjin Wu et al [9] conducted field measure-ments of the groundwater head and the building settlementduring excavation and analyzed the influence range ofdewatering and the relationship between the drawdownhead and the settlement To predict the behavior of landsubsidence due to groundwater extraction Zhang et al [10]established a three-dimensional numerical model consid-ering the confined aquifer and soft deposit and then ana-lyzed and compared between the calculated result andmeasured value -is paper mainly takes the dewateringproject of an open cut foundation pit of a metro station onChengdu Metro Line 6 as an example -e results of groundsubsidence around the foundation pit calculated by usingtheoretical formulae and numerical simulation FLAC3D arerespectively compared with the on-site monitoring data-e design scheme of the foundation pit dewatering is alsoproposed and the ground subsidence curve caused bydewatering is also compared -us the results proposed inthis paper can be used as a reference and guidance for similarprojects under similar geological conditions
2 Design and Calculation of Dewatering inFoundation Pit
21 Calculation of Dewatering in Foundation Pit in Single SoilLayer under Waterproofing Enclosure Structure It is seen
2 Advances in Civil Engineering
from references [11ndash13] that if the boundary of the seepagefield is impervious the flow line in the flow net is parallel tothe boundary and while the seepage field is equal to thewater head boundary the flow line is orthogonal to theseepage boundary -erefore the seepage field around thefoundation pit under the geological conditions of single-layered soil is shown in Figure 2
Both the enclosure structure and the bottom im-pervious layer can be regarded as impervious boundariesand the horizontal seepage velocity of the groundwater atthe bottom of the enclosure structure is far greater thanits vertical one so the flow of groundwater at differentdepths below the bottom of the center axis of the en-closure structure is approximately regarded as horizontalflow that is laminar flow -erefore the water head lineat the bottom of the central axis of the enclosurestructure is vertical -us according to these vertical flowlines the seepage field around the foundation pit is di-vided into two seepage fields respectively one is insidethe foundation pit and the other is outside the foundationpit Water inflow from the two seepage fields can then besolved separately It is known that the groundwateroutside the pit provides water inflow for groundwaterinside the pit thus the water inflow Q1 inside the pit isequal to the water inflow Q2 outside the pit namely
Q1 Q2 (1)
-e radius of influence is defined as the maximumdistance at which the drawdowns can be detected with theusual measure devices in the field [14] -e most commonway to find the radius of influence is the use of empiricalformulae [15ndash17] such as Sichardtrsquos formula as well asKusakinrsquos formula Furthermore related influence factorslike time t and radius of the pit re are also taken into accountin the formulae by some scholars [15ndash17] In this project thedesign is based on the Chinese Code According to theChinese Technical Specification for Retaining and Protec-tion of Building Foundation Excavations (JGJ 120-2012)[18] the radius of influence for phreatic aquifers in the
foundation pit can be calculated according to the followingequation
R 2Sw
Hk
radic (2)
where R is the radius of influence m Sw denotes the depth ofdewatering in the well point m H indicates the thickness ofthe phreatic aquifer eg original water table in the foun-dation pit m and k refers to the permeability coefficient ofthe ground md
In order to analyze the water inflow inside and outsidethe foundation pit two conditions are taken into account asfollows
① If considering the enclosure structure as the wall of awell point the entire foundation pit can then beconsidered as a submersible incomplete well and thewater inflow outside the foundation pit away fromthe boundary can be calculated approximately byusing the normative formula presented in theTechnical Specification JGJ 120-2012 [18] As for acircular or a rectangular pit with length-width ratioless than 20 the water inflow Q2 is calculatedaccording to the following equation [19]
H
h
S
Phreatic aquifer
Dewatering funnel curveEnclosure structure
Well pointGround
Impermeable stratum
Groundwater level
S whprime
Figure 1 Schematic diagram of dewatering design in the open cut foundation pit
Water level in the pit
Impermeablestratum
Phreatic aquifer
Groundwater level
Enclosure structure
Ground
Flow line
S w
H
hprime
Figure 2 Seepage field distribution around a single-layered soilfoundation pit
Advances in Civil Engineering 3
Q2 1366k H2 minus h2
m1113872 1113873
lg 1 + Rr0( 1113857( 1113857 + hm minus l( 1113857l( 1113857lg 1 + 02 hmr0( 1113857( 1113857
(3)
hm H + hprime
2 (4)
where r0 stands for the equivalent radius of thefoundation pit m it is calculated according tor0 0565
A0
1113968A0 denotes the foundation pit area m2
hprime indicates the water head at the bottom of the centralaxis of the enclosure structure m and l means thelength of the water inlet part of the dewatering well m
② -e enclosure structure and the bottom boundary areboth impervious layers According to Darcyrsquos seepageexperimental conditions the seepage field distribu-tion in Figure 3 is simplified to be a one-dimensionalflow field distribution as shown in Figure 4
-at is in the assumption that the groundwater in the pitone-dimensionally flows in a circular glass tube and itsatisfies Darcyrsquos flow law water inflow Q1 into the pit isderived theoretically as follows
Q1 kA hprime minus h( 1113857
L (5)
A V
L (6)
V πr20 l2 + l3( 1113857 (7)
L 2l2 + l3 + r0( 1113857
2 (8)
H l1 + l2 + l3 (9)
h l2 + l3 (10)
Sw H minus hprime (11)
where h denotes water head height in the foundation pit afterdewatering m l1 denotes water table drawdown in thefoundation pit m l2 refers to the distance from the watertable to the bottom of the enclosure structure after dew-atering in the foundation pit m l3 indicates the distancefrom the bottom of the enclosure structure to the imperviouslayer m and A V and L refers to the cross-sectional area ofthe seepage field m2 the total volume of seepage m3 andaverage seepage path m respectively
Simultaneous solution is obtained from equations (1) to(11) and then the following equation is obtained
4πr20 l2 + l3( 1113857 hprime minus h( 1113857
2l2 + l3 + r0( 11138572
03415 4H2 minus H + hprime( 11138572
1113960 1113961
lg 1 + 2 H minus hprime( 1113857Hk
radic( 1113857r0( 11138571113858 1113859 + H + hprime minus 2l( 11138572l( 1113857lg 1 + H + hprime( 111385710r0( 1113857( 1113857
(12)
l 2 (l)
l 3l 1
r0
Initial groundwater level
Water levelin the pit
Enclosure structure
H
h
S whprime
Flow line
Figure 3 Seepage field distribution in the pit
r 0l 3
l 2
L
h
hprime
l 2 (l)
Figure 4 Simplification of the seepage path in the pit
4 Advances in Civil Engineering
As for a real open cut foundation pit it is seen fromequation (12) that there is only one unknown variable inequation (12) that is hprime the water head at the bottom of thecentral axis of the enclosure structure inside the pit -iswater head at the bottom of the central axis of the enclosurestructure inside the pit can be iteratively obtained so that theradius of influence of dewatering in the foundation pit andthe water inflow in the foundation pit can also be obtained
22 Calculation of Dewatering in Foundation Pit in Multi-layered Soil under Waterproofing Enclosure Structure Forthe calculation of water inflow in the foundation pit con-sidering the waterproofing effect of the enclosure structureunder the geological conditions of the multilayered soil thestratification calculation method is adopted to calculate thewater inflow of each layer of soil separately and algebraiccalculation is performed to obtain the total water inflow inthe pit Generally speaking there are many soil layers inactual foundation pits It is very cumbersome and time-consuming to use this method -erefore the geologicalconditions of the multilayered soils are simplified to be asingle formation and the permeability coefficient is averagedfor calculation -ree soil layers are used to illustrate thismethod as shown in Figure 5
-e permeability coefficient is calculated as follows
k k1h1 + k2h2 + k3h3
h1 + h2 + h3 (13)
where h1 h2 and h3 denote the thicknesses of the three soillayers respectively m and k1 k2 and k3 stand for thepermeability coefficients respectively corresponding to thethree soil layers md
23 eoretical Design of Dewatering In the actual projectthe average permeability coefficient of the multilayered soilsis firstly obtained according to equation (13) and then byusing equation (12) engineering parameters are substitutedand simplified to obtain the transcendental equation abouthprime -is equation can only be solved by means of a computerso it is solved with Matlab using the dichotomy By inputtingthe program in Matlab the water head hprime at the bottom ofthe central axis of the enclosure structure can be obtainedand then both the depth Sw of lowering water level in the wellpoint and radius of influence R of the phreatic aquifer infoundation pit can then be obtained
From equations (8) to (11) the equation for calculatingthe water inflow Q1 in the pit is derived as follows
Q1 kA hprime minus h( 1113857
L4kπr20 l2 + l3( 1113857 hprime minus h( 1113857
2l2 + l3 + r0( 11138572 (14)
-e water inflow of a single well is calculated as follows[18]
q0 120πrslk
3radic
(15)
where q0 represents the water inflow capacity of a single wellm3d rs denotes the filter radius m l stands for the length of
the inlet part of the filter m and k denotes the permeabilitycoefficient of the aquifer md
-e number of dewatering wells is calculated as followsaccording to the Chinese Technical Code for GroundwaterControl in Building and Municipal Engineering (JGJT111-2016) [19] if the safety level of the foundation pit isassessed to be in Grade I and the complexity of thefoundation pit is evaluated to be complicated and then thecalculation coefficient ε in equation (16) gets the value of12 -e number of well points is obtained from the fol-lowing equation
n εQ1
q0 (16)
D L
n (17)
where D denotes the space between well points m L rep-resents the circumference of the foundation pit m and nmeans the number of well points
-erefore the layout of the dewatering well points in theactual foundation pit can be obtained from the precedingequations
3 Case Study of Dewatering in Foundation Pit
31 Engineering Background -is paper depends on adeep foundation pit of a subway station on ChengduMetro Line 6 -e station is an underground three-storied island platform station its east side is closelyadjacent to a street-facing commercial store that has 2-3stories of brick-concrete structure In addition a 220 kVpower cable tunnel constructed with conventionalmining method is buried on its east side -e power cabletunnel is 14 m away from the sidewall of the station -ewest side of the subway station closely approaches privatehouses and public shops on the ground surface and theouter edge of a shop on the west side lies within aminimum distance of 17 m away from the foundationpit -e station is a 130 m wide island platform stationwith a standard cross-sectional width of 225 m and itstotal length is 2429 m long on the right side and 2221 mlong on the left side -e depth of the soil on its roof isabout 398 m and the depth of the bottom is about2664 m -e station and its surrounding environmentare shown in Figure 6 According to the hydrogeologicalconditions for this project there are two types ofgroundwater in the site one is the perched water in thebackfill layer above the clay layer and the other is thepore water in the quaternary sand and pebble layer -eprimary geotechnical investigation showed that stablewater table measured in the site was 500sim640 m inOctober 2015 and the detailed investigation showed thatit was 540sim670 m in October 2016 Obviously there isminor difference in the two results of water tables so thewater level in the site is based on the results of the de-tailed geotechnical investigation -e site geotechnicalproperties and its distribution are also shown in Table 1
Advances in Civil Engineering 5
32 Design of Dewatering Well Points Since the subwaystation is closely located to the shops and buildings on its twosides and the underground pipelines are densely packed inpower cable tunnel and municipal sewages there are noother spare places to install dewatering wells outside thefoundation pit -erefore to avoid the impact of dewateringon the surrounding environment well points are used to thelower groundwater level inside the foundation pit prior to itsexcavation
-e groundwater of the construction site belongs to theQuaternary existing in the sandy pebble pore phreaticaquifer -e thickness of the phreatic aquifer is less than30m and the bottom floor of the station is located in thecompacted pebble layer -e purpose of dewatering is tolower the water table in the foundation pit to 1m below itsbottom so that normal construction of the subway stationcan be fulfilled without groundwater -e open cut foun-dation pit of the station is 225m long and 24m wide with a
Family wingsEntrance F
Entrance GEntrance 2Shops
Entrance DEntrance E
Foundation pit
N
Seco
nd ri
ng
Figure 6 Station location and its surrounding environment
⑧ Medium dense pebblelayer 1sim214 1233 255times10minus 2 32
⑨ Compacted pebblelayer Not drilled Not drilled 255times10minus 2 43
h 1h 2
h 3
k1
k2
k3
Ground
Groundwater level
Soil layer 1
Soil layer 2
Soil layer 3
Phreatic aquifer
Impermeable stratum
Figure 5 Geological conditions of multilayered soil
6 Advances in Civil Engineering
length-width ratio of 9375lt 20-e depth of the foundationpit is about 2664m For the convenience of calculation it isset to 27m -e diameter of the dewatering well is 600mmand the well bottom is 35m lower than the one of thestation-e length of the filter pipe is 2m-e distance fromthe bottom of the filter pipe to the impervious layer is 2mand the stable water table is considered to be 6m
From the theoretical design and calculation of Section 2 itis known that 17 well points are to be laid around the foun-dation pit Referring to the Chinese Technical Code forGroundwater Control in Building and Municipal Engineering(JGJT111-2016) [19] and considering an idealized situation inwhichwells can be easily bored around the open cut foundationpit the dewatering wells are arranged at the same distancealong the foundation pit After a well is installed at the center ofthe foundation pit the remaining 16 well points are installedevenly at the inner edge of the pit Since it is a long and narrowopen cut foundation pit the wells can be placed at the inneredge of the long side of the foundation pit and the spacebetween them is about 25m -e specific layout of well pointsinside the open cut foundation pit is shown in Figure 7
33 Comparison between Calculations with and withoutconsidering the Waterproof Effect of Enclosure StructureAccording to the Chinese Technical Code for groundwatercontrol in building and municipal engineering [18] if groupwell points are simplified to be large one the total waterinflow from the incomplete well points in the phreaticaquifer which is calculated by using equations (3) and (4) isreplaced with the following equation
hm H + h
2 (18)
-e parameters in equation (18) are the same as that inthe formulae as stated above If the waterproofing effect ofthe enclosure structure is not considered then the depth ofwater level lowered by well points is expressed below
Sw H minus h (19)
Substituting aforementioned engineering data intoequations (3) and (16)ndash(18) respectively it is seen that thewater inflow from incomplete well point Q3 in the foun-dation pit is
Q3 Q2 1209532m3 (20)
the radius of influence of the foundation pit Rprime is
Rprime 2Sw
Hk
radicasymp 105233m (21)
and the number of well points nprime is certainly obtainedaccording to
nprime 12Q2
q0asymp 23 (22)
After a well point is installed at the center of the pit theremaining 22 well points can then be set evenly at the edge ofthe foundation pit Since it is a long and narrow foundationpit the well points are evenly installed at the edge of the long
side L of the foundation pit and the space Dprime between themis
Dprime L
nasymp 2045m (23)
Compared with the waterproofing effect of the enclosurestructure the water inflow in the foundation pit is893651209532 asymp 074 times of the conventional algorithmproposed in the Chinese Specification and the dewateringradius of influence is 54099105233 asymp 051 times of theconventional algorithm in the Specification -e number ofwell points is 1723 asymp 074 times of the conventional algo-rithm in the Specification and the space between them is252045 asymp 122 times of the conventional algorithm in theSpecification -rough comparison if the enclosure struc-ture of the foundation pit is used as a waterproof curtainthen the waterproofing effect of the enclosure structureshould not be ignored when calculating water inflow insidethe open cut foundation pit
34 Arrangement of Points Monitoring Ground SubsidenceIn order to make further analysis on the ground subsidencecaused by dewatering in the foundation pit the groundsubsidence of the typical positions around the foundation pitis monitored -e ground subsidence monitoring points areset up according to the actual condition of the open cutfoundation pit In the actual project the ground subsidencemonitoring points are arranged around the foundation pitFor the consideration of symmetry and the convenience ofmeasuring six points at the midline of the long side of thefoundation pit are selected and they are 16-6 16-5 16-4 16-3 16-2 and 16-1 respectively the distance between the wellpoints and pit wall is set within 8m 12m 16m 20m 24mand 28m respectively -e monitoring points are shown inFigure 8 which are located at the center axis of the edge lineof the pit Step-by-step dewatering is carried out in the actualproject and the depth of dewatering for every step is set to6m 5m 5m and 6m respectively and the total stepamounts to 22m
4 Theoretical Calculation of GroundSubsidence Caused by Dewatering inFoundation Pit
41 Normative Calculation of Ground Subsidence Caused byDewatering in Foundation Pit
411 Dupuitrsquos Assumption and Derivation of DewateringFunnel Curve Dewatering in the foundation pit will defi-nitely produce a falling funnel curve around the pit and thegroundwater may flow into the dewatering well inside thefoundation pit French scholar Dupuit first studied thesteady well flow put forward Dupuitrsquos assumption andderived the dewatering funnel curve -e hypothesis con-siders a cylindrical homogeneous phreatic aquifer withisotropic and horizontally waterproofing bottom floor afixed water head outside the aquifer a complete pumpingwell in the center no vertical infiltration recharge and
Advances in Civil Engineering 7
evaporation and a steady seepage subject to the linear law[20] -e dewatering well point can develop a dewateringfunnel curve around it Groundwater flows to the well after acertain time and the dewatering curve can reach a steadystate Assuming that the well point dewatering is a stablephreatic well flow without the group well effect the center ofthe well bottom is set to be the origin and the abscissa is setto be the positive x-axis as shown in Figure 9
According to Dupuitrsquos assumption the water flowequation of the stable phreatic well is obtained below [20]
Q kIA 2πrhkdh
dr (24)
If we separate the variable in equation (24) and take anyone point on the dewatering funnel curve then followingequations are obtained by integrating the equation from thepoint to its boundary
r x h z
r R h H
⎧⎪⎨
⎪⎩
⎫⎪⎬
⎪⎭⟶ 1113946
R
x
Q
2πk
1rdr 1113946
H
zh dh (25)
and thenQ
2πkln
R
x12
H2
minus z2
1113872 1113873 (26)
If the boundary condition meets
x r0
z h0 + l(27)
and substituting equation (27) into equation (26) then thefollowing equation is obtained
Q
2πkln
R
r012
H2
minus h0 + l( 11138572
1113960 1113961 (28)
-e solution of simultaneous equations (26) and (28) isused to obtain the following dewatering funnel curveequation of the well point
z2
H2
minus H2
minus h0 + l( 11138572
1113960 1113961ln(Rx)
ln Rr0( 1113857 (29)
where r0 denotes the radius of wells m h0 represents thelength of the inlet part of the filter pipe m R denotes theradius of influence m H represents the height from thebottom of the well to the initial groundwater level m k is thepermeability coefficient of the aquifer md A is the side areaof the dewatering well m2 I denotes the groundwaterseepage field hydraulic gradient Q stands for the boreholewater yield m3d and l means the distance between thebottom of well and the impermeable layer m-e remainingsymbols are shown in Figure 9
412 Ground Subsidence at Any Point outside the Foun-dation Pit At present the calculation of ground subsi-dence around the foundation pit after dewatering isgenerally carried out using the stratification summationmethod given in the Technical Specification for Retainingand Protection of Building Foundation Excavations (JGJ120-2012) [18] -e ground subsidence can be calculated bysummarizing the compression of each layer of soil Firstlywe can calculate the additional effective stress caused bydewatering and then calculate the ground subsidence usingstress calculation
(1) When the soil layer numbered as i is above the initialgroundwater level the effective stress is
Δσziprime 0 (30)
(2) If the soil layer numbered as i is located between thepostdewatering groundwater level and the initialgroundwater level then the effective stress is
Δσziprime cwz (31)
Foundation pit
Pit wall 16-616-516-416-316-216-1
Figure 8 Schematic diagram of the ground subsidence monitoring points around the foundation pit
Well point
Foundation pit wall
Figure 7 Schematic diagram of the layout of the dewatering well
8 Advances in Civil Engineering
(3) When the soil layer numbered as i is below thegroundwater level after dewatering then the effectivestress is
Δσziprime λicwsi (32)
-e soil compression caused by dewatering is as follows
s ψw 1113944Δσziprime Δhi
Esi
(33)
where cw means the bulk density of water kNm3 z denotesthe vertical distance from the midpoint of the soil layer i tothe initial groundwater level m and λi represents the cal-culation coefficient it should be based on the analysis ofgroundwater seepage If the analysis data are not availablethen its value should be based on local engineering expe-rience si refers to the depth of lowering groundwater levelcorresponding to the calculation profile m s denotes theground compression in the calculation profile m and ψw
means the empirical coefficient of subsidence calculationbased on local engineering experience If no experience isavailable then the value is set to be 1 Δσzi
prime denotes the meanadditional stress of the soil layer i under the ground surfacecaused by dewatering kPa Δhi means the thickness of thesoil layer i m and Esi
denotes the compressive modulus ofthe soil layer i kPa
42 Calculation of Ground Subsidence Caused by Dewateringin Foundation Pit under Seepage Force -e pumping anddrainage of the dewatering well will cause the change ofthe underground seepage field which will generate a newseepage field and lead to the variation of the stress fieldaround the well -erefore the seepage force is the maincause of soil consolidation and settlement -e seepage ofgroundwater causes the dissipation of pore water pres-sure resulting in an increase in effective stress -at isadditional stress is generated in the soil the direction ofwhich is vertically downward in addition it produces a
horizontal component Deformation can be caused by theimpact of seepage force -e additional stress namely thevertical component of the seepage force will cause theground subsidence [21] -e horizontal component of theseepage force will cause the lateral deformation of the soilAccording to reference [22] the seepage direction of anywater head at any point is tangent to the phreatic surfaceat that point pointing to the well axis as shown inFigure 10
Wu and Zhu [22] performed related research andproposed a new algorithm of ground subsidence caused byseepage force Yang and Zhao [23] also used this method tocalculate ground subsidence -is section draws lessonsfrom their research to discuss the dewatering in open cutfoundation pit of a subway station on Chengdu Metro Line6
-e stratum is divided into three parts dry soil zonedewatered zone and saturated zone -e dry soil zone isalways above the groundwater level before and afterdewatering which does not contain groundwater andseepage force during dewatering -us additional stressdoes not appear in this zone and the subsidence valuecaused by dewatering is 0 -e other two layers are S1 andS2 respectively as shown in Figure 10 -e soil layer S1 inthe dewatering process is drained -e zone S2 is alwaysbelow the groundwater level and is saturated In thissection the soil subsidence in the dewatered and saturatedzones is calculated separately -e seepage forces in thesetwo areas possess horizontal components as shown inFigure 10
Assuming that the water head that keeps a distance of x0from itself to the well axis is z0 as shown in Figure 10according to the dewatering funnel curve equation (29) inSection 41 the height of the falling funnel curve is obtained-e direction of the seepage force is actually in the straightline vector on the curve which goes through the point(x0 z0) and is tangent to the falling funnel curve and pointsto the well In order to find out the slope of the straight linethe following equation is obtained by performing the de-rivative of equation (29) at point x x0
R
H
h 0
x
z
r0 l
Ground
Impermeable stratum
Well point
Groundwater level
Figure 9 Dewatering funnel curve for the phreatic incomplete well point
-e equation of the line that passes through the point(x0 z0) and is tangential to the curve after the dewateringbecomes stable is assumed to be
y x tan α + b (35)
-e slope of the equation is
tan α zprime x0( 1113857 (36)
According to the trigonometric function conversion
sin α tan α
1 + tan2 α
radic (37)
-en the following equation is obtained
sin α zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (38)
-erefore the vertical component of additional pressureof the surrounding stratum caused by dewatering in thefoundation pit is expressed as
ΔPy ΔP sin α ΔPzprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (39)
where ΔPx denotes the horizontal component of the seepageforce ΔPy denotes the vertical component of the seepageforce and ΔP is the seepage force
-e angle α in equation (39) denotes the one existsbetween the horizontal component of the additional pres-sure and the additional pressure caused by dewatering in thefoundation pit
Based on equations (30) to (33) the additional stresses inthe dewatering zone and the saturation zone are calculatedseparately -e rewriting is carried out on the basis ofequation (34) which derives the ground subsidence causedby dewatering in the foundation pit considering the action ofseepage force -e formula after rewriting is as follows
s ψw 1113944Δσziprime Δhi
Esi
zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (40)
Substituting equation (34) into equation (40) yields thefollowing equation
Figure 10 Simplified calculation model considering seepage force
10 Advances in Civil Engineering
If the ground subsidence S1 and S2 corresponding to thedewatered and saturated zones are separately figured outthen the total ground subsidence is obtained and illustratedin the following equation
S S1 + S2 (42)
where the symbols are illustrated in Section 41
5 Numerical Simulation
With fast development of urban construction various in-frastructures such as subway station high-speed railwaytunnel underground parking lot and basement are beingbuilt especially the construction of deep foundation pits-e geological condition varies in the foundation pit and thesurrounding environment is becoming more and morecomplex after construction -eoretical analysis and em-pirical calculation methods are no longer suitable forpractical projects Nowadays actual projects are generallybuilt ahead of theoretical research and calculation methodsof numerical simulation provide an effective approach forthe design and construction of practical foundation pitAlthough numerical simulation method has not been pro-posed for very long time it has become the most commonmethod used in structural analysis and calculation so farMany scholars [24ndash31] have also used FLAC3D to conductfluid-solid coupling analysis of dewatering in the foundationpit Finite difference software FLAC3D is also employed inthis paper to carry out three-dimensional numerical sim-ulation of the whole process of the environment changecaused by dewatering in the open cut foundation pit of ametro station on Chengdu Metro Line 6 -e calculationresults are compared with the one obtained from theoreticalcalculation and on-site monitoring which are used to makefurther study on the surrounding ground subsidence due todewatering in the open cut foundation pit
51 Physical and Mechanical Parameters According to thegeological properties of the actual project the calculationparameters are obtained and shown in Table 2
52 Establishment of 3D Model and Initial Stress BalanceBased on the engineering geological and hydrologicalconditions a three-dimensional stratigraphic model isestablished From the geological section of the site (Fig-ures 11 and 12) it can be seen that each of soil layers is nearlyhorizontal and almost parallel to each other So in order toestablish the model and conduct the calculation conve-niently [26] the geological layers in the model are simplified-e foundation pit and wells are set up in the stratigraphicmodel according to dewatering scheme According to theprinciple of Saint Venant in elastic mechanics in order toeliminate the boundary influence of the foundation pit oncalculation results the calculation model is extended to 3sim5times of the depth of the pit from the periphery of thefoundation pit on the plane -e depth is set to the im-pervious interface and the entire calculation domain is inthe volume of 420mtimes 198mtimes 72m Referring to
geotechnical mapping and geological properties the modelis divided into 9 strata and the generalized permeabilitycoefficient and stratum thickness are shown in Table 1
According to stratum distribution and initial conditionsthe calculation domain is divided into grids by consideringthe monitoring points the dewatering wells and the dia-phragm wall considering the geological survey In themeshing the grid of the calculation domain is locally refinedin addition the grids around the foundation pit are alsorefined but grids far away from the pit are sparsely meshedconsidering Saint Venantrsquos principle [26] So the entiremodel is then divided into 42 layers 78 rows 300 columnsand a total of 982800 elements and the established three-dimensional model is shown in Figure 13
After the 3D model is set up the initial stress balance isneeded to be applied firstly and the corresponding seepageand displacement boundary conditions are also applied -enormal displacement and the bottom displacement of thefour sidewalls and bottom of the model are restricted andthe horizontal displacement of the wall of the dewateringwell is also constrained -e bottom filter pipe of thedewatering well belongs to the seepage boundary which isachieved by applying the pore water pressure After nu-merical simulation the pore water pressure of initialequilibrium is shown in Figure 14 -e pore water pressurein the initial state is evenly applied on the strata
53 Numerical Simulation of Fluid-Solid Coupling of GroundSubsidence Caused by Dewatering in Foundation PitAfter the initial equilibrium of the numerical model theeffect of single well and group wells were firstly carried outseparately -e stepwise dewatering was conducted underthe conditions of single well and group wells separatelywithout considering excavation -e calculated isograms ofthe dewatering at all levels and the subsidence of themonitoring points are analyzed
531 Numerical Simulation Analysis of Single WellDewatering Considering the symmetry of the well pointthe well point at the center of the calculation domain isselected when carrying out single well simulation -e po-sition of the single well on the model is shown in Figure 15-e isograms showing subsidence and pore water pressure atvarious drawdowns are also shown in Figures 16ndash23
It is seen from Figures 16ndash23 that the influence range ofdewatering well increases with the increase of drawdownAfter the well is dewatered the pore water pressure formsthe dewatering funnel surface -e bigger the drawdown isthe deeper the surface becomes Moreover due to the wa-terproofing effect of the diaphragm wall the stratum insidethe pit bulges but the stratum outside the pit subsides -isis because the groundwater flows through the bottom of thediaphragm wall into the pit -e increase of the pore waterpressure in the pit causes the additional stress to increasewhich leads to the upheaval of the soil layer in the pit -edecrease of the pore water pressure outside the pit causes theeffective stress of the outer stratum to decrease resulting inthe formation of downward additional stress in the outer
Advances in Civil Engineering 11
stratum of the pit which eventually leads to the consoli-dation and settlement of the outer stratum
532 Numerical Simulation Analysis of Group WellsDewatering When conducting the effect of group wells onground subsidence the calculation is based on the well pointlayout diagram designed in Section 3 -e layout of thegroup wells in the model is shown in Figure 24-e isogramsshowing ground subsidence and pore water pressure cal-culated using FLAC3D are shown in Figures 25ndash32
It is seen from the isograms shown in Figures 25ndash32 thatthe deeper the groundwater drawdown the greater theimpact on the surrounding ground is and the lower thefunnel surface formed by the pore water pressure Comparedwith the isogram under the effect of single well in Section 52the influence of dewatering by group wells is much largerGroup wells dewatering has a great impact on the pore waterpressure and strata deformation inside the foundation pit
that is the influence of group wells on ground subsidenceinside the pit is obvious and should not be ignored -esubsidence and pore water pressure isogram under differentdrawdowns are symmetrically distributed As shown in thesingle well effect the strata inside the foundation pit bulgeduring the dewatering of group wells but the strata outsidethe foundation pit subside-is is mainly due to the result ofthe movement of groundwater outside the pit to the dew-atering well inside the pit
533 Effect of Group Wells According to the results ob-tained from 3D simulation the subsidence of the sixmonitoring points on the sides of the foundation pit due todifferent drawdowns caused by the single well and the groupwells are shown in Figure 33
Grade I drawdown including Grade II Grade III andGrade IV drawdown means that the dewatering depth ofgroundwater level is in 6m 11m 16m and 22m respectively
Table 2 Physical and mechanical parameters of strata
Name of stratum Modulus of deformation E (MPa) Poissonrsquos ratio Density (gcm3) Permeability coefficient (md)Miscellaneous fill 2 035 18 125Silty clay 4 029 196 005Clayey silt 4 030 194 015Fine sand 4 028 185 3Medium sand 40 026 19 10Loose pebble layer 18 025 2 22Slightly condensed pebble layer 20 023 21 22Medium condensed pebble layer 28 020 22 22Compacted pebble layer 38 017 23 22
Miscellaneous fill Silty clayClayey siltFine sandMedium sand
12345
Figure 11 Geological section of the territory
12 Advances in Civil Engineering
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
from references [11ndash13] that if the boundary of the seepagefield is impervious the flow line in the flow net is parallel tothe boundary and while the seepage field is equal to thewater head boundary the flow line is orthogonal to theseepage boundary -erefore the seepage field around thefoundation pit under the geological conditions of single-layered soil is shown in Figure 2
Both the enclosure structure and the bottom im-pervious layer can be regarded as impervious boundariesand the horizontal seepage velocity of the groundwater atthe bottom of the enclosure structure is far greater thanits vertical one so the flow of groundwater at differentdepths below the bottom of the center axis of the en-closure structure is approximately regarded as horizontalflow that is laminar flow -erefore the water head lineat the bottom of the central axis of the enclosurestructure is vertical -us according to these vertical flowlines the seepage field around the foundation pit is di-vided into two seepage fields respectively one is insidethe foundation pit and the other is outside the foundationpit Water inflow from the two seepage fields can then besolved separately It is known that the groundwateroutside the pit provides water inflow for groundwaterinside the pit thus the water inflow Q1 inside the pit isequal to the water inflow Q2 outside the pit namely
Q1 Q2 (1)
-e radius of influence is defined as the maximumdistance at which the drawdowns can be detected with theusual measure devices in the field [14] -e most commonway to find the radius of influence is the use of empiricalformulae [15ndash17] such as Sichardtrsquos formula as well asKusakinrsquos formula Furthermore related influence factorslike time t and radius of the pit re are also taken into accountin the formulae by some scholars [15ndash17] In this project thedesign is based on the Chinese Code According to theChinese Technical Specification for Retaining and Protec-tion of Building Foundation Excavations (JGJ 120-2012)[18] the radius of influence for phreatic aquifers in the
foundation pit can be calculated according to the followingequation
R 2Sw
Hk
radic (2)
where R is the radius of influence m Sw denotes the depth ofdewatering in the well point m H indicates the thickness ofthe phreatic aquifer eg original water table in the foun-dation pit m and k refers to the permeability coefficient ofthe ground md
In order to analyze the water inflow inside and outsidethe foundation pit two conditions are taken into account asfollows
① If considering the enclosure structure as the wall of awell point the entire foundation pit can then beconsidered as a submersible incomplete well and thewater inflow outside the foundation pit away fromthe boundary can be calculated approximately byusing the normative formula presented in theTechnical Specification JGJ 120-2012 [18] As for acircular or a rectangular pit with length-width ratioless than 20 the water inflow Q2 is calculatedaccording to the following equation [19]
H
h
S
Phreatic aquifer
Dewatering funnel curveEnclosure structure
Well pointGround
Impermeable stratum
Groundwater level
S whprime
Figure 1 Schematic diagram of dewatering design in the open cut foundation pit
Water level in the pit
Impermeablestratum
Phreatic aquifer
Groundwater level
Enclosure structure
Ground
Flow line
S w
H
hprime
Figure 2 Seepage field distribution around a single-layered soilfoundation pit
Advances in Civil Engineering 3
Q2 1366k H2 minus h2
m1113872 1113873
lg 1 + Rr0( 1113857( 1113857 + hm minus l( 1113857l( 1113857lg 1 + 02 hmr0( 1113857( 1113857
(3)
hm H + hprime
2 (4)
where r0 stands for the equivalent radius of thefoundation pit m it is calculated according tor0 0565
A0
1113968A0 denotes the foundation pit area m2
hprime indicates the water head at the bottom of the centralaxis of the enclosure structure m and l means thelength of the water inlet part of the dewatering well m
② -e enclosure structure and the bottom boundary areboth impervious layers According to Darcyrsquos seepageexperimental conditions the seepage field distribu-tion in Figure 3 is simplified to be a one-dimensionalflow field distribution as shown in Figure 4
-at is in the assumption that the groundwater in the pitone-dimensionally flows in a circular glass tube and itsatisfies Darcyrsquos flow law water inflow Q1 into the pit isderived theoretically as follows
Q1 kA hprime minus h( 1113857
L (5)
A V
L (6)
V πr20 l2 + l3( 1113857 (7)
L 2l2 + l3 + r0( 1113857
2 (8)
H l1 + l2 + l3 (9)
h l2 + l3 (10)
Sw H minus hprime (11)
where h denotes water head height in the foundation pit afterdewatering m l1 denotes water table drawdown in thefoundation pit m l2 refers to the distance from the watertable to the bottom of the enclosure structure after dew-atering in the foundation pit m l3 indicates the distancefrom the bottom of the enclosure structure to the imperviouslayer m and A V and L refers to the cross-sectional area ofthe seepage field m2 the total volume of seepage m3 andaverage seepage path m respectively
Simultaneous solution is obtained from equations (1) to(11) and then the following equation is obtained
4πr20 l2 + l3( 1113857 hprime minus h( 1113857
2l2 + l3 + r0( 11138572
03415 4H2 minus H + hprime( 11138572
1113960 1113961
lg 1 + 2 H minus hprime( 1113857Hk
radic( 1113857r0( 11138571113858 1113859 + H + hprime minus 2l( 11138572l( 1113857lg 1 + H + hprime( 111385710r0( 1113857( 1113857
(12)
l 2 (l)
l 3l 1
r0
Initial groundwater level
Water levelin the pit
Enclosure structure
H
h
S whprime
Flow line
Figure 3 Seepage field distribution in the pit
r 0l 3
l 2
L
h
hprime
l 2 (l)
Figure 4 Simplification of the seepage path in the pit
4 Advances in Civil Engineering
As for a real open cut foundation pit it is seen fromequation (12) that there is only one unknown variable inequation (12) that is hprime the water head at the bottom of thecentral axis of the enclosure structure inside the pit -iswater head at the bottom of the central axis of the enclosurestructure inside the pit can be iteratively obtained so that theradius of influence of dewatering in the foundation pit andthe water inflow in the foundation pit can also be obtained
22 Calculation of Dewatering in Foundation Pit in Multi-layered Soil under Waterproofing Enclosure Structure Forthe calculation of water inflow in the foundation pit con-sidering the waterproofing effect of the enclosure structureunder the geological conditions of the multilayered soil thestratification calculation method is adopted to calculate thewater inflow of each layer of soil separately and algebraiccalculation is performed to obtain the total water inflow inthe pit Generally speaking there are many soil layers inactual foundation pits It is very cumbersome and time-consuming to use this method -erefore the geologicalconditions of the multilayered soils are simplified to be asingle formation and the permeability coefficient is averagedfor calculation -ree soil layers are used to illustrate thismethod as shown in Figure 5
-e permeability coefficient is calculated as follows
k k1h1 + k2h2 + k3h3
h1 + h2 + h3 (13)
where h1 h2 and h3 denote the thicknesses of the three soillayers respectively m and k1 k2 and k3 stand for thepermeability coefficients respectively corresponding to thethree soil layers md
23 eoretical Design of Dewatering In the actual projectthe average permeability coefficient of the multilayered soilsis firstly obtained according to equation (13) and then byusing equation (12) engineering parameters are substitutedand simplified to obtain the transcendental equation abouthprime -is equation can only be solved by means of a computerso it is solved with Matlab using the dichotomy By inputtingthe program in Matlab the water head hprime at the bottom ofthe central axis of the enclosure structure can be obtainedand then both the depth Sw of lowering water level in the wellpoint and radius of influence R of the phreatic aquifer infoundation pit can then be obtained
From equations (8) to (11) the equation for calculatingthe water inflow Q1 in the pit is derived as follows
Q1 kA hprime minus h( 1113857
L4kπr20 l2 + l3( 1113857 hprime minus h( 1113857
2l2 + l3 + r0( 11138572 (14)
-e water inflow of a single well is calculated as follows[18]
q0 120πrslk
3radic
(15)
where q0 represents the water inflow capacity of a single wellm3d rs denotes the filter radius m l stands for the length of
the inlet part of the filter m and k denotes the permeabilitycoefficient of the aquifer md
-e number of dewatering wells is calculated as followsaccording to the Chinese Technical Code for GroundwaterControl in Building and Municipal Engineering (JGJT111-2016) [19] if the safety level of the foundation pit isassessed to be in Grade I and the complexity of thefoundation pit is evaluated to be complicated and then thecalculation coefficient ε in equation (16) gets the value of12 -e number of well points is obtained from the fol-lowing equation
n εQ1
q0 (16)
D L
n (17)
where D denotes the space between well points m L rep-resents the circumference of the foundation pit m and nmeans the number of well points
-erefore the layout of the dewatering well points in theactual foundation pit can be obtained from the precedingequations
3 Case Study of Dewatering in Foundation Pit
31 Engineering Background -is paper depends on adeep foundation pit of a subway station on ChengduMetro Line 6 -e station is an underground three-storied island platform station its east side is closelyadjacent to a street-facing commercial store that has 2-3stories of brick-concrete structure In addition a 220 kVpower cable tunnel constructed with conventionalmining method is buried on its east side -e power cabletunnel is 14 m away from the sidewall of the station -ewest side of the subway station closely approaches privatehouses and public shops on the ground surface and theouter edge of a shop on the west side lies within aminimum distance of 17 m away from the foundationpit -e station is a 130 m wide island platform stationwith a standard cross-sectional width of 225 m and itstotal length is 2429 m long on the right side and 2221 mlong on the left side -e depth of the soil on its roof isabout 398 m and the depth of the bottom is about2664 m -e station and its surrounding environmentare shown in Figure 6 According to the hydrogeologicalconditions for this project there are two types ofgroundwater in the site one is the perched water in thebackfill layer above the clay layer and the other is thepore water in the quaternary sand and pebble layer -eprimary geotechnical investigation showed that stablewater table measured in the site was 500sim640 m inOctober 2015 and the detailed investigation showed thatit was 540sim670 m in October 2016 Obviously there isminor difference in the two results of water tables so thewater level in the site is based on the results of the de-tailed geotechnical investigation -e site geotechnicalproperties and its distribution are also shown in Table 1
Advances in Civil Engineering 5
32 Design of Dewatering Well Points Since the subwaystation is closely located to the shops and buildings on its twosides and the underground pipelines are densely packed inpower cable tunnel and municipal sewages there are noother spare places to install dewatering wells outside thefoundation pit -erefore to avoid the impact of dewateringon the surrounding environment well points are used to thelower groundwater level inside the foundation pit prior to itsexcavation
-e groundwater of the construction site belongs to theQuaternary existing in the sandy pebble pore phreaticaquifer -e thickness of the phreatic aquifer is less than30m and the bottom floor of the station is located in thecompacted pebble layer -e purpose of dewatering is tolower the water table in the foundation pit to 1m below itsbottom so that normal construction of the subway stationcan be fulfilled without groundwater -e open cut foun-dation pit of the station is 225m long and 24m wide with a
Family wingsEntrance F
Entrance GEntrance 2Shops
Entrance DEntrance E
Foundation pit
N
Seco
nd ri
ng
Figure 6 Station location and its surrounding environment
⑧ Medium dense pebblelayer 1sim214 1233 255times10minus 2 32
⑨ Compacted pebblelayer Not drilled Not drilled 255times10minus 2 43
h 1h 2
h 3
k1
k2
k3
Ground
Groundwater level
Soil layer 1
Soil layer 2
Soil layer 3
Phreatic aquifer
Impermeable stratum
Figure 5 Geological conditions of multilayered soil
6 Advances in Civil Engineering
length-width ratio of 9375lt 20-e depth of the foundationpit is about 2664m For the convenience of calculation it isset to 27m -e diameter of the dewatering well is 600mmand the well bottom is 35m lower than the one of thestation-e length of the filter pipe is 2m-e distance fromthe bottom of the filter pipe to the impervious layer is 2mand the stable water table is considered to be 6m
From the theoretical design and calculation of Section 2 itis known that 17 well points are to be laid around the foun-dation pit Referring to the Chinese Technical Code forGroundwater Control in Building and Municipal Engineering(JGJT111-2016) [19] and considering an idealized situation inwhichwells can be easily bored around the open cut foundationpit the dewatering wells are arranged at the same distancealong the foundation pit After a well is installed at the center ofthe foundation pit the remaining 16 well points are installedevenly at the inner edge of the pit Since it is a long and narrowopen cut foundation pit the wells can be placed at the inneredge of the long side of the foundation pit and the spacebetween them is about 25m -e specific layout of well pointsinside the open cut foundation pit is shown in Figure 7
33 Comparison between Calculations with and withoutconsidering the Waterproof Effect of Enclosure StructureAccording to the Chinese Technical Code for groundwatercontrol in building and municipal engineering [18] if groupwell points are simplified to be large one the total waterinflow from the incomplete well points in the phreaticaquifer which is calculated by using equations (3) and (4) isreplaced with the following equation
hm H + h
2 (18)
-e parameters in equation (18) are the same as that inthe formulae as stated above If the waterproofing effect ofthe enclosure structure is not considered then the depth ofwater level lowered by well points is expressed below
Sw H minus h (19)
Substituting aforementioned engineering data intoequations (3) and (16)ndash(18) respectively it is seen that thewater inflow from incomplete well point Q3 in the foun-dation pit is
Q3 Q2 1209532m3 (20)
the radius of influence of the foundation pit Rprime is
Rprime 2Sw
Hk
radicasymp 105233m (21)
and the number of well points nprime is certainly obtainedaccording to
nprime 12Q2
q0asymp 23 (22)
After a well point is installed at the center of the pit theremaining 22 well points can then be set evenly at the edge ofthe foundation pit Since it is a long and narrow foundationpit the well points are evenly installed at the edge of the long
side L of the foundation pit and the space Dprime between themis
Dprime L
nasymp 2045m (23)
Compared with the waterproofing effect of the enclosurestructure the water inflow in the foundation pit is893651209532 asymp 074 times of the conventional algorithmproposed in the Chinese Specification and the dewateringradius of influence is 54099105233 asymp 051 times of theconventional algorithm in the Specification -e number ofwell points is 1723 asymp 074 times of the conventional algo-rithm in the Specification and the space between them is252045 asymp 122 times of the conventional algorithm in theSpecification -rough comparison if the enclosure struc-ture of the foundation pit is used as a waterproof curtainthen the waterproofing effect of the enclosure structureshould not be ignored when calculating water inflow insidethe open cut foundation pit
34 Arrangement of Points Monitoring Ground SubsidenceIn order to make further analysis on the ground subsidencecaused by dewatering in the foundation pit the groundsubsidence of the typical positions around the foundation pitis monitored -e ground subsidence monitoring points areset up according to the actual condition of the open cutfoundation pit In the actual project the ground subsidencemonitoring points are arranged around the foundation pitFor the consideration of symmetry and the convenience ofmeasuring six points at the midline of the long side of thefoundation pit are selected and they are 16-6 16-5 16-4 16-3 16-2 and 16-1 respectively the distance between the wellpoints and pit wall is set within 8m 12m 16m 20m 24mand 28m respectively -e monitoring points are shown inFigure 8 which are located at the center axis of the edge lineof the pit Step-by-step dewatering is carried out in the actualproject and the depth of dewatering for every step is set to6m 5m 5m and 6m respectively and the total stepamounts to 22m
4 Theoretical Calculation of GroundSubsidence Caused by Dewatering inFoundation Pit
41 Normative Calculation of Ground Subsidence Caused byDewatering in Foundation Pit
411 Dupuitrsquos Assumption and Derivation of DewateringFunnel Curve Dewatering in the foundation pit will defi-nitely produce a falling funnel curve around the pit and thegroundwater may flow into the dewatering well inside thefoundation pit French scholar Dupuit first studied thesteady well flow put forward Dupuitrsquos assumption andderived the dewatering funnel curve -e hypothesis con-siders a cylindrical homogeneous phreatic aquifer withisotropic and horizontally waterproofing bottom floor afixed water head outside the aquifer a complete pumpingwell in the center no vertical infiltration recharge and
Advances in Civil Engineering 7
evaporation and a steady seepage subject to the linear law[20] -e dewatering well point can develop a dewateringfunnel curve around it Groundwater flows to the well after acertain time and the dewatering curve can reach a steadystate Assuming that the well point dewatering is a stablephreatic well flow without the group well effect the center ofthe well bottom is set to be the origin and the abscissa is setto be the positive x-axis as shown in Figure 9
According to Dupuitrsquos assumption the water flowequation of the stable phreatic well is obtained below [20]
Q kIA 2πrhkdh
dr (24)
If we separate the variable in equation (24) and take anyone point on the dewatering funnel curve then followingequations are obtained by integrating the equation from thepoint to its boundary
r x h z
r R h H
⎧⎪⎨
⎪⎩
⎫⎪⎬
⎪⎭⟶ 1113946
R
x
Q
2πk
1rdr 1113946
H
zh dh (25)
and thenQ
2πkln
R
x12
H2
minus z2
1113872 1113873 (26)
If the boundary condition meets
x r0
z h0 + l(27)
and substituting equation (27) into equation (26) then thefollowing equation is obtained
Q
2πkln
R
r012
H2
minus h0 + l( 11138572
1113960 1113961 (28)
-e solution of simultaneous equations (26) and (28) isused to obtain the following dewatering funnel curveequation of the well point
z2
H2
minus H2
minus h0 + l( 11138572
1113960 1113961ln(Rx)
ln Rr0( 1113857 (29)
where r0 denotes the radius of wells m h0 represents thelength of the inlet part of the filter pipe m R denotes theradius of influence m H represents the height from thebottom of the well to the initial groundwater level m k is thepermeability coefficient of the aquifer md A is the side areaof the dewatering well m2 I denotes the groundwaterseepage field hydraulic gradient Q stands for the boreholewater yield m3d and l means the distance between thebottom of well and the impermeable layer m-e remainingsymbols are shown in Figure 9
412 Ground Subsidence at Any Point outside the Foun-dation Pit At present the calculation of ground subsi-dence around the foundation pit after dewatering isgenerally carried out using the stratification summationmethod given in the Technical Specification for Retainingand Protection of Building Foundation Excavations (JGJ120-2012) [18] -e ground subsidence can be calculated bysummarizing the compression of each layer of soil Firstlywe can calculate the additional effective stress caused bydewatering and then calculate the ground subsidence usingstress calculation
(1) When the soil layer numbered as i is above the initialgroundwater level the effective stress is
Δσziprime 0 (30)
(2) If the soil layer numbered as i is located between thepostdewatering groundwater level and the initialgroundwater level then the effective stress is
Δσziprime cwz (31)
Foundation pit
Pit wall 16-616-516-416-316-216-1
Figure 8 Schematic diagram of the ground subsidence monitoring points around the foundation pit
Well point
Foundation pit wall
Figure 7 Schematic diagram of the layout of the dewatering well
8 Advances in Civil Engineering
(3) When the soil layer numbered as i is below thegroundwater level after dewatering then the effectivestress is
Δσziprime λicwsi (32)
-e soil compression caused by dewatering is as follows
s ψw 1113944Δσziprime Δhi
Esi
(33)
where cw means the bulk density of water kNm3 z denotesthe vertical distance from the midpoint of the soil layer i tothe initial groundwater level m and λi represents the cal-culation coefficient it should be based on the analysis ofgroundwater seepage If the analysis data are not availablethen its value should be based on local engineering expe-rience si refers to the depth of lowering groundwater levelcorresponding to the calculation profile m s denotes theground compression in the calculation profile m and ψw
means the empirical coefficient of subsidence calculationbased on local engineering experience If no experience isavailable then the value is set to be 1 Δσzi
prime denotes the meanadditional stress of the soil layer i under the ground surfacecaused by dewatering kPa Δhi means the thickness of thesoil layer i m and Esi
denotes the compressive modulus ofthe soil layer i kPa
42 Calculation of Ground Subsidence Caused by Dewateringin Foundation Pit under Seepage Force -e pumping anddrainage of the dewatering well will cause the change ofthe underground seepage field which will generate a newseepage field and lead to the variation of the stress fieldaround the well -erefore the seepage force is the maincause of soil consolidation and settlement -e seepage ofgroundwater causes the dissipation of pore water pres-sure resulting in an increase in effective stress -at isadditional stress is generated in the soil the direction ofwhich is vertically downward in addition it produces a
horizontal component Deformation can be caused by theimpact of seepage force -e additional stress namely thevertical component of the seepage force will cause theground subsidence [21] -e horizontal component of theseepage force will cause the lateral deformation of the soilAccording to reference [22] the seepage direction of anywater head at any point is tangent to the phreatic surfaceat that point pointing to the well axis as shown inFigure 10
Wu and Zhu [22] performed related research andproposed a new algorithm of ground subsidence caused byseepage force Yang and Zhao [23] also used this method tocalculate ground subsidence -is section draws lessonsfrom their research to discuss the dewatering in open cutfoundation pit of a subway station on Chengdu Metro Line6
-e stratum is divided into three parts dry soil zonedewatered zone and saturated zone -e dry soil zone isalways above the groundwater level before and afterdewatering which does not contain groundwater andseepage force during dewatering -us additional stressdoes not appear in this zone and the subsidence valuecaused by dewatering is 0 -e other two layers are S1 andS2 respectively as shown in Figure 10 -e soil layer S1 inthe dewatering process is drained -e zone S2 is alwaysbelow the groundwater level and is saturated In thissection the soil subsidence in the dewatered and saturatedzones is calculated separately -e seepage forces in thesetwo areas possess horizontal components as shown inFigure 10
Assuming that the water head that keeps a distance of x0from itself to the well axis is z0 as shown in Figure 10according to the dewatering funnel curve equation (29) inSection 41 the height of the falling funnel curve is obtained-e direction of the seepage force is actually in the straightline vector on the curve which goes through the point(x0 z0) and is tangent to the falling funnel curve and pointsto the well In order to find out the slope of the straight linethe following equation is obtained by performing the de-rivative of equation (29) at point x x0
R
H
h 0
x
z
r0 l
Ground
Impermeable stratum
Well point
Groundwater level
Figure 9 Dewatering funnel curve for the phreatic incomplete well point
-e equation of the line that passes through the point(x0 z0) and is tangential to the curve after the dewateringbecomes stable is assumed to be
y x tan α + b (35)
-e slope of the equation is
tan α zprime x0( 1113857 (36)
According to the trigonometric function conversion
sin α tan α
1 + tan2 α
radic (37)
-en the following equation is obtained
sin α zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (38)
-erefore the vertical component of additional pressureof the surrounding stratum caused by dewatering in thefoundation pit is expressed as
ΔPy ΔP sin α ΔPzprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (39)
where ΔPx denotes the horizontal component of the seepageforce ΔPy denotes the vertical component of the seepageforce and ΔP is the seepage force
-e angle α in equation (39) denotes the one existsbetween the horizontal component of the additional pres-sure and the additional pressure caused by dewatering in thefoundation pit
Based on equations (30) to (33) the additional stresses inthe dewatering zone and the saturation zone are calculatedseparately -e rewriting is carried out on the basis ofequation (34) which derives the ground subsidence causedby dewatering in the foundation pit considering the action ofseepage force -e formula after rewriting is as follows
s ψw 1113944Δσziprime Δhi
Esi
zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (40)
Substituting equation (34) into equation (40) yields thefollowing equation
Figure 10 Simplified calculation model considering seepage force
10 Advances in Civil Engineering
If the ground subsidence S1 and S2 corresponding to thedewatered and saturated zones are separately figured outthen the total ground subsidence is obtained and illustratedin the following equation
S S1 + S2 (42)
where the symbols are illustrated in Section 41
5 Numerical Simulation
With fast development of urban construction various in-frastructures such as subway station high-speed railwaytunnel underground parking lot and basement are beingbuilt especially the construction of deep foundation pits-e geological condition varies in the foundation pit and thesurrounding environment is becoming more and morecomplex after construction -eoretical analysis and em-pirical calculation methods are no longer suitable forpractical projects Nowadays actual projects are generallybuilt ahead of theoretical research and calculation methodsof numerical simulation provide an effective approach forthe design and construction of practical foundation pitAlthough numerical simulation method has not been pro-posed for very long time it has become the most commonmethod used in structural analysis and calculation so farMany scholars [24ndash31] have also used FLAC3D to conductfluid-solid coupling analysis of dewatering in the foundationpit Finite difference software FLAC3D is also employed inthis paper to carry out three-dimensional numerical sim-ulation of the whole process of the environment changecaused by dewatering in the open cut foundation pit of ametro station on Chengdu Metro Line 6 -e calculationresults are compared with the one obtained from theoreticalcalculation and on-site monitoring which are used to makefurther study on the surrounding ground subsidence due todewatering in the open cut foundation pit
51 Physical and Mechanical Parameters According to thegeological properties of the actual project the calculationparameters are obtained and shown in Table 2
52 Establishment of 3D Model and Initial Stress BalanceBased on the engineering geological and hydrologicalconditions a three-dimensional stratigraphic model isestablished From the geological section of the site (Fig-ures 11 and 12) it can be seen that each of soil layers is nearlyhorizontal and almost parallel to each other So in order toestablish the model and conduct the calculation conve-niently [26] the geological layers in the model are simplified-e foundation pit and wells are set up in the stratigraphicmodel according to dewatering scheme According to theprinciple of Saint Venant in elastic mechanics in order toeliminate the boundary influence of the foundation pit oncalculation results the calculation model is extended to 3sim5times of the depth of the pit from the periphery of thefoundation pit on the plane -e depth is set to the im-pervious interface and the entire calculation domain is inthe volume of 420mtimes 198mtimes 72m Referring to
geotechnical mapping and geological properties the modelis divided into 9 strata and the generalized permeabilitycoefficient and stratum thickness are shown in Table 1
According to stratum distribution and initial conditionsthe calculation domain is divided into grids by consideringthe monitoring points the dewatering wells and the dia-phragm wall considering the geological survey In themeshing the grid of the calculation domain is locally refinedin addition the grids around the foundation pit are alsorefined but grids far away from the pit are sparsely meshedconsidering Saint Venantrsquos principle [26] So the entiremodel is then divided into 42 layers 78 rows 300 columnsand a total of 982800 elements and the established three-dimensional model is shown in Figure 13
After the 3D model is set up the initial stress balance isneeded to be applied firstly and the corresponding seepageand displacement boundary conditions are also applied -enormal displacement and the bottom displacement of thefour sidewalls and bottom of the model are restricted andthe horizontal displacement of the wall of the dewateringwell is also constrained -e bottom filter pipe of thedewatering well belongs to the seepage boundary which isachieved by applying the pore water pressure After nu-merical simulation the pore water pressure of initialequilibrium is shown in Figure 14 -e pore water pressurein the initial state is evenly applied on the strata
53 Numerical Simulation of Fluid-Solid Coupling of GroundSubsidence Caused by Dewatering in Foundation PitAfter the initial equilibrium of the numerical model theeffect of single well and group wells were firstly carried outseparately -e stepwise dewatering was conducted underthe conditions of single well and group wells separatelywithout considering excavation -e calculated isograms ofthe dewatering at all levels and the subsidence of themonitoring points are analyzed
531 Numerical Simulation Analysis of Single WellDewatering Considering the symmetry of the well pointthe well point at the center of the calculation domain isselected when carrying out single well simulation -e po-sition of the single well on the model is shown in Figure 15-e isograms showing subsidence and pore water pressure atvarious drawdowns are also shown in Figures 16ndash23
It is seen from Figures 16ndash23 that the influence range ofdewatering well increases with the increase of drawdownAfter the well is dewatered the pore water pressure formsthe dewatering funnel surface -e bigger the drawdown isthe deeper the surface becomes Moreover due to the wa-terproofing effect of the diaphragm wall the stratum insidethe pit bulges but the stratum outside the pit subsides -isis because the groundwater flows through the bottom of thediaphragm wall into the pit -e increase of the pore waterpressure in the pit causes the additional stress to increasewhich leads to the upheaval of the soil layer in the pit -edecrease of the pore water pressure outside the pit causes theeffective stress of the outer stratum to decrease resulting inthe formation of downward additional stress in the outer
Advances in Civil Engineering 11
stratum of the pit which eventually leads to the consoli-dation and settlement of the outer stratum
532 Numerical Simulation Analysis of Group WellsDewatering When conducting the effect of group wells onground subsidence the calculation is based on the well pointlayout diagram designed in Section 3 -e layout of thegroup wells in the model is shown in Figure 24-e isogramsshowing ground subsidence and pore water pressure cal-culated using FLAC3D are shown in Figures 25ndash32
It is seen from the isograms shown in Figures 25ndash32 thatthe deeper the groundwater drawdown the greater theimpact on the surrounding ground is and the lower thefunnel surface formed by the pore water pressure Comparedwith the isogram under the effect of single well in Section 52the influence of dewatering by group wells is much largerGroup wells dewatering has a great impact on the pore waterpressure and strata deformation inside the foundation pit
that is the influence of group wells on ground subsidenceinside the pit is obvious and should not be ignored -esubsidence and pore water pressure isogram under differentdrawdowns are symmetrically distributed As shown in thesingle well effect the strata inside the foundation pit bulgeduring the dewatering of group wells but the strata outsidethe foundation pit subside-is is mainly due to the result ofthe movement of groundwater outside the pit to the dew-atering well inside the pit
533 Effect of Group Wells According to the results ob-tained from 3D simulation the subsidence of the sixmonitoring points on the sides of the foundation pit due todifferent drawdowns caused by the single well and the groupwells are shown in Figure 33
Grade I drawdown including Grade II Grade III andGrade IV drawdown means that the dewatering depth ofgroundwater level is in 6m 11m 16m and 22m respectively
Table 2 Physical and mechanical parameters of strata
Name of stratum Modulus of deformation E (MPa) Poissonrsquos ratio Density (gcm3) Permeability coefficient (md)Miscellaneous fill 2 035 18 125Silty clay 4 029 196 005Clayey silt 4 030 194 015Fine sand 4 028 185 3Medium sand 40 026 19 10Loose pebble layer 18 025 2 22Slightly condensed pebble layer 20 023 21 22Medium condensed pebble layer 28 020 22 22Compacted pebble layer 38 017 23 22
Miscellaneous fill Silty clayClayey siltFine sandMedium sand
12345
Figure 11 Geological section of the territory
12 Advances in Civil Engineering
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
Q2 1366k H2 minus h2
m1113872 1113873
lg 1 + Rr0( 1113857( 1113857 + hm minus l( 1113857l( 1113857lg 1 + 02 hmr0( 1113857( 1113857
(3)
hm H + hprime
2 (4)
where r0 stands for the equivalent radius of thefoundation pit m it is calculated according tor0 0565
A0
1113968A0 denotes the foundation pit area m2
hprime indicates the water head at the bottom of the centralaxis of the enclosure structure m and l means thelength of the water inlet part of the dewatering well m
② -e enclosure structure and the bottom boundary areboth impervious layers According to Darcyrsquos seepageexperimental conditions the seepage field distribu-tion in Figure 3 is simplified to be a one-dimensionalflow field distribution as shown in Figure 4
-at is in the assumption that the groundwater in the pitone-dimensionally flows in a circular glass tube and itsatisfies Darcyrsquos flow law water inflow Q1 into the pit isderived theoretically as follows
Q1 kA hprime minus h( 1113857
L (5)
A V
L (6)
V πr20 l2 + l3( 1113857 (7)
L 2l2 + l3 + r0( 1113857
2 (8)
H l1 + l2 + l3 (9)
h l2 + l3 (10)
Sw H minus hprime (11)
where h denotes water head height in the foundation pit afterdewatering m l1 denotes water table drawdown in thefoundation pit m l2 refers to the distance from the watertable to the bottom of the enclosure structure after dew-atering in the foundation pit m l3 indicates the distancefrom the bottom of the enclosure structure to the imperviouslayer m and A V and L refers to the cross-sectional area ofthe seepage field m2 the total volume of seepage m3 andaverage seepage path m respectively
Simultaneous solution is obtained from equations (1) to(11) and then the following equation is obtained
4πr20 l2 + l3( 1113857 hprime minus h( 1113857
2l2 + l3 + r0( 11138572
03415 4H2 minus H + hprime( 11138572
1113960 1113961
lg 1 + 2 H minus hprime( 1113857Hk
radic( 1113857r0( 11138571113858 1113859 + H + hprime minus 2l( 11138572l( 1113857lg 1 + H + hprime( 111385710r0( 1113857( 1113857
(12)
l 2 (l)
l 3l 1
r0
Initial groundwater level
Water levelin the pit
Enclosure structure
H
h
S whprime
Flow line
Figure 3 Seepage field distribution in the pit
r 0l 3
l 2
L
h
hprime
l 2 (l)
Figure 4 Simplification of the seepage path in the pit
4 Advances in Civil Engineering
As for a real open cut foundation pit it is seen fromequation (12) that there is only one unknown variable inequation (12) that is hprime the water head at the bottom of thecentral axis of the enclosure structure inside the pit -iswater head at the bottom of the central axis of the enclosurestructure inside the pit can be iteratively obtained so that theradius of influence of dewatering in the foundation pit andthe water inflow in the foundation pit can also be obtained
22 Calculation of Dewatering in Foundation Pit in Multi-layered Soil under Waterproofing Enclosure Structure Forthe calculation of water inflow in the foundation pit con-sidering the waterproofing effect of the enclosure structureunder the geological conditions of the multilayered soil thestratification calculation method is adopted to calculate thewater inflow of each layer of soil separately and algebraiccalculation is performed to obtain the total water inflow inthe pit Generally speaking there are many soil layers inactual foundation pits It is very cumbersome and time-consuming to use this method -erefore the geologicalconditions of the multilayered soils are simplified to be asingle formation and the permeability coefficient is averagedfor calculation -ree soil layers are used to illustrate thismethod as shown in Figure 5
-e permeability coefficient is calculated as follows
k k1h1 + k2h2 + k3h3
h1 + h2 + h3 (13)
where h1 h2 and h3 denote the thicknesses of the three soillayers respectively m and k1 k2 and k3 stand for thepermeability coefficients respectively corresponding to thethree soil layers md
23 eoretical Design of Dewatering In the actual projectthe average permeability coefficient of the multilayered soilsis firstly obtained according to equation (13) and then byusing equation (12) engineering parameters are substitutedand simplified to obtain the transcendental equation abouthprime -is equation can only be solved by means of a computerso it is solved with Matlab using the dichotomy By inputtingthe program in Matlab the water head hprime at the bottom ofthe central axis of the enclosure structure can be obtainedand then both the depth Sw of lowering water level in the wellpoint and radius of influence R of the phreatic aquifer infoundation pit can then be obtained
From equations (8) to (11) the equation for calculatingthe water inflow Q1 in the pit is derived as follows
Q1 kA hprime minus h( 1113857
L4kπr20 l2 + l3( 1113857 hprime minus h( 1113857
2l2 + l3 + r0( 11138572 (14)
-e water inflow of a single well is calculated as follows[18]
q0 120πrslk
3radic
(15)
where q0 represents the water inflow capacity of a single wellm3d rs denotes the filter radius m l stands for the length of
the inlet part of the filter m and k denotes the permeabilitycoefficient of the aquifer md
-e number of dewatering wells is calculated as followsaccording to the Chinese Technical Code for GroundwaterControl in Building and Municipal Engineering (JGJT111-2016) [19] if the safety level of the foundation pit isassessed to be in Grade I and the complexity of thefoundation pit is evaluated to be complicated and then thecalculation coefficient ε in equation (16) gets the value of12 -e number of well points is obtained from the fol-lowing equation
n εQ1
q0 (16)
D L
n (17)
where D denotes the space between well points m L rep-resents the circumference of the foundation pit m and nmeans the number of well points
-erefore the layout of the dewatering well points in theactual foundation pit can be obtained from the precedingequations
3 Case Study of Dewatering in Foundation Pit
31 Engineering Background -is paper depends on adeep foundation pit of a subway station on ChengduMetro Line 6 -e station is an underground three-storied island platform station its east side is closelyadjacent to a street-facing commercial store that has 2-3stories of brick-concrete structure In addition a 220 kVpower cable tunnel constructed with conventionalmining method is buried on its east side -e power cabletunnel is 14 m away from the sidewall of the station -ewest side of the subway station closely approaches privatehouses and public shops on the ground surface and theouter edge of a shop on the west side lies within aminimum distance of 17 m away from the foundationpit -e station is a 130 m wide island platform stationwith a standard cross-sectional width of 225 m and itstotal length is 2429 m long on the right side and 2221 mlong on the left side -e depth of the soil on its roof isabout 398 m and the depth of the bottom is about2664 m -e station and its surrounding environmentare shown in Figure 6 According to the hydrogeologicalconditions for this project there are two types ofgroundwater in the site one is the perched water in thebackfill layer above the clay layer and the other is thepore water in the quaternary sand and pebble layer -eprimary geotechnical investigation showed that stablewater table measured in the site was 500sim640 m inOctober 2015 and the detailed investigation showed thatit was 540sim670 m in October 2016 Obviously there isminor difference in the two results of water tables so thewater level in the site is based on the results of the de-tailed geotechnical investigation -e site geotechnicalproperties and its distribution are also shown in Table 1
Advances in Civil Engineering 5
32 Design of Dewatering Well Points Since the subwaystation is closely located to the shops and buildings on its twosides and the underground pipelines are densely packed inpower cable tunnel and municipal sewages there are noother spare places to install dewatering wells outside thefoundation pit -erefore to avoid the impact of dewateringon the surrounding environment well points are used to thelower groundwater level inside the foundation pit prior to itsexcavation
-e groundwater of the construction site belongs to theQuaternary existing in the sandy pebble pore phreaticaquifer -e thickness of the phreatic aquifer is less than30m and the bottom floor of the station is located in thecompacted pebble layer -e purpose of dewatering is tolower the water table in the foundation pit to 1m below itsbottom so that normal construction of the subway stationcan be fulfilled without groundwater -e open cut foun-dation pit of the station is 225m long and 24m wide with a
Family wingsEntrance F
Entrance GEntrance 2Shops
Entrance DEntrance E
Foundation pit
N
Seco
nd ri
ng
Figure 6 Station location and its surrounding environment
⑧ Medium dense pebblelayer 1sim214 1233 255times10minus 2 32
⑨ Compacted pebblelayer Not drilled Not drilled 255times10minus 2 43
h 1h 2
h 3
k1
k2
k3
Ground
Groundwater level
Soil layer 1
Soil layer 2
Soil layer 3
Phreatic aquifer
Impermeable stratum
Figure 5 Geological conditions of multilayered soil
6 Advances in Civil Engineering
length-width ratio of 9375lt 20-e depth of the foundationpit is about 2664m For the convenience of calculation it isset to 27m -e diameter of the dewatering well is 600mmand the well bottom is 35m lower than the one of thestation-e length of the filter pipe is 2m-e distance fromthe bottom of the filter pipe to the impervious layer is 2mand the stable water table is considered to be 6m
From the theoretical design and calculation of Section 2 itis known that 17 well points are to be laid around the foun-dation pit Referring to the Chinese Technical Code forGroundwater Control in Building and Municipal Engineering(JGJT111-2016) [19] and considering an idealized situation inwhichwells can be easily bored around the open cut foundationpit the dewatering wells are arranged at the same distancealong the foundation pit After a well is installed at the center ofthe foundation pit the remaining 16 well points are installedevenly at the inner edge of the pit Since it is a long and narrowopen cut foundation pit the wells can be placed at the inneredge of the long side of the foundation pit and the spacebetween them is about 25m -e specific layout of well pointsinside the open cut foundation pit is shown in Figure 7
33 Comparison between Calculations with and withoutconsidering the Waterproof Effect of Enclosure StructureAccording to the Chinese Technical Code for groundwatercontrol in building and municipal engineering [18] if groupwell points are simplified to be large one the total waterinflow from the incomplete well points in the phreaticaquifer which is calculated by using equations (3) and (4) isreplaced with the following equation
hm H + h
2 (18)
-e parameters in equation (18) are the same as that inthe formulae as stated above If the waterproofing effect ofthe enclosure structure is not considered then the depth ofwater level lowered by well points is expressed below
Sw H minus h (19)
Substituting aforementioned engineering data intoequations (3) and (16)ndash(18) respectively it is seen that thewater inflow from incomplete well point Q3 in the foun-dation pit is
Q3 Q2 1209532m3 (20)
the radius of influence of the foundation pit Rprime is
Rprime 2Sw
Hk
radicasymp 105233m (21)
and the number of well points nprime is certainly obtainedaccording to
nprime 12Q2
q0asymp 23 (22)
After a well point is installed at the center of the pit theremaining 22 well points can then be set evenly at the edge ofthe foundation pit Since it is a long and narrow foundationpit the well points are evenly installed at the edge of the long
side L of the foundation pit and the space Dprime between themis
Dprime L
nasymp 2045m (23)
Compared with the waterproofing effect of the enclosurestructure the water inflow in the foundation pit is893651209532 asymp 074 times of the conventional algorithmproposed in the Chinese Specification and the dewateringradius of influence is 54099105233 asymp 051 times of theconventional algorithm in the Specification -e number ofwell points is 1723 asymp 074 times of the conventional algo-rithm in the Specification and the space between them is252045 asymp 122 times of the conventional algorithm in theSpecification -rough comparison if the enclosure struc-ture of the foundation pit is used as a waterproof curtainthen the waterproofing effect of the enclosure structureshould not be ignored when calculating water inflow insidethe open cut foundation pit
34 Arrangement of Points Monitoring Ground SubsidenceIn order to make further analysis on the ground subsidencecaused by dewatering in the foundation pit the groundsubsidence of the typical positions around the foundation pitis monitored -e ground subsidence monitoring points areset up according to the actual condition of the open cutfoundation pit In the actual project the ground subsidencemonitoring points are arranged around the foundation pitFor the consideration of symmetry and the convenience ofmeasuring six points at the midline of the long side of thefoundation pit are selected and they are 16-6 16-5 16-4 16-3 16-2 and 16-1 respectively the distance between the wellpoints and pit wall is set within 8m 12m 16m 20m 24mand 28m respectively -e monitoring points are shown inFigure 8 which are located at the center axis of the edge lineof the pit Step-by-step dewatering is carried out in the actualproject and the depth of dewatering for every step is set to6m 5m 5m and 6m respectively and the total stepamounts to 22m
4 Theoretical Calculation of GroundSubsidence Caused by Dewatering inFoundation Pit
41 Normative Calculation of Ground Subsidence Caused byDewatering in Foundation Pit
411 Dupuitrsquos Assumption and Derivation of DewateringFunnel Curve Dewatering in the foundation pit will defi-nitely produce a falling funnel curve around the pit and thegroundwater may flow into the dewatering well inside thefoundation pit French scholar Dupuit first studied thesteady well flow put forward Dupuitrsquos assumption andderived the dewatering funnel curve -e hypothesis con-siders a cylindrical homogeneous phreatic aquifer withisotropic and horizontally waterproofing bottom floor afixed water head outside the aquifer a complete pumpingwell in the center no vertical infiltration recharge and
Advances in Civil Engineering 7
evaporation and a steady seepage subject to the linear law[20] -e dewatering well point can develop a dewateringfunnel curve around it Groundwater flows to the well after acertain time and the dewatering curve can reach a steadystate Assuming that the well point dewatering is a stablephreatic well flow without the group well effect the center ofthe well bottom is set to be the origin and the abscissa is setto be the positive x-axis as shown in Figure 9
According to Dupuitrsquos assumption the water flowequation of the stable phreatic well is obtained below [20]
Q kIA 2πrhkdh
dr (24)
If we separate the variable in equation (24) and take anyone point on the dewatering funnel curve then followingequations are obtained by integrating the equation from thepoint to its boundary
r x h z
r R h H
⎧⎪⎨
⎪⎩
⎫⎪⎬
⎪⎭⟶ 1113946
R
x
Q
2πk
1rdr 1113946
H
zh dh (25)
and thenQ
2πkln
R
x12
H2
minus z2
1113872 1113873 (26)
If the boundary condition meets
x r0
z h0 + l(27)
and substituting equation (27) into equation (26) then thefollowing equation is obtained
Q
2πkln
R
r012
H2
minus h0 + l( 11138572
1113960 1113961 (28)
-e solution of simultaneous equations (26) and (28) isused to obtain the following dewatering funnel curveequation of the well point
z2
H2
minus H2
minus h0 + l( 11138572
1113960 1113961ln(Rx)
ln Rr0( 1113857 (29)
where r0 denotes the radius of wells m h0 represents thelength of the inlet part of the filter pipe m R denotes theradius of influence m H represents the height from thebottom of the well to the initial groundwater level m k is thepermeability coefficient of the aquifer md A is the side areaof the dewatering well m2 I denotes the groundwaterseepage field hydraulic gradient Q stands for the boreholewater yield m3d and l means the distance between thebottom of well and the impermeable layer m-e remainingsymbols are shown in Figure 9
412 Ground Subsidence at Any Point outside the Foun-dation Pit At present the calculation of ground subsi-dence around the foundation pit after dewatering isgenerally carried out using the stratification summationmethod given in the Technical Specification for Retainingand Protection of Building Foundation Excavations (JGJ120-2012) [18] -e ground subsidence can be calculated bysummarizing the compression of each layer of soil Firstlywe can calculate the additional effective stress caused bydewatering and then calculate the ground subsidence usingstress calculation
(1) When the soil layer numbered as i is above the initialgroundwater level the effective stress is
Δσziprime 0 (30)
(2) If the soil layer numbered as i is located between thepostdewatering groundwater level and the initialgroundwater level then the effective stress is
Δσziprime cwz (31)
Foundation pit
Pit wall 16-616-516-416-316-216-1
Figure 8 Schematic diagram of the ground subsidence monitoring points around the foundation pit
Well point
Foundation pit wall
Figure 7 Schematic diagram of the layout of the dewatering well
8 Advances in Civil Engineering
(3) When the soil layer numbered as i is below thegroundwater level after dewatering then the effectivestress is
Δσziprime λicwsi (32)
-e soil compression caused by dewatering is as follows
s ψw 1113944Δσziprime Δhi
Esi
(33)
where cw means the bulk density of water kNm3 z denotesthe vertical distance from the midpoint of the soil layer i tothe initial groundwater level m and λi represents the cal-culation coefficient it should be based on the analysis ofgroundwater seepage If the analysis data are not availablethen its value should be based on local engineering expe-rience si refers to the depth of lowering groundwater levelcorresponding to the calculation profile m s denotes theground compression in the calculation profile m and ψw
means the empirical coefficient of subsidence calculationbased on local engineering experience If no experience isavailable then the value is set to be 1 Δσzi
prime denotes the meanadditional stress of the soil layer i under the ground surfacecaused by dewatering kPa Δhi means the thickness of thesoil layer i m and Esi
denotes the compressive modulus ofthe soil layer i kPa
42 Calculation of Ground Subsidence Caused by Dewateringin Foundation Pit under Seepage Force -e pumping anddrainage of the dewatering well will cause the change ofthe underground seepage field which will generate a newseepage field and lead to the variation of the stress fieldaround the well -erefore the seepage force is the maincause of soil consolidation and settlement -e seepage ofgroundwater causes the dissipation of pore water pres-sure resulting in an increase in effective stress -at isadditional stress is generated in the soil the direction ofwhich is vertically downward in addition it produces a
horizontal component Deformation can be caused by theimpact of seepage force -e additional stress namely thevertical component of the seepage force will cause theground subsidence [21] -e horizontal component of theseepage force will cause the lateral deformation of the soilAccording to reference [22] the seepage direction of anywater head at any point is tangent to the phreatic surfaceat that point pointing to the well axis as shown inFigure 10
Wu and Zhu [22] performed related research andproposed a new algorithm of ground subsidence caused byseepage force Yang and Zhao [23] also used this method tocalculate ground subsidence -is section draws lessonsfrom their research to discuss the dewatering in open cutfoundation pit of a subway station on Chengdu Metro Line6
-e stratum is divided into three parts dry soil zonedewatered zone and saturated zone -e dry soil zone isalways above the groundwater level before and afterdewatering which does not contain groundwater andseepage force during dewatering -us additional stressdoes not appear in this zone and the subsidence valuecaused by dewatering is 0 -e other two layers are S1 andS2 respectively as shown in Figure 10 -e soil layer S1 inthe dewatering process is drained -e zone S2 is alwaysbelow the groundwater level and is saturated In thissection the soil subsidence in the dewatered and saturatedzones is calculated separately -e seepage forces in thesetwo areas possess horizontal components as shown inFigure 10
Assuming that the water head that keeps a distance of x0from itself to the well axis is z0 as shown in Figure 10according to the dewatering funnel curve equation (29) inSection 41 the height of the falling funnel curve is obtained-e direction of the seepage force is actually in the straightline vector on the curve which goes through the point(x0 z0) and is tangent to the falling funnel curve and pointsto the well In order to find out the slope of the straight linethe following equation is obtained by performing the de-rivative of equation (29) at point x x0
R
H
h 0
x
z
r0 l
Ground
Impermeable stratum
Well point
Groundwater level
Figure 9 Dewatering funnel curve for the phreatic incomplete well point
-e equation of the line that passes through the point(x0 z0) and is tangential to the curve after the dewateringbecomes stable is assumed to be
y x tan α + b (35)
-e slope of the equation is
tan α zprime x0( 1113857 (36)
According to the trigonometric function conversion
sin α tan α
1 + tan2 α
radic (37)
-en the following equation is obtained
sin α zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (38)
-erefore the vertical component of additional pressureof the surrounding stratum caused by dewatering in thefoundation pit is expressed as
ΔPy ΔP sin α ΔPzprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (39)
where ΔPx denotes the horizontal component of the seepageforce ΔPy denotes the vertical component of the seepageforce and ΔP is the seepage force
-e angle α in equation (39) denotes the one existsbetween the horizontal component of the additional pres-sure and the additional pressure caused by dewatering in thefoundation pit
Based on equations (30) to (33) the additional stresses inthe dewatering zone and the saturation zone are calculatedseparately -e rewriting is carried out on the basis ofequation (34) which derives the ground subsidence causedby dewatering in the foundation pit considering the action ofseepage force -e formula after rewriting is as follows
s ψw 1113944Δσziprime Δhi
Esi
zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (40)
Substituting equation (34) into equation (40) yields thefollowing equation
Figure 10 Simplified calculation model considering seepage force
10 Advances in Civil Engineering
If the ground subsidence S1 and S2 corresponding to thedewatered and saturated zones are separately figured outthen the total ground subsidence is obtained and illustratedin the following equation
S S1 + S2 (42)
where the symbols are illustrated in Section 41
5 Numerical Simulation
With fast development of urban construction various in-frastructures such as subway station high-speed railwaytunnel underground parking lot and basement are beingbuilt especially the construction of deep foundation pits-e geological condition varies in the foundation pit and thesurrounding environment is becoming more and morecomplex after construction -eoretical analysis and em-pirical calculation methods are no longer suitable forpractical projects Nowadays actual projects are generallybuilt ahead of theoretical research and calculation methodsof numerical simulation provide an effective approach forthe design and construction of practical foundation pitAlthough numerical simulation method has not been pro-posed for very long time it has become the most commonmethod used in structural analysis and calculation so farMany scholars [24ndash31] have also used FLAC3D to conductfluid-solid coupling analysis of dewatering in the foundationpit Finite difference software FLAC3D is also employed inthis paper to carry out three-dimensional numerical sim-ulation of the whole process of the environment changecaused by dewatering in the open cut foundation pit of ametro station on Chengdu Metro Line 6 -e calculationresults are compared with the one obtained from theoreticalcalculation and on-site monitoring which are used to makefurther study on the surrounding ground subsidence due todewatering in the open cut foundation pit
51 Physical and Mechanical Parameters According to thegeological properties of the actual project the calculationparameters are obtained and shown in Table 2
52 Establishment of 3D Model and Initial Stress BalanceBased on the engineering geological and hydrologicalconditions a three-dimensional stratigraphic model isestablished From the geological section of the site (Fig-ures 11 and 12) it can be seen that each of soil layers is nearlyhorizontal and almost parallel to each other So in order toestablish the model and conduct the calculation conve-niently [26] the geological layers in the model are simplified-e foundation pit and wells are set up in the stratigraphicmodel according to dewatering scheme According to theprinciple of Saint Venant in elastic mechanics in order toeliminate the boundary influence of the foundation pit oncalculation results the calculation model is extended to 3sim5times of the depth of the pit from the periphery of thefoundation pit on the plane -e depth is set to the im-pervious interface and the entire calculation domain is inthe volume of 420mtimes 198mtimes 72m Referring to
geotechnical mapping and geological properties the modelis divided into 9 strata and the generalized permeabilitycoefficient and stratum thickness are shown in Table 1
According to stratum distribution and initial conditionsthe calculation domain is divided into grids by consideringthe monitoring points the dewatering wells and the dia-phragm wall considering the geological survey In themeshing the grid of the calculation domain is locally refinedin addition the grids around the foundation pit are alsorefined but grids far away from the pit are sparsely meshedconsidering Saint Venantrsquos principle [26] So the entiremodel is then divided into 42 layers 78 rows 300 columnsand a total of 982800 elements and the established three-dimensional model is shown in Figure 13
After the 3D model is set up the initial stress balance isneeded to be applied firstly and the corresponding seepageand displacement boundary conditions are also applied -enormal displacement and the bottom displacement of thefour sidewalls and bottom of the model are restricted andthe horizontal displacement of the wall of the dewateringwell is also constrained -e bottom filter pipe of thedewatering well belongs to the seepage boundary which isachieved by applying the pore water pressure After nu-merical simulation the pore water pressure of initialequilibrium is shown in Figure 14 -e pore water pressurein the initial state is evenly applied on the strata
53 Numerical Simulation of Fluid-Solid Coupling of GroundSubsidence Caused by Dewatering in Foundation PitAfter the initial equilibrium of the numerical model theeffect of single well and group wells were firstly carried outseparately -e stepwise dewatering was conducted underthe conditions of single well and group wells separatelywithout considering excavation -e calculated isograms ofthe dewatering at all levels and the subsidence of themonitoring points are analyzed
531 Numerical Simulation Analysis of Single WellDewatering Considering the symmetry of the well pointthe well point at the center of the calculation domain isselected when carrying out single well simulation -e po-sition of the single well on the model is shown in Figure 15-e isograms showing subsidence and pore water pressure atvarious drawdowns are also shown in Figures 16ndash23
It is seen from Figures 16ndash23 that the influence range ofdewatering well increases with the increase of drawdownAfter the well is dewatered the pore water pressure formsthe dewatering funnel surface -e bigger the drawdown isthe deeper the surface becomes Moreover due to the wa-terproofing effect of the diaphragm wall the stratum insidethe pit bulges but the stratum outside the pit subsides -isis because the groundwater flows through the bottom of thediaphragm wall into the pit -e increase of the pore waterpressure in the pit causes the additional stress to increasewhich leads to the upheaval of the soil layer in the pit -edecrease of the pore water pressure outside the pit causes theeffective stress of the outer stratum to decrease resulting inthe formation of downward additional stress in the outer
Advances in Civil Engineering 11
stratum of the pit which eventually leads to the consoli-dation and settlement of the outer stratum
532 Numerical Simulation Analysis of Group WellsDewatering When conducting the effect of group wells onground subsidence the calculation is based on the well pointlayout diagram designed in Section 3 -e layout of thegroup wells in the model is shown in Figure 24-e isogramsshowing ground subsidence and pore water pressure cal-culated using FLAC3D are shown in Figures 25ndash32
It is seen from the isograms shown in Figures 25ndash32 thatthe deeper the groundwater drawdown the greater theimpact on the surrounding ground is and the lower thefunnel surface formed by the pore water pressure Comparedwith the isogram under the effect of single well in Section 52the influence of dewatering by group wells is much largerGroup wells dewatering has a great impact on the pore waterpressure and strata deformation inside the foundation pit
that is the influence of group wells on ground subsidenceinside the pit is obvious and should not be ignored -esubsidence and pore water pressure isogram under differentdrawdowns are symmetrically distributed As shown in thesingle well effect the strata inside the foundation pit bulgeduring the dewatering of group wells but the strata outsidethe foundation pit subside-is is mainly due to the result ofthe movement of groundwater outside the pit to the dew-atering well inside the pit
533 Effect of Group Wells According to the results ob-tained from 3D simulation the subsidence of the sixmonitoring points on the sides of the foundation pit due todifferent drawdowns caused by the single well and the groupwells are shown in Figure 33
Grade I drawdown including Grade II Grade III andGrade IV drawdown means that the dewatering depth ofgroundwater level is in 6m 11m 16m and 22m respectively
Table 2 Physical and mechanical parameters of strata
Name of stratum Modulus of deformation E (MPa) Poissonrsquos ratio Density (gcm3) Permeability coefficient (md)Miscellaneous fill 2 035 18 125Silty clay 4 029 196 005Clayey silt 4 030 194 015Fine sand 4 028 185 3Medium sand 40 026 19 10Loose pebble layer 18 025 2 22Slightly condensed pebble layer 20 023 21 22Medium condensed pebble layer 28 020 22 22Compacted pebble layer 38 017 23 22
Miscellaneous fill Silty clayClayey siltFine sandMedium sand
12345
Figure 11 Geological section of the territory
12 Advances in Civil Engineering
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
As for a real open cut foundation pit it is seen fromequation (12) that there is only one unknown variable inequation (12) that is hprime the water head at the bottom of thecentral axis of the enclosure structure inside the pit -iswater head at the bottom of the central axis of the enclosurestructure inside the pit can be iteratively obtained so that theradius of influence of dewatering in the foundation pit andthe water inflow in the foundation pit can also be obtained
22 Calculation of Dewatering in Foundation Pit in Multi-layered Soil under Waterproofing Enclosure Structure Forthe calculation of water inflow in the foundation pit con-sidering the waterproofing effect of the enclosure structureunder the geological conditions of the multilayered soil thestratification calculation method is adopted to calculate thewater inflow of each layer of soil separately and algebraiccalculation is performed to obtain the total water inflow inthe pit Generally speaking there are many soil layers inactual foundation pits It is very cumbersome and time-consuming to use this method -erefore the geologicalconditions of the multilayered soils are simplified to be asingle formation and the permeability coefficient is averagedfor calculation -ree soil layers are used to illustrate thismethod as shown in Figure 5
-e permeability coefficient is calculated as follows
k k1h1 + k2h2 + k3h3
h1 + h2 + h3 (13)
where h1 h2 and h3 denote the thicknesses of the three soillayers respectively m and k1 k2 and k3 stand for thepermeability coefficients respectively corresponding to thethree soil layers md
23 eoretical Design of Dewatering In the actual projectthe average permeability coefficient of the multilayered soilsis firstly obtained according to equation (13) and then byusing equation (12) engineering parameters are substitutedand simplified to obtain the transcendental equation abouthprime -is equation can only be solved by means of a computerso it is solved with Matlab using the dichotomy By inputtingthe program in Matlab the water head hprime at the bottom ofthe central axis of the enclosure structure can be obtainedand then both the depth Sw of lowering water level in the wellpoint and radius of influence R of the phreatic aquifer infoundation pit can then be obtained
From equations (8) to (11) the equation for calculatingthe water inflow Q1 in the pit is derived as follows
Q1 kA hprime minus h( 1113857
L4kπr20 l2 + l3( 1113857 hprime minus h( 1113857
2l2 + l3 + r0( 11138572 (14)
-e water inflow of a single well is calculated as follows[18]
q0 120πrslk
3radic
(15)
where q0 represents the water inflow capacity of a single wellm3d rs denotes the filter radius m l stands for the length of
the inlet part of the filter m and k denotes the permeabilitycoefficient of the aquifer md
-e number of dewatering wells is calculated as followsaccording to the Chinese Technical Code for GroundwaterControl in Building and Municipal Engineering (JGJT111-2016) [19] if the safety level of the foundation pit isassessed to be in Grade I and the complexity of thefoundation pit is evaluated to be complicated and then thecalculation coefficient ε in equation (16) gets the value of12 -e number of well points is obtained from the fol-lowing equation
n εQ1
q0 (16)
D L
n (17)
where D denotes the space between well points m L rep-resents the circumference of the foundation pit m and nmeans the number of well points
-erefore the layout of the dewatering well points in theactual foundation pit can be obtained from the precedingequations
3 Case Study of Dewatering in Foundation Pit
31 Engineering Background -is paper depends on adeep foundation pit of a subway station on ChengduMetro Line 6 -e station is an underground three-storied island platform station its east side is closelyadjacent to a street-facing commercial store that has 2-3stories of brick-concrete structure In addition a 220 kVpower cable tunnel constructed with conventionalmining method is buried on its east side -e power cabletunnel is 14 m away from the sidewall of the station -ewest side of the subway station closely approaches privatehouses and public shops on the ground surface and theouter edge of a shop on the west side lies within aminimum distance of 17 m away from the foundationpit -e station is a 130 m wide island platform stationwith a standard cross-sectional width of 225 m and itstotal length is 2429 m long on the right side and 2221 mlong on the left side -e depth of the soil on its roof isabout 398 m and the depth of the bottom is about2664 m -e station and its surrounding environmentare shown in Figure 6 According to the hydrogeologicalconditions for this project there are two types ofgroundwater in the site one is the perched water in thebackfill layer above the clay layer and the other is thepore water in the quaternary sand and pebble layer -eprimary geotechnical investigation showed that stablewater table measured in the site was 500sim640 m inOctober 2015 and the detailed investigation showed thatit was 540sim670 m in October 2016 Obviously there isminor difference in the two results of water tables so thewater level in the site is based on the results of the de-tailed geotechnical investigation -e site geotechnicalproperties and its distribution are also shown in Table 1
Advances in Civil Engineering 5
32 Design of Dewatering Well Points Since the subwaystation is closely located to the shops and buildings on its twosides and the underground pipelines are densely packed inpower cable tunnel and municipal sewages there are noother spare places to install dewatering wells outside thefoundation pit -erefore to avoid the impact of dewateringon the surrounding environment well points are used to thelower groundwater level inside the foundation pit prior to itsexcavation
-e groundwater of the construction site belongs to theQuaternary existing in the sandy pebble pore phreaticaquifer -e thickness of the phreatic aquifer is less than30m and the bottom floor of the station is located in thecompacted pebble layer -e purpose of dewatering is tolower the water table in the foundation pit to 1m below itsbottom so that normal construction of the subway stationcan be fulfilled without groundwater -e open cut foun-dation pit of the station is 225m long and 24m wide with a
Family wingsEntrance F
Entrance GEntrance 2Shops
Entrance DEntrance E
Foundation pit
N
Seco
nd ri
ng
Figure 6 Station location and its surrounding environment
⑧ Medium dense pebblelayer 1sim214 1233 255times10minus 2 32
⑨ Compacted pebblelayer Not drilled Not drilled 255times10minus 2 43
h 1h 2
h 3
k1
k2
k3
Ground
Groundwater level
Soil layer 1
Soil layer 2
Soil layer 3
Phreatic aquifer
Impermeable stratum
Figure 5 Geological conditions of multilayered soil
6 Advances in Civil Engineering
length-width ratio of 9375lt 20-e depth of the foundationpit is about 2664m For the convenience of calculation it isset to 27m -e diameter of the dewatering well is 600mmand the well bottom is 35m lower than the one of thestation-e length of the filter pipe is 2m-e distance fromthe bottom of the filter pipe to the impervious layer is 2mand the stable water table is considered to be 6m
From the theoretical design and calculation of Section 2 itis known that 17 well points are to be laid around the foun-dation pit Referring to the Chinese Technical Code forGroundwater Control in Building and Municipal Engineering(JGJT111-2016) [19] and considering an idealized situation inwhichwells can be easily bored around the open cut foundationpit the dewatering wells are arranged at the same distancealong the foundation pit After a well is installed at the center ofthe foundation pit the remaining 16 well points are installedevenly at the inner edge of the pit Since it is a long and narrowopen cut foundation pit the wells can be placed at the inneredge of the long side of the foundation pit and the spacebetween them is about 25m -e specific layout of well pointsinside the open cut foundation pit is shown in Figure 7
33 Comparison between Calculations with and withoutconsidering the Waterproof Effect of Enclosure StructureAccording to the Chinese Technical Code for groundwatercontrol in building and municipal engineering [18] if groupwell points are simplified to be large one the total waterinflow from the incomplete well points in the phreaticaquifer which is calculated by using equations (3) and (4) isreplaced with the following equation
hm H + h
2 (18)
-e parameters in equation (18) are the same as that inthe formulae as stated above If the waterproofing effect ofthe enclosure structure is not considered then the depth ofwater level lowered by well points is expressed below
Sw H minus h (19)
Substituting aforementioned engineering data intoequations (3) and (16)ndash(18) respectively it is seen that thewater inflow from incomplete well point Q3 in the foun-dation pit is
Q3 Q2 1209532m3 (20)
the radius of influence of the foundation pit Rprime is
Rprime 2Sw
Hk
radicasymp 105233m (21)
and the number of well points nprime is certainly obtainedaccording to
nprime 12Q2
q0asymp 23 (22)
After a well point is installed at the center of the pit theremaining 22 well points can then be set evenly at the edge ofthe foundation pit Since it is a long and narrow foundationpit the well points are evenly installed at the edge of the long
side L of the foundation pit and the space Dprime between themis
Dprime L
nasymp 2045m (23)
Compared with the waterproofing effect of the enclosurestructure the water inflow in the foundation pit is893651209532 asymp 074 times of the conventional algorithmproposed in the Chinese Specification and the dewateringradius of influence is 54099105233 asymp 051 times of theconventional algorithm in the Specification -e number ofwell points is 1723 asymp 074 times of the conventional algo-rithm in the Specification and the space between them is252045 asymp 122 times of the conventional algorithm in theSpecification -rough comparison if the enclosure struc-ture of the foundation pit is used as a waterproof curtainthen the waterproofing effect of the enclosure structureshould not be ignored when calculating water inflow insidethe open cut foundation pit
34 Arrangement of Points Monitoring Ground SubsidenceIn order to make further analysis on the ground subsidencecaused by dewatering in the foundation pit the groundsubsidence of the typical positions around the foundation pitis monitored -e ground subsidence monitoring points areset up according to the actual condition of the open cutfoundation pit In the actual project the ground subsidencemonitoring points are arranged around the foundation pitFor the consideration of symmetry and the convenience ofmeasuring six points at the midline of the long side of thefoundation pit are selected and they are 16-6 16-5 16-4 16-3 16-2 and 16-1 respectively the distance between the wellpoints and pit wall is set within 8m 12m 16m 20m 24mand 28m respectively -e monitoring points are shown inFigure 8 which are located at the center axis of the edge lineof the pit Step-by-step dewatering is carried out in the actualproject and the depth of dewatering for every step is set to6m 5m 5m and 6m respectively and the total stepamounts to 22m
4 Theoretical Calculation of GroundSubsidence Caused by Dewatering inFoundation Pit
41 Normative Calculation of Ground Subsidence Caused byDewatering in Foundation Pit
411 Dupuitrsquos Assumption and Derivation of DewateringFunnel Curve Dewatering in the foundation pit will defi-nitely produce a falling funnel curve around the pit and thegroundwater may flow into the dewatering well inside thefoundation pit French scholar Dupuit first studied thesteady well flow put forward Dupuitrsquos assumption andderived the dewatering funnel curve -e hypothesis con-siders a cylindrical homogeneous phreatic aquifer withisotropic and horizontally waterproofing bottom floor afixed water head outside the aquifer a complete pumpingwell in the center no vertical infiltration recharge and
Advances in Civil Engineering 7
evaporation and a steady seepage subject to the linear law[20] -e dewatering well point can develop a dewateringfunnel curve around it Groundwater flows to the well after acertain time and the dewatering curve can reach a steadystate Assuming that the well point dewatering is a stablephreatic well flow without the group well effect the center ofthe well bottom is set to be the origin and the abscissa is setto be the positive x-axis as shown in Figure 9
According to Dupuitrsquos assumption the water flowequation of the stable phreatic well is obtained below [20]
Q kIA 2πrhkdh
dr (24)
If we separate the variable in equation (24) and take anyone point on the dewatering funnel curve then followingequations are obtained by integrating the equation from thepoint to its boundary
r x h z
r R h H
⎧⎪⎨
⎪⎩
⎫⎪⎬
⎪⎭⟶ 1113946
R
x
Q
2πk
1rdr 1113946
H
zh dh (25)
and thenQ
2πkln
R
x12
H2
minus z2
1113872 1113873 (26)
If the boundary condition meets
x r0
z h0 + l(27)
and substituting equation (27) into equation (26) then thefollowing equation is obtained
Q
2πkln
R
r012
H2
minus h0 + l( 11138572
1113960 1113961 (28)
-e solution of simultaneous equations (26) and (28) isused to obtain the following dewatering funnel curveequation of the well point
z2
H2
minus H2
minus h0 + l( 11138572
1113960 1113961ln(Rx)
ln Rr0( 1113857 (29)
where r0 denotes the radius of wells m h0 represents thelength of the inlet part of the filter pipe m R denotes theradius of influence m H represents the height from thebottom of the well to the initial groundwater level m k is thepermeability coefficient of the aquifer md A is the side areaof the dewatering well m2 I denotes the groundwaterseepage field hydraulic gradient Q stands for the boreholewater yield m3d and l means the distance between thebottom of well and the impermeable layer m-e remainingsymbols are shown in Figure 9
412 Ground Subsidence at Any Point outside the Foun-dation Pit At present the calculation of ground subsi-dence around the foundation pit after dewatering isgenerally carried out using the stratification summationmethod given in the Technical Specification for Retainingand Protection of Building Foundation Excavations (JGJ120-2012) [18] -e ground subsidence can be calculated bysummarizing the compression of each layer of soil Firstlywe can calculate the additional effective stress caused bydewatering and then calculate the ground subsidence usingstress calculation
(1) When the soil layer numbered as i is above the initialgroundwater level the effective stress is
Δσziprime 0 (30)
(2) If the soil layer numbered as i is located between thepostdewatering groundwater level and the initialgroundwater level then the effective stress is
Δσziprime cwz (31)
Foundation pit
Pit wall 16-616-516-416-316-216-1
Figure 8 Schematic diagram of the ground subsidence monitoring points around the foundation pit
Well point
Foundation pit wall
Figure 7 Schematic diagram of the layout of the dewatering well
8 Advances in Civil Engineering
(3) When the soil layer numbered as i is below thegroundwater level after dewatering then the effectivestress is
Δσziprime λicwsi (32)
-e soil compression caused by dewatering is as follows
s ψw 1113944Δσziprime Δhi
Esi
(33)
where cw means the bulk density of water kNm3 z denotesthe vertical distance from the midpoint of the soil layer i tothe initial groundwater level m and λi represents the cal-culation coefficient it should be based on the analysis ofgroundwater seepage If the analysis data are not availablethen its value should be based on local engineering expe-rience si refers to the depth of lowering groundwater levelcorresponding to the calculation profile m s denotes theground compression in the calculation profile m and ψw
means the empirical coefficient of subsidence calculationbased on local engineering experience If no experience isavailable then the value is set to be 1 Δσzi
prime denotes the meanadditional stress of the soil layer i under the ground surfacecaused by dewatering kPa Δhi means the thickness of thesoil layer i m and Esi
denotes the compressive modulus ofthe soil layer i kPa
42 Calculation of Ground Subsidence Caused by Dewateringin Foundation Pit under Seepage Force -e pumping anddrainage of the dewatering well will cause the change ofthe underground seepage field which will generate a newseepage field and lead to the variation of the stress fieldaround the well -erefore the seepage force is the maincause of soil consolidation and settlement -e seepage ofgroundwater causes the dissipation of pore water pres-sure resulting in an increase in effective stress -at isadditional stress is generated in the soil the direction ofwhich is vertically downward in addition it produces a
horizontal component Deformation can be caused by theimpact of seepage force -e additional stress namely thevertical component of the seepage force will cause theground subsidence [21] -e horizontal component of theseepage force will cause the lateral deformation of the soilAccording to reference [22] the seepage direction of anywater head at any point is tangent to the phreatic surfaceat that point pointing to the well axis as shown inFigure 10
Wu and Zhu [22] performed related research andproposed a new algorithm of ground subsidence caused byseepage force Yang and Zhao [23] also used this method tocalculate ground subsidence -is section draws lessonsfrom their research to discuss the dewatering in open cutfoundation pit of a subway station on Chengdu Metro Line6
-e stratum is divided into three parts dry soil zonedewatered zone and saturated zone -e dry soil zone isalways above the groundwater level before and afterdewatering which does not contain groundwater andseepage force during dewatering -us additional stressdoes not appear in this zone and the subsidence valuecaused by dewatering is 0 -e other two layers are S1 andS2 respectively as shown in Figure 10 -e soil layer S1 inthe dewatering process is drained -e zone S2 is alwaysbelow the groundwater level and is saturated In thissection the soil subsidence in the dewatered and saturatedzones is calculated separately -e seepage forces in thesetwo areas possess horizontal components as shown inFigure 10
Assuming that the water head that keeps a distance of x0from itself to the well axis is z0 as shown in Figure 10according to the dewatering funnel curve equation (29) inSection 41 the height of the falling funnel curve is obtained-e direction of the seepage force is actually in the straightline vector on the curve which goes through the point(x0 z0) and is tangent to the falling funnel curve and pointsto the well In order to find out the slope of the straight linethe following equation is obtained by performing the de-rivative of equation (29) at point x x0
R
H
h 0
x
z
r0 l
Ground
Impermeable stratum
Well point
Groundwater level
Figure 9 Dewatering funnel curve for the phreatic incomplete well point
-e equation of the line that passes through the point(x0 z0) and is tangential to the curve after the dewateringbecomes stable is assumed to be
y x tan α + b (35)
-e slope of the equation is
tan α zprime x0( 1113857 (36)
According to the trigonometric function conversion
sin α tan α
1 + tan2 α
radic (37)
-en the following equation is obtained
sin α zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (38)
-erefore the vertical component of additional pressureof the surrounding stratum caused by dewatering in thefoundation pit is expressed as
ΔPy ΔP sin α ΔPzprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (39)
where ΔPx denotes the horizontal component of the seepageforce ΔPy denotes the vertical component of the seepageforce and ΔP is the seepage force
-e angle α in equation (39) denotes the one existsbetween the horizontal component of the additional pres-sure and the additional pressure caused by dewatering in thefoundation pit
Based on equations (30) to (33) the additional stresses inthe dewatering zone and the saturation zone are calculatedseparately -e rewriting is carried out on the basis ofequation (34) which derives the ground subsidence causedby dewatering in the foundation pit considering the action ofseepage force -e formula after rewriting is as follows
s ψw 1113944Δσziprime Δhi
Esi
zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (40)
Substituting equation (34) into equation (40) yields thefollowing equation
Figure 10 Simplified calculation model considering seepage force
10 Advances in Civil Engineering
If the ground subsidence S1 and S2 corresponding to thedewatered and saturated zones are separately figured outthen the total ground subsidence is obtained and illustratedin the following equation
S S1 + S2 (42)
where the symbols are illustrated in Section 41
5 Numerical Simulation
With fast development of urban construction various in-frastructures such as subway station high-speed railwaytunnel underground parking lot and basement are beingbuilt especially the construction of deep foundation pits-e geological condition varies in the foundation pit and thesurrounding environment is becoming more and morecomplex after construction -eoretical analysis and em-pirical calculation methods are no longer suitable forpractical projects Nowadays actual projects are generallybuilt ahead of theoretical research and calculation methodsof numerical simulation provide an effective approach forthe design and construction of practical foundation pitAlthough numerical simulation method has not been pro-posed for very long time it has become the most commonmethod used in structural analysis and calculation so farMany scholars [24ndash31] have also used FLAC3D to conductfluid-solid coupling analysis of dewatering in the foundationpit Finite difference software FLAC3D is also employed inthis paper to carry out three-dimensional numerical sim-ulation of the whole process of the environment changecaused by dewatering in the open cut foundation pit of ametro station on Chengdu Metro Line 6 -e calculationresults are compared with the one obtained from theoreticalcalculation and on-site monitoring which are used to makefurther study on the surrounding ground subsidence due todewatering in the open cut foundation pit
51 Physical and Mechanical Parameters According to thegeological properties of the actual project the calculationparameters are obtained and shown in Table 2
52 Establishment of 3D Model and Initial Stress BalanceBased on the engineering geological and hydrologicalconditions a three-dimensional stratigraphic model isestablished From the geological section of the site (Fig-ures 11 and 12) it can be seen that each of soil layers is nearlyhorizontal and almost parallel to each other So in order toestablish the model and conduct the calculation conve-niently [26] the geological layers in the model are simplified-e foundation pit and wells are set up in the stratigraphicmodel according to dewatering scheme According to theprinciple of Saint Venant in elastic mechanics in order toeliminate the boundary influence of the foundation pit oncalculation results the calculation model is extended to 3sim5times of the depth of the pit from the periphery of thefoundation pit on the plane -e depth is set to the im-pervious interface and the entire calculation domain is inthe volume of 420mtimes 198mtimes 72m Referring to
geotechnical mapping and geological properties the modelis divided into 9 strata and the generalized permeabilitycoefficient and stratum thickness are shown in Table 1
According to stratum distribution and initial conditionsthe calculation domain is divided into grids by consideringthe monitoring points the dewatering wells and the dia-phragm wall considering the geological survey In themeshing the grid of the calculation domain is locally refinedin addition the grids around the foundation pit are alsorefined but grids far away from the pit are sparsely meshedconsidering Saint Venantrsquos principle [26] So the entiremodel is then divided into 42 layers 78 rows 300 columnsand a total of 982800 elements and the established three-dimensional model is shown in Figure 13
After the 3D model is set up the initial stress balance isneeded to be applied firstly and the corresponding seepageand displacement boundary conditions are also applied -enormal displacement and the bottom displacement of thefour sidewalls and bottom of the model are restricted andthe horizontal displacement of the wall of the dewateringwell is also constrained -e bottom filter pipe of thedewatering well belongs to the seepage boundary which isachieved by applying the pore water pressure After nu-merical simulation the pore water pressure of initialequilibrium is shown in Figure 14 -e pore water pressurein the initial state is evenly applied on the strata
53 Numerical Simulation of Fluid-Solid Coupling of GroundSubsidence Caused by Dewatering in Foundation PitAfter the initial equilibrium of the numerical model theeffect of single well and group wells were firstly carried outseparately -e stepwise dewatering was conducted underthe conditions of single well and group wells separatelywithout considering excavation -e calculated isograms ofthe dewatering at all levels and the subsidence of themonitoring points are analyzed
531 Numerical Simulation Analysis of Single WellDewatering Considering the symmetry of the well pointthe well point at the center of the calculation domain isselected when carrying out single well simulation -e po-sition of the single well on the model is shown in Figure 15-e isograms showing subsidence and pore water pressure atvarious drawdowns are also shown in Figures 16ndash23
It is seen from Figures 16ndash23 that the influence range ofdewatering well increases with the increase of drawdownAfter the well is dewatered the pore water pressure formsthe dewatering funnel surface -e bigger the drawdown isthe deeper the surface becomes Moreover due to the wa-terproofing effect of the diaphragm wall the stratum insidethe pit bulges but the stratum outside the pit subsides -isis because the groundwater flows through the bottom of thediaphragm wall into the pit -e increase of the pore waterpressure in the pit causes the additional stress to increasewhich leads to the upheaval of the soil layer in the pit -edecrease of the pore water pressure outside the pit causes theeffective stress of the outer stratum to decrease resulting inthe formation of downward additional stress in the outer
Advances in Civil Engineering 11
stratum of the pit which eventually leads to the consoli-dation and settlement of the outer stratum
532 Numerical Simulation Analysis of Group WellsDewatering When conducting the effect of group wells onground subsidence the calculation is based on the well pointlayout diagram designed in Section 3 -e layout of thegroup wells in the model is shown in Figure 24-e isogramsshowing ground subsidence and pore water pressure cal-culated using FLAC3D are shown in Figures 25ndash32
It is seen from the isograms shown in Figures 25ndash32 thatthe deeper the groundwater drawdown the greater theimpact on the surrounding ground is and the lower thefunnel surface formed by the pore water pressure Comparedwith the isogram under the effect of single well in Section 52the influence of dewatering by group wells is much largerGroup wells dewatering has a great impact on the pore waterpressure and strata deformation inside the foundation pit
that is the influence of group wells on ground subsidenceinside the pit is obvious and should not be ignored -esubsidence and pore water pressure isogram under differentdrawdowns are symmetrically distributed As shown in thesingle well effect the strata inside the foundation pit bulgeduring the dewatering of group wells but the strata outsidethe foundation pit subside-is is mainly due to the result ofthe movement of groundwater outside the pit to the dew-atering well inside the pit
533 Effect of Group Wells According to the results ob-tained from 3D simulation the subsidence of the sixmonitoring points on the sides of the foundation pit due todifferent drawdowns caused by the single well and the groupwells are shown in Figure 33
Grade I drawdown including Grade II Grade III andGrade IV drawdown means that the dewatering depth ofgroundwater level is in 6m 11m 16m and 22m respectively
Table 2 Physical and mechanical parameters of strata
Name of stratum Modulus of deformation E (MPa) Poissonrsquos ratio Density (gcm3) Permeability coefficient (md)Miscellaneous fill 2 035 18 125Silty clay 4 029 196 005Clayey silt 4 030 194 015Fine sand 4 028 185 3Medium sand 40 026 19 10Loose pebble layer 18 025 2 22Slightly condensed pebble layer 20 023 21 22Medium condensed pebble layer 28 020 22 22Compacted pebble layer 38 017 23 22
Miscellaneous fill Silty clayClayey siltFine sandMedium sand
12345
Figure 11 Geological section of the territory
12 Advances in Civil Engineering
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
32 Design of Dewatering Well Points Since the subwaystation is closely located to the shops and buildings on its twosides and the underground pipelines are densely packed inpower cable tunnel and municipal sewages there are noother spare places to install dewatering wells outside thefoundation pit -erefore to avoid the impact of dewateringon the surrounding environment well points are used to thelower groundwater level inside the foundation pit prior to itsexcavation
-e groundwater of the construction site belongs to theQuaternary existing in the sandy pebble pore phreaticaquifer -e thickness of the phreatic aquifer is less than30m and the bottom floor of the station is located in thecompacted pebble layer -e purpose of dewatering is tolower the water table in the foundation pit to 1m below itsbottom so that normal construction of the subway stationcan be fulfilled without groundwater -e open cut foun-dation pit of the station is 225m long and 24m wide with a
Family wingsEntrance F
Entrance GEntrance 2Shops
Entrance DEntrance E
Foundation pit
N
Seco
nd ri
ng
Figure 6 Station location and its surrounding environment
⑧ Medium dense pebblelayer 1sim214 1233 255times10minus 2 32
⑨ Compacted pebblelayer Not drilled Not drilled 255times10minus 2 43
h 1h 2
h 3
k1
k2
k3
Ground
Groundwater level
Soil layer 1
Soil layer 2
Soil layer 3
Phreatic aquifer
Impermeable stratum
Figure 5 Geological conditions of multilayered soil
6 Advances in Civil Engineering
length-width ratio of 9375lt 20-e depth of the foundationpit is about 2664m For the convenience of calculation it isset to 27m -e diameter of the dewatering well is 600mmand the well bottom is 35m lower than the one of thestation-e length of the filter pipe is 2m-e distance fromthe bottom of the filter pipe to the impervious layer is 2mand the stable water table is considered to be 6m
From the theoretical design and calculation of Section 2 itis known that 17 well points are to be laid around the foun-dation pit Referring to the Chinese Technical Code forGroundwater Control in Building and Municipal Engineering(JGJT111-2016) [19] and considering an idealized situation inwhichwells can be easily bored around the open cut foundationpit the dewatering wells are arranged at the same distancealong the foundation pit After a well is installed at the center ofthe foundation pit the remaining 16 well points are installedevenly at the inner edge of the pit Since it is a long and narrowopen cut foundation pit the wells can be placed at the inneredge of the long side of the foundation pit and the spacebetween them is about 25m -e specific layout of well pointsinside the open cut foundation pit is shown in Figure 7
33 Comparison between Calculations with and withoutconsidering the Waterproof Effect of Enclosure StructureAccording to the Chinese Technical Code for groundwatercontrol in building and municipal engineering [18] if groupwell points are simplified to be large one the total waterinflow from the incomplete well points in the phreaticaquifer which is calculated by using equations (3) and (4) isreplaced with the following equation
hm H + h
2 (18)
-e parameters in equation (18) are the same as that inthe formulae as stated above If the waterproofing effect ofthe enclosure structure is not considered then the depth ofwater level lowered by well points is expressed below
Sw H minus h (19)
Substituting aforementioned engineering data intoequations (3) and (16)ndash(18) respectively it is seen that thewater inflow from incomplete well point Q3 in the foun-dation pit is
Q3 Q2 1209532m3 (20)
the radius of influence of the foundation pit Rprime is
Rprime 2Sw
Hk
radicasymp 105233m (21)
and the number of well points nprime is certainly obtainedaccording to
nprime 12Q2
q0asymp 23 (22)
After a well point is installed at the center of the pit theremaining 22 well points can then be set evenly at the edge ofthe foundation pit Since it is a long and narrow foundationpit the well points are evenly installed at the edge of the long
side L of the foundation pit and the space Dprime between themis
Dprime L
nasymp 2045m (23)
Compared with the waterproofing effect of the enclosurestructure the water inflow in the foundation pit is893651209532 asymp 074 times of the conventional algorithmproposed in the Chinese Specification and the dewateringradius of influence is 54099105233 asymp 051 times of theconventional algorithm in the Specification -e number ofwell points is 1723 asymp 074 times of the conventional algo-rithm in the Specification and the space between them is252045 asymp 122 times of the conventional algorithm in theSpecification -rough comparison if the enclosure struc-ture of the foundation pit is used as a waterproof curtainthen the waterproofing effect of the enclosure structureshould not be ignored when calculating water inflow insidethe open cut foundation pit
34 Arrangement of Points Monitoring Ground SubsidenceIn order to make further analysis on the ground subsidencecaused by dewatering in the foundation pit the groundsubsidence of the typical positions around the foundation pitis monitored -e ground subsidence monitoring points areset up according to the actual condition of the open cutfoundation pit In the actual project the ground subsidencemonitoring points are arranged around the foundation pitFor the consideration of symmetry and the convenience ofmeasuring six points at the midline of the long side of thefoundation pit are selected and they are 16-6 16-5 16-4 16-3 16-2 and 16-1 respectively the distance between the wellpoints and pit wall is set within 8m 12m 16m 20m 24mand 28m respectively -e monitoring points are shown inFigure 8 which are located at the center axis of the edge lineof the pit Step-by-step dewatering is carried out in the actualproject and the depth of dewatering for every step is set to6m 5m 5m and 6m respectively and the total stepamounts to 22m
4 Theoretical Calculation of GroundSubsidence Caused by Dewatering inFoundation Pit
41 Normative Calculation of Ground Subsidence Caused byDewatering in Foundation Pit
411 Dupuitrsquos Assumption and Derivation of DewateringFunnel Curve Dewatering in the foundation pit will defi-nitely produce a falling funnel curve around the pit and thegroundwater may flow into the dewatering well inside thefoundation pit French scholar Dupuit first studied thesteady well flow put forward Dupuitrsquos assumption andderived the dewatering funnel curve -e hypothesis con-siders a cylindrical homogeneous phreatic aquifer withisotropic and horizontally waterproofing bottom floor afixed water head outside the aquifer a complete pumpingwell in the center no vertical infiltration recharge and
Advances in Civil Engineering 7
evaporation and a steady seepage subject to the linear law[20] -e dewatering well point can develop a dewateringfunnel curve around it Groundwater flows to the well after acertain time and the dewatering curve can reach a steadystate Assuming that the well point dewatering is a stablephreatic well flow without the group well effect the center ofthe well bottom is set to be the origin and the abscissa is setto be the positive x-axis as shown in Figure 9
According to Dupuitrsquos assumption the water flowequation of the stable phreatic well is obtained below [20]
Q kIA 2πrhkdh
dr (24)
If we separate the variable in equation (24) and take anyone point on the dewatering funnel curve then followingequations are obtained by integrating the equation from thepoint to its boundary
r x h z
r R h H
⎧⎪⎨
⎪⎩
⎫⎪⎬
⎪⎭⟶ 1113946
R
x
Q
2πk
1rdr 1113946
H
zh dh (25)
and thenQ
2πkln
R
x12
H2
minus z2
1113872 1113873 (26)
If the boundary condition meets
x r0
z h0 + l(27)
and substituting equation (27) into equation (26) then thefollowing equation is obtained
Q
2πkln
R
r012
H2
minus h0 + l( 11138572
1113960 1113961 (28)
-e solution of simultaneous equations (26) and (28) isused to obtain the following dewatering funnel curveequation of the well point
z2
H2
minus H2
minus h0 + l( 11138572
1113960 1113961ln(Rx)
ln Rr0( 1113857 (29)
where r0 denotes the radius of wells m h0 represents thelength of the inlet part of the filter pipe m R denotes theradius of influence m H represents the height from thebottom of the well to the initial groundwater level m k is thepermeability coefficient of the aquifer md A is the side areaof the dewatering well m2 I denotes the groundwaterseepage field hydraulic gradient Q stands for the boreholewater yield m3d and l means the distance between thebottom of well and the impermeable layer m-e remainingsymbols are shown in Figure 9
412 Ground Subsidence at Any Point outside the Foun-dation Pit At present the calculation of ground subsi-dence around the foundation pit after dewatering isgenerally carried out using the stratification summationmethod given in the Technical Specification for Retainingand Protection of Building Foundation Excavations (JGJ120-2012) [18] -e ground subsidence can be calculated bysummarizing the compression of each layer of soil Firstlywe can calculate the additional effective stress caused bydewatering and then calculate the ground subsidence usingstress calculation
(1) When the soil layer numbered as i is above the initialgroundwater level the effective stress is
Δσziprime 0 (30)
(2) If the soil layer numbered as i is located between thepostdewatering groundwater level and the initialgroundwater level then the effective stress is
Δσziprime cwz (31)
Foundation pit
Pit wall 16-616-516-416-316-216-1
Figure 8 Schematic diagram of the ground subsidence monitoring points around the foundation pit
Well point
Foundation pit wall
Figure 7 Schematic diagram of the layout of the dewatering well
8 Advances in Civil Engineering
(3) When the soil layer numbered as i is below thegroundwater level after dewatering then the effectivestress is
Δσziprime λicwsi (32)
-e soil compression caused by dewatering is as follows
s ψw 1113944Δσziprime Δhi
Esi
(33)
where cw means the bulk density of water kNm3 z denotesthe vertical distance from the midpoint of the soil layer i tothe initial groundwater level m and λi represents the cal-culation coefficient it should be based on the analysis ofgroundwater seepage If the analysis data are not availablethen its value should be based on local engineering expe-rience si refers to the depth of lowering groundwater levelcorresponding to the calculation profile m s denotes theground compression in the calculation profile m and ψw
means the empirical coefficient of subsidence calculationbased on local engineering experience If no experience isavailable then the value is set to be 1 Δσzi
prime denotes the meanadditional stress of the soil layer i under the ground surfacecaused by dewatering kPa Δhi means the thickness of thesoil layer i m and Esi
denotes the compressive modulus ofthe soil layer i kPa
42 Calculation of Ground Subsidence Caused by Dewateringin Foundation Pit under Seepage Force -e pumping anddrainage of the dewatering well will cause the change ofthe underground seepage field which will generate a newseepage field and lead to the variation of the stress fieldaround the well -erefore the seepage force is the maincause of soil consolidation and settlement -e seepage ofgroundwater causes the dissipation of pore water pres-sure resulting in an increase in effective stress -at isadditional stress is generated in the soil the direction ofwhich is vertically downward in addition it produces a
horizontal component Deformation can be caused by theimpact of seepage force -e additional stress namely thevertical component of the seepage force will cause theground subsidence [21] -e horizontal component of theseepage force will cause the lateral deformation of the soilAccording to reference [22] the seepage direction of anywater head at any point is tangent to the phreatic surfaceat that point pointing to the well axis as shown inFigure 10
Wu and Zhu [22] performed related research andproposed a new algorithm of ground subsidence caused byseepage force Yang and Zhao [23] also used this method tocalculate ground subsidence -is section draws lessonsfrom their research to discuss the dewatering in open cutfoundation pit of a subway station on Chengdu Metro Line6
-e stratum is divided into three parts dry soil zonedewatered zone and saturated zone -e dry soil zone isalways above the groundwater level before and afterdewatering which does not contain groundwater andseepage force during dewatering -us additional stressdoes not appear in this zone and the subsidence valuecaused by dewatering is 0 -e other two layers are S1 andS2 respectively as shown in Figure 10 -e soil layer S1 inthe dewatering process is drained -e zone S2 is alwaysbelow the groundwater level and is saturated In thissection the soil subsidence in the dewatered and saturatedzones is calculated separately -e seepage forces in thesetwo areas possess horizontal components as shown inFigure 10
Assuming that the water head that keeps a distance of x0from itself to the well axis is z0 as shown in Figure 10according to the dewatering funnel curve equation (29) inSection 41 the height of the falling funnel curve is obtained-e direction of the seepage force is actually in the straightline vector on the curve which goes through the point(x0 z0) and is tangent to the falling funnel curve and pointsto the well In order to find out the slope of the straight linethe following equation is obtained by performing the de-rivative of equation (29) at point x x0
R
H
h 0
x
z
r0 l
Ground
Impermeable stratum
Well point
Groundwater level
Figure 9 Dewatering funnel curve for the phreatic incomplete well point
-e equation of the line that passes through the point(x0 z0) and is tangential to the curve after the dewateringbecomes stable is assumed to be
y x tan α + b (35)
-e slope of the equation is
tan α zprime x0( 1113857 (36)
According to the trigonometric function conversion
sin α tan α
1 + tan2 α
radic (37)
-en the following equation is obtained
sin α zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (38)
-erefore the vertical component of additional pressureof the surrounding stratum caused by dewatering in thefoundation pit is expressed as
ΔPy ΔP sin α ΔPzprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (39)
where ΔPx denotes the horizontal component of the seepageforce ΔPy denotes the vertical component of the seepageforce and ΔP is the seepage force
-e angle α in equation (39) denotes the one existsbetween the horizontal component of the additional pres-sure and the additional pressure caused by dewatering in thefoundation pit
Based on equations (30) to (33) the additional stresses inthe dewatering zone and the saturation zone are calculatedseparately -e rewriting is carried out on the basis ofequation (34) which derives the ground subsidence causedby dewatering in the foundation pit considering the action ofseepage force -e formula after rewriting is as follows
s ψw 1113944Δσziprime Δhi
Esi
zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (40)
Substituting equation (34) into equation (40) yields thefollowing equation
Figure 10 Simplified calculation model considering seepage force
10 Advances in Civil Engineering
If the ground subsidence S1 and S2 corresponding to thedewatered and saturated zones are separately figured outthen the total ground subsidence is obtained and illustratedin the following equation
S S1 + S2 (42)
where the symbols are illustrated in Section 41
5 Numerical Simulation
With fast development of urban construction various in-frastructures such as subway station high-speed railwaytunnel underground parking lot and basement are beingbuilt especially the construction of deep foundation pits-e geological condition varies in the foundation pit and thesurrounding environment is becoming more and morecomplex after construction -eoretical analysis and em-pirical calculation methods are no longer suitable forpractical projects Nowadays actual projects are generallybuilt ahead of theoretical research and calculation methodsof numerical simulation provide an effective approach forthe design and construction of practical foundation pitAlthough numerical simulation method has not been pro-posed for very long time it has become the most commonmethod used in structural analysis and calculation so farMany scholars [24ndash31] have also used FLAC3D to conductfluid-solid coupling analysis of dewatering in the foundationpit Finite difference software FLAC3D is also employed inthis paper to carry out three-dimensional numerical sim-ulation of the whole process of the environment changecaused by dewatering in the open cut foundation pit of ametro station on Chengdu Metro Line 6 -e calculationresults are compared with the one obtained from theoreticalcalculation and on-site monitoring which are used to makefurther study on the surrounding ground subsidence due todewatering in the open cut foundation pit
51 Physical and Mechanical Parameters According to thegeological properties of the actual project the calculationparameters are obtained and shown in Table 2
52 Establishment of 3D Model and Initial Stress BalanceBased on the engineering geological and hydrologicalconditions a three-dimensional stratigraphic model isestablished From the geological section of the site (Fig-ures 11 and 12) it can be seen that each of soil layers is nearlyhorizontal and almost parallel to each other So in order toestablish the model and conduct the calculation conve-niently [26] the geological layers in the model are simplified-e foundation pit and wells are set up in the stratigraphicmodel according to dewatering scheme According to theprinciple of Saint Venant in elastic mechanics in order toeliminate the boundary influence of the foundation pit oncalculation results the calculation model is extended to 3sim5times of the depth of the pit from the periphery of thefoundation pit on the plane -e depth is set to the im-pervious interface and the entire calculation domain is inthe volume of 420mtimes 198mtimes 72m Referring to
geotechnical mapping and geological properties the modelis divided into 9 strata and the generalized permeabilitycoefficient and stratum thickness are shown in Table 1
According to stratum distribution and initial conditionsthe calculation domain is divided into grids by consideringthe monitoring points the dewatering wells and the dia-phragm wall considering the geological survey In themeshing the grid of the calculation domain is locally refinedin addition the grids around the foundation pit are alsorefined but grids far away from the pit are sparsely meshedconsidering Saint Venantrsquos principle [26] So the entiremodel is then divided into 42 layers 78 rows 300 columnsand a total of 982800 elements and the established three-dimensional model is shown in Figure 13
After the 3D model is set up the initial stress balance isneeded to be applied firstly and the corresponding seepageand displacement boundary conditions are also applied -enormal displacement and the bottom displacement of thefour sidewalls and bottom of the model are restricted andthe horizontal displacement of the wall of the dewateringwell is also constrained -e bottom filter pipe of thedewatering well belongs to the seepage boundary which isachieved by applying the pore water pressure After nu-merical simulation the pore water pressure of initialequilibrium is shown in Figure 14 -e pore water pressurein the initial state is evenly applied on the strata
53 Numerical Simulation of Fluid-Solid Coupling of GroundSubsidence Caused by Dewatering in Foundation PitAfter the initial equilibrium of the numerical model theeffect of single well and group wells were firstly carried outseparately -e stepwise dewatering was conducted underthe conditions of single well and group wells separatelywithout considering excavation -e calculated isograms ofthe dewatering at all levels and the subsidence of themonitoring points are analyzed
531 Numerical Simulation Analysis of Single WellDewatering Considering the symmetry of the well pointthe well point at the center of the calculation domain isselected when carrying out single well simulation -e po-sition of the single well on the model is shown in Figure 15-e isograms showing subsidence and pore water pressure atvarious drawdowns are also shown in Figures 16ndash23
It is seen from Figures 16ndash23 that the influence range ofdewatering well increases with the increase of drawdownAfter the well is dewatered the pore water pressure formsthe dewatering funnel surface -e bigger the drawdown isthe deeper the surface becomes Moreover due to the wa-terproofing effect of the diaphragm wall the stratum insidethe pit bulges but the stratum outside the pit subsides -isis because the groundwater flows through the bottom of thediaphragm wall into the pit -e increase of the pore waterpressure in the pit causes the additional stress to increasewhich leads to the upheaval of the soil layer in the pit -edecrease of the pore water pressure outside the pit causes theeffective stress of the outer stratum to decrease resulting inthe formation of downward additional stress in the outer
Advances in Civil Engineering 11
stratum of the pit which eventually leads to the consoli-dation and settlement of the outer stratum
532 Numerical Simulation Analysis of Group WellsDewatering When conducting the effect of group wells onground subsidence the calculation is based on the well pointlayout diagram designed in Section 3 -e layout of thegroup wells in the model is shown in Figure 24-e isogramsshowing ground subsidence and pore water pressure cal-culated using FLAC3D are shown in Figures 25ndash32
It is seen from the isograms shown in Figures 25ndash32 thatthe deeper the groundwater drawdown the greater theimpact on the surrounding ground is and the lower thefunnel surface formed by the pore water pressure Comparedwith the isogram under the effect of single well in Section 52the influence of dewatering by group wells is much largerGroup wells dewatering has a great impact on the pore waterpressure and strata deformation inside the foundation pit
that is the influence of group wells on ground subsidenceinside the pit is obvious and should not be ignored -esubsidence and pore water pressure isogram under differentdrawdowns are symmetrically distributed As shown in thesingle well effect the strata inside the foundation pit bulgeduring the dewatering of group wells but the strata outsidethe foundation pit subside-is is mainly due to the result ofthe movement of groundwater outside the pit to the dew-atering well inside the pit
533 Effect of Group Wells According to the results ob-tained from 3D simulation the subsidence of the sixmonitoring points on the sides of the foundation pit due todifferent drawdowns caused by the single well and the groupwells are shown in Figure 33
Grade I drawdown including Grade II Grade III andGrade IV drawdown means that the dewatering depth ofgroundwater level is in 6m 11m 16m and 22m respectively
Table 2 Physical and mechanical parameters of strata
Name of stratum Modulus of deformation E (MPa) Poissonrsquos ratio Density (gcm3) Permeability coefficient (md)Miscellaneous fill 2 035 18 125Silty clay 4 029 196 005Clayey silt 4 030 194 015Fine sand 4 028 185 3Medium sand 40 026 19 10Loose pebble layer 18 025 2 22Slightly condensed pebble layer 20 023 21 22Medium condensed pebble layer 28 020 22 22Compacted pebble layer 38 017 23 22
Miscellaneous fill Silty clayClayey siltFine sandMedium sand
12345
Figure 11 Geological section of the territory
12 Advances in Civil Engineering
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
length-width ratio of 9375lt 20-e depth of the foundationpit is about 2664m For the convenience of calculation it isset to 27m -e diameter of the dewatering well is 600mmand the well bottom is 35m lower than the one of thestation-e length of the filter pipe is 2m-e distance fromthe bottom of the filter pipe to the impervious layer is 2mand the stable water table is considered to be 6m
From the theoretical design and calculation of Section 2 itis known that 17 well points are to be laid around the foun-dation pit Referring to the Chinese Technical Code forGroundwater Control in Building and Municipal Engineering(JGJT111-2016) [19] and considering an idealized situation inwhichwells can be easily bored around the open cut foundationpit the dewatering wells are arranged at the same distancealong the foundation pit After a well is installed at the center ofthe foundation pit the remaining 16 well points are installedevenly at the inner edge of the pit Since it is a long and narrowopen cut foundation pit the wells can be placed at the inneredge of the long side of the foundation pit and the spacebetween them is about 25m -e specific layout of well pointsinside the open cut foundation pit is shown in Figure 7
33 Comparison between Calculations with and withoutconsidering the Waterproof Effect of Enclosure StructureAccording to the Chinese Technical Code for groundwatercontrol in building and municipal engineering [18] if groupwell points are simplified to be large one the total waterinflow from the incomplete well points in the phreaticaquifer which is calculated by using equations (3) and (4) isreplaced with the following equation
hm H + h
2 (18)
-e parameters in equation (18) are the same as that inthe formulae as stated above If the waterproofing effect ofthe enclosure structure is not considered then the depth ofwater level lowered by well points is expressed below
Sw H minus h (19)
Substituting aforementioned engineering data intoequations (3) and (16)ndash(18) respectively it is seen that thewater inflow from incomplete well point Q3 in the foun-dation pit is
Q3 Q2 1209532m3 (20)
the radius of influence of the foundation pit Rprime is
Rprime 2Sw
Hk
radicasymp 105233m (21)
and the number of well points nprime is certainly obtainedaccording to
nprime 12Q2
q0asymp 23 (22)
After a well point is installed at the center of the pit theremaining 22 well points can then be set evenly at the edge ofthe foundation pit Since it is a long and narrow foundationpit the well points are evenly installed at the edge of the long
side L of the foundation pit and the space Dprime between themis
Dprime L
nasymp 2045m (23)
Compared with the waterproofing effect of the enclosurestructure the water inflow in the foundation pit is893651209532 asymp 074 times of the conventional algorithmproposed in the Chinese Specification and the dewateringradius of influence is 54099105233 asymp 051 times of theconventional algorithm in the Specification -e number ofwell points is 1723 asymp 074 times of the conventional algo-rithm in the Specification and the space between them is252045 asymp 122 times of the conventional algorithm in theSpecification -rough comparison if the enclosure struc-ture of the foundation pit is used as a waterproof curtainthen the waterproofing effect of the enclosure structureshould not be ignored when calculating water inflow insidethe open cut foundation pit
34 Arrangement of Points Monitoring Ground SubsidenceIn order to make further analysis on the ground subsidencecaused by dewatering in the foundation pit the groundsubsidence of the typical positions around the foundation pitis monitored -e ground subsidence monitoring points areset up according to the actual condition of the open cutfoundation pit In the actual project the ground subsidencemonitoring points are arranged around the foundation pitFor the consideration of symmetry and the convenience ofmeasuring six points at the midline of the long side of thefoundation pit are selected and they are 16-6 16-5 16-4 16-3 16-2 and 16-1 respectively the distance between the wellpoints and pit wall is set within 8m 12m 16m 20m 24mand 28m respectively -e monitoring points are shown inFigure 8 which are located at the center axis of the edge lineof the pit Step-by-step dewatering is carried out in the actualproject and the depth of dewatering for every step is set to6m 5m 5m and 6m respectively and the total stepamounts to 22m
4 Theoretical Calculation of GroundSubsidence Caused by Dewatering inFoundation Pit
41 Normative Calculation of Ground Subsidence Caused byDewatering in Foundation Pit
411 Dupuitrsquos Assumption and Derivation of DewateringFunnel Curve Dewatering in the foundation pit will defi-nitely produce a falling funnel curve around the pit and thegroundwater may flow into the dewatering well inside thefoundation pit French scholar Dupuit first studied thesteady well flow put forward Dupuitrsquos assumption andderived the dewatering funnel curve -e hypothesis con-siders a cylindrical homogeneous phreatic aquifer withisotropic and horizontally waterproofing bottom floor afixed water head outside the aquifer a complete pumpingwell in the center no vertical infiltration recharge and
Advances in Civil Engineering 7
evaporation and a steady seepage subject to the linear law[20] -e dewatering well point can develop a dewateringfunnel curve around it Groundwater flows to the well after acertain time and the dewatering curve can reach a steadystate Assuming that the well point dewatering is a stablephreatic well flow without the group well effect the center ofthe well bottom is set to be the origin and the abscissa is setto be the positive x-axis as shown in Figure 9
According to Dupuitrsquos assumption the water flowequation of the stable phreatic well is obtained below [20]
Q kIA 2πrhkdh
dr (24)
If we separate the variable in equation (24) and take anyone point on the dewatering funnel curve then followingequations are obtained by integrating the equation from thepoint to its boundary
r x h z
r R h H
⎧⎪⎨
⎪⎩
⎫⎪⎬
⎪⎭⟶ 1113946
R
x
Q
2πk
1rdr 1113946
H
zh dh (25)
and thenQ
2πkln
R
x12
H2
minus z2
1113872 1113873 (26)
If the boundary condition meets
x r0
z h0 + l(27)
and substituting equation (27) into equation (26) then thefollowing equation is obtained
Q
2πkln
R
r012
H2
minus h0 + l( 11138572
1113960 1113961 (28)
-e solution of simultaneous equations (26) and (28) isused to obtain the following dewatering funnel curveequation of the well point
z2
H2
minus H2
minus h0 + l( 11138572
1113960 1113961ln(Rx)
ln Rr0( 1113857 (29)
where r0 denotes the radius of wells m h0 represents thelength of the inlet part of the filter pipe m R denotes theradius of influence m H represents the height from thebottom of the well to the initial groundwater level m k is thepermeability coefficient of the aquifer md A is the side areaof the dewatering well m2 I denotes the groundwaterseepage field hydraulic gradient Q stands for the boreholewater yield m3d and l means the distance between thebottom of well and the impermeable layer m-e remainingsymbols are shown in Figure 9
412 Ground Subsidence at Any Point outside the Foun-dation Pit At present the calculation of ground subsi-dence around the foundation pit after dewatering isgenerally carried out using the stratification summationmethod given in the Technical Specification for Retainingand Protection of Building Foundation Excavations (JGJ120-2012) [18] -e ground subsidence can be calculated bysummarizing the compression of each layer of soil Firstlywe can calculate the additional effective stress caused bydewatering and then calculate the ground subsidence usingstress calculation
(1) When the soil layer numbered as i is above the initialgroundwater level the effective stress is
Δσziprime 0 (30)
(2) If the soil layer numbered as i is located between thepostdewatering groundwater level and the initialgroundwater level then the effective stress is
Δσziprime cwz (31)
Foundation pit
Pit wall 16-616-516-416-316-216-1
Figure 8 Schematic diagram of the ground subsidence monitoring points around the foundation pit
Well point
Foundation pit wall
Figure 7 Schematic diagram of the layout of the dewatering well
8 Advances in Civil Engineering
(3) When the soil layer numbered as i is below thegroundwater level after dewatering then the effectivestress is
Δσziprime λicwsi (32)
-e soil compression caused by dewatering is as follows
s ψw 1113944Δσziprime Δhi
Esi
(33)
where cw means the bulk density of water kNm3 z denotesthe vertical distance from the midpoint of the soil layer i tothe initial groundwater level m and λi represents the cal-culation coefficient it should be based on the analysis ofgroundwater seepage If the analysis data are not availablethen its value should be based on local engineering expe-rience si refers to the depth of lowering groundwater levelcorresponding to the calculation profile m s denotes theground compression in the calculation profile m and ψw
means the empirical coefficient of subsidence calculationbased on local engineering experience If no experience isavailable then the value is set to be 1 Δσzi
prime denotes the meanadditional stress of the soil layer i under the ground surfacecaused by dewatering kPa Δhi means the thickness of thesoil layer i m and Esi
denotes the compressive modulus ofthe soil layer i kPa
42 Calculation of Ground Subsidence Caused by Dewateringin Foundation Pit under Seepage Force -e pumping anddrainage of the dewatering well will cause the change ofthe underground seepage field which will generate a newseepage field and lead to the variation of the stress fieldaround the well -erefore the seepage force is the maincause of soil consolidation and settlement -e seepage ofgroundwater causes the dissipation of pore water pres-sure resulting in an increase in effective stress -at isadditional stress is generated in the soil the direction ofwhich is vertically downward in addition it produces a
horizontal component Deformation can be caused by theimpact of seepage force -e additional stress namely thevertical component of the seepage force will cause theground subsidence [21] -e horizontal component of theseepage force will cause the lateral deformation of the soilAccording to reference [22] the seepage direction of anywater head at any point is tangent to the phreatic surfaceat that point pointing to the well axis as shown inFigure 10
Wu and Zhu [22] performed related research andproposed a new algorithm of ground subsidence caused byseepage force Yang and Zhao [23] also used this method tocalculate ground subsidence -is section draws lessonsfrom their research to discuss the dewatering in open cutfoundation pit of a subway station on Chengdu Metro Line6
-e stratum is divided into three parts dry soil zonedewatered zone and saturated zone -e dry soil zone isalways above the groundwater level before and afterdewatering which does not contain groundwater andseepage force during dewatering -us additional stressdoes not appear in this zone and the subsidence valuecaused by dewatering is 0 -e other two layers are S1 andS2 respectively as shown in Figure 10 -e soil layer S1 inthe dewatering process is drained -e zone S2 is alwaysbelow the groundwater level and is saturated In thissection the soil subsidence in the dewatered and saturatedzones is calculated separately -e seepage forces in thesetwo areas possess horizontal components as shown inFigure 10
Assuming that the water head that keeps a distance of x0from itself to the well axis is z0 as shown in Figure 10according to the dewatering funnel curve equation (29) inSection 41 the height of the falling funnel curve is obtained-e direction of the seepage force is actually in the straightline vector on the curve which goes through the point(x0 z0) and is tangent to the falling funnel curve and pointsto the well In order to find out the slope of the straight linethe following equation is obtained by performing the de-rivative of equation (29) at point x x0
R
H
h 0
x
z
r0 l
Ground
Impermeable stratum
Well point
Groundwater level
Figure 9 Dewatering funnel curve for the phreatic incomplete well point
-e equation of the line that passes through the point(x0 z0) and is tangential to the curve after the dewateringbecomes stable is assumed to be
y x tan α + b (35)
-e slope of the equation is
tan α zprime x0( 1113857 (36)
According to the trigonometric function conversion
sin α tan α
1 + tan2 α
radic (37)
-en the following equation is obtained
sin α zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (38)
-erefore the vertical component of additional pressureof the surrounding stratum caused by dewatering in thefoundation pit is expressed as
ΔPy ΔP sin α ΔPzprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (39)
where ΔPx denotes the horizontal component of the seepageforce ΔPy denotes the vertical component of the seepageforce and ΔP is the seepage force
-e angle α in equation (39) denotes the one existsbetween the horizontal component of the additional pres-sure and the additional pressure caused by dewatering in thefoundation pit
Based on equations (30) to (33) the additional stresses inthe dewatering zone and the saturation zone are calculatedseparately -e rewriting is carried out on the basis ofequation (34) which derives the ground subsidence causedby dewatering in the foundation pit considering the action ofseepage force -e formula after rewriting is as follows
s ψw 1113944Δσziprime Δhi
Esi
zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (40)
Substituting equation (34) into equation (40) yields thefollowing equation
Figure 10 Simplified calculation model considering seepage force
10 Advances in Civil Engineering
If the ground subsidence S1 and S2 corresponding to thedewatered and saturated zones are separately figured outthen the total ground subsidence is obtained and illustratedin the following equation
S S1 + S2 (42)
where the symbols are illustrated in Section 41
5 Numerical Simulation
With fast development of urban construction various in-frastructures such as subway station high-speed railwaytunnel underground parking lot and basement are beingbuilt especially the construction of deep foundation pits-e geological condition varies in the foundation pit and thesurrounding environment is becoming more and morecomplex after construction -eoretical analysis and em-pirical calculation methods are no longer suitable forpractical projects Nowadays actual projects are generallybuilt ahead of theoretical research and calculation methodsof numerical simulation provide an effective approach forthe design and construction of practical foundation pitAlthough numerical simulation method has not been pro-posed for very long time it has become the most commonmethod used in structural analysis and calculation so farMany scholars [24ndash31] have also used FLAC3D to conductfluid-solid coupling analysis of dewatering in the foundationpit Finite difference software FLAC3D is also employed inthis paper to carry out three-dimensional numerical sim-ulation of the whole process of the environment changecaused by dewatering in the open cut foundation pit of ametro station on Chengdu Metro Line 6 -e calculationresults are compared with the one obtained from theoreticalcalculation and on-site monitoring which are used to makefurther study on the surrounding ground subsidence due todewatering in the open cut foundation pit
51 Physical and Mechanical Parameters According to thegeological properties of the actual project the calculationparameters are obtained and shown in Table 2
52 Establishment of 3D Model and Initial Stress BalanceBased on the engineering geological and hydrologicalconditions a three-dimensional stratigraphic model isestablished From the geological section of the site (Fig-ures 11 and 12) it can be seen that each of soil layers is nearlyhorizontal and almost parallel to each other So in order toestablish the model and conduct the calculation conve-niently [26] the geological layers in the model are simplified-e foundation pit and wells are set up in the stratigraphicmodel according to dewatering scheme According to theprinciple of Saint Venant in elastic mechanics in order toeliminate the boundary influence of the foundation pit oncalculation results the calculation model is extended to 3sim5times of the depth of the pit from the periphery of thefoundation pit on the plane -e depth is set to the im-pervious interface and the entire calculation domain is inthe volume of 420mtimes 198mtimes 72m Referring to
geotechnical mapping and geological properties the modelis divided into 9 strata and the generalized permeabilitycoefficient and stratum thickness are shown in Table 1
According to stratum distribution and initial conditionsthe calculation domain is divided into grids by consideringthe monitoring points the dewatering wells and the dia-phragm wall considering the geological survey In themeshing the grid of the calculation domain is locally refinedin addition the grids around the foundation pit are alsorefined but grids far away from the pit are sparsely meshedconsidering Saint Venantrsquos principle [26] So the entiremodel is then divided into 42 layers 78 rows 300 columnsand a total of 982800 elements and the established three-dimensional model is shown in Figure 13
After the 3D model is set up the initial stress balance isneeded to be applied firstly and the corresponding seepageand displacement boundary conditions are also applied -enormal displacement and the bottom displacement of thefour sidewalls and bottom of the model are restricted andthe horizontal displacement of the wall of the dewateringwell is also constrained -e bottom filter pipe of thedewatering well belongs to the seepage boundary which isachieved by applying the pore water pressure After nu-merical simulation the pore water pressure of initialequilibrium is shown in Figure 14 -e pore water pressurein the initial state is evenly applied on the strata
53 Numerical Simulation of Fluid-Solid Coupling of GroundSubsidence Caused by Dewatering in Foundation PitAfter the initial equilibrium of the numerical model theeffect of single well and group wells were firstly carried outseparately -e stepwise dewatering was conducted underthe conditions of single well and group wells separatelywithout considering excavation -e calculated isograms ofthe dewatering at all levels and the subsidence of themonitoring points are analyzed
531 Numerical Simulation Analysis of Single WellDewatering Considering the symmetry of the well pointthe well point at the center of the calculation domain isselected when carrying out single well simulation -e po-sition of the single well on the model is shown in Figure 15-e isograms showing subsidence and pore water pressure atvarious drawdowns are also shown in Figures 16ndash23
It is seen from Figures 16ndash23 that the influence range ofdewatering well increases with the increase of drawdownAfter the well is dewatered the pore water pressure formsthe dewatering funnel surface -e bigger the drawdown isthe deeper the surface becomes Moreover due to the wa-terproofing effect of the diaphragm wall the stratum insidethe pit bulges but the stratum outside the pit subsides -isis because the groundwater flows through the bottom of thediaphragm wall into the pit -e increase of the pore waterpressure in the pit causes the additional stress to increasewhich leads to the upheaval of the soil layer in the pit -edecrease of the pore water pressure outside the pit causes theeffective stress of the outer stratum to decrease resulting inthe formation of downward additional stress in the outer
Advances in Civil Engineering 11
stratum of the pit which eventually leads to the consoli-dation and settlement of the outer stratum
532 Numerical Simulation Analysis of Group WellsDewatering When conducting the effect of group wells onground subsidence the calculation is based on the well pointlayout diagram designed in Section 3 -e layout of thegroup wells in the model is shown in Figure 24-e isogramsshowing ground subsidence and pore water pressure cal-culated using FLAC3D are shown in Figures 25ndash32
It is seen from the isograms shown in Figures 25ndash32 thatthe deeper the groundwater drawdown the greater theimpact on the surrounding ground is and the lower thefunnel surface formed by the pore water pressure Comparedwith the isogram under the effect of single well in Section 52the influence of dewatering by group wells is much largerGroup wells dewatering has a great impact on the pore waterpressure and strata deformation inside the foundation pit
that is the influence of group wells on ground subsidenceinside the pit is obvious and should not be ignored -esubsidence and pore water pressure isogram under differentdrawdowns are symmetrically distributed As shown in thesingle well effect the strata inside the foundation pit bulgeduring the dewatering of group wells but the strata outsidethe foundation pit subside-is is mainly due to the result ofthe movement of groundwater outside the pit to the dew-atering well inside the pit
533 Effect of Group Wells According to the results ob-tained from 3D simulation the subsidence of the sixmonitoring points on the sides of the foundation pit due todifferent drawdowns caused by the single well and the groupwells are shown in Figure 33
Grade I drawdown including Grade II Grade III andGrade IV drawdown means that the dewatering depth ofgroundwater level is in 6m 11m 16m and 22m respectively
Table 2 Physical and mechanical parameters of strata
Name of stratum Modulus of deformation E (MPa) Poissonrsquos ratio Density (gcm3) Permeability coefficient (md)Miscellaneous fill 2 035 18 125Silty clay 4 029 196 005Clayey silt 4 030 194 015Fine sand 4 028 185 3Medium sand 40 026 19 10Loose pebble layer 18 025 2 22Slightly condensed pebble layer 20 023 21 22Medium condensed pebble layer 28 020 22 22Compacted pebble layer 38 017 23 22
Miscellaneous fill Silty clayClayey siltFine sandMedium sand
12345
Figure 11 Geological section of the territory
12 Advances in Civil Engineering
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
evaporation and a steady seepage subject to the linear law[20] -e dewatering well point can develop a dewateringfunnel curve around it Groundwater flows to the well after acertain time and the dewatering curve can reach a steadystate Assuming that the well point dewatering is a stablephreatic well flow without the group well effect the center ofthe well bottom is set to be the origin and the abscissa is setto be the positive x-axis as shown in Figure 9
According to Dupuitrsquos assumption the water flowequation of the stable phreatic well is obtained below [20]
Q kIA 2πrhkdh
dr (24)
If we separate the variable in equation (24) and take anyone point on the dewatering funnel curve then followingequations are obtained by integrating the equation from thepoint to its boundary
r x h z
r R h H
⎧⎪⎨
⎪⎩
⎫⎪⎬
⎪⎭⟶ 1113946
R
x
Q
2πk
1rdr 1113946
H
zh dh (25)
and thenQ
2πkln
R
x12
H2
minus z2
1113872 1113873 (26)
If the boundary condition meets
x r0
z h0 + l(27)
and substituting equation (27) into equation (26) then thefollowing equation is obtained
Q
2πkln
R
r012
H2
minus h0 + l( 11138572
1113960 1113961 (28)
-e solution of simultaneous equations (26) and (28) isused to obtain the following dewatering funnel curveequation of the well point
z2
H2
minus H2
minus h0 + l( 11138572
1113960 1113961ln(Rx)
ln Rr0( 1113857 (29)
where r0 denotes the radius of wells m h0 represents thelength of the inlet part of the filter pipe m R denotes theradius of influence m H represents the height from thebottom of the well to the initial groundwater level m k is thepermeability coefficient of the aquifer md A is the side areaof the dewatering well m2 I denotes the groundwaterseepage field hydraulic gradient Q stands for the boreholewater yield m3d and l means the distance between thebottom of well and the impermeable layer m-e remainingsymbols are shown in Figure 9
412 Ground Subsidence at Any Point outside the Foun-dation Pit At present the calculation of ground subsi-dence around the foundation pit after dewatering isgenerally carried out using the stratification summationmethod given in the Technical Specification for Retainingand Protection of Building Foundation Excavations (JGJ120-2012) [18] -e ground subsidence can be calculated bysummarizing the compression of each layer of soil Firstlywe can calculate the additional effective stress caused bydewatering and then calculate the ground subsidence usingstress calculation
(1) When the soil layer numbered as i is above the initialgroundwater level the effective stress is
Δσziprime 0 (30)
(2) If the soil layer numbered as i is located between thepostdewatering groundwater level and the initialgroundwater level then the effective stress is
Δσziprime cwz (31)
Foundation pit
Pit wall 16-616-516-416-316-216-1
Figure 8 Schematic diagram of the ground subsidence monitoring points around the foundation pit
Well point
Foundation pit wall
Figure 7 Schematic diagram of the layout of the dewatering well
8 Advances in Civil Engineering
(3) When the soil layer numbered as i is below thegroundwater level after dewatering then the effectivestress is
Δσziprime λicwsi (32)
-e soil compression caused by dewatering is as follows
s ψw 1113944Δσziprime Δhi
Esi
(33)
where cw means the bulk density of water kNm3 z denotesthe vertical distance from the midpoint of the soil layer i tothe initial groundwater level m and λi represents the cal-culation coefficient it should be based on the analysis ofgroundwater seepage If the analysis data are not availablethen its value should be based on local engineering expe-rience si refers to the depth of lowering groundwater levelcorresponding to the calculation profile m s denotes theground compression in the calculation profile m and ψw
means the empirical coefficient of subsidence calculationbased on local engineering experience If no experience isavailable then the value is set to be 1 Δσzi
prime denotes the meanadditional stress of the soil layer i under the ground surfacecaused by dewatering kPa Δhi means the thickness of thesoil layer i m and Esi
denotes the compressive modulus ofthe soil layer i kPa
42 Calculation of Ground Subsidence Caused by Dewateringin Foundation Pit under Seepage Force -e pumping anddrainage of the dewatering well will cause the change ofthe underground seepage field which will generate a newseepage field and lead to the variation of the stress fieldaround the well -erefore the seepage force is the maincause of soil consolidation and settlement -e seepage ofgroundwater causes the dissipation of pore water pres-sure resulting in an increase in effective stress -at isadditional stress is generated in the soil the direction ofwhich is vertically downward in addition it produces a
horizontal component Deformation can be caused by theimpact of seepage force -e additional stress namely thevertical component of the seepage force will cause theground subsidence [21] -e horizontal component of theseepage force will cause the lateral deformation of the soilAccording to reference [22] the seepage direction of anywater head at any point is tangent to the phreatic surfaceat that point pointing to the well axis as shown inFigure 10
Wu and Zhu [22] performed related research andproposed a new algorithm of ground subsidence caused byseepage force Yang and Zhao [23] also used this method tocalculate ground subsidence -is section draws lessonsfrom their research to discuss the dewatering in open cutfoundation pit of a subway station on Chengdu Metro Line6
-e stratum is divided into three parts dry soil zonedewatered zone and saturated zone -e dry soil zone isalways above the groundwater level before and afterdewatering which does not contain groundwater andseepage force during dewatering -us additional stressdoes not appear in this zone and the subsidence valuecaused by dewatering is 0 -e other two layers are S1 andS2 respectively as shown in Figure 10 -e soil layer S1 inthe dewatering process is drained -e zone S2 is alwaysbelow the groundwater level and is saturated In thissection the soil subsidence in the dewatered and saturatedzones is calculated separately -e seepage forces in thesetwo areas possess horizontal components as shown inFigure 10
Assuming that the water head that keeps a distance of x0from itself to the well axis is z0 as shown in Figure 10according to the dewatering funnel curve equation (29) inSection 41 the height of the falling funnel curve is obtained-e direction of the seepage force is actually in the straightline vector on the curve which goes through the point(x0 z0) and is tangent to the falling funnel curve and pointsto the well In order to find out the slope of the straight linethe following equation is obtained by performing the de-rivative of equation (29) at point x x0
R
H
h 0
x
z
r0 l
Ground
Impermeable stratum
Well point
Groundwater level
Figure 9 Dewatering funnel curve for the phreatic incomplete well point
-e equation of the line that passes through the point(x0 z0) and is tangential to the curve after the dewateringbecomes stable is assumed to be
y x tan α + b (35)
-e slope of the equation is
tan α zprime x0( 1113857 (36)
According to the trigonometric function conversion
sin α tan α
1 + tan2 α
radic (37)
-en the following equation is obtained
sin α zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (38)
-erefore the vertical component of additional pressureof the surrounding stratum caused by dewatering in thefoundation pit is expressed as
ΔPy ΔP sin α ΔPzprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (39)
where ΔPx denotes the horizontal component of the seepageforce ΔPy denotes the vertical component of the seepageforce and ΔP is the seepage force
-e angle α in equation (39) denotes the one existsbetween the horizontal component of the additional pres-sure and the additional pressure caused by dewatering in thefoundation pit
Based on equations (30) to (33) the additional stresses inthe dewatering zone and the saturation zone are calculatedseparately -e rewriting is carried out on the basis ofequation (34) which derives the ground subsidence causedby dewatering in the foundation pit considering the action ofseepage force -e formula after rewriting is as follows
s ψw 1113944Δσziprime Δhi
Esi
zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (40)
Substituting equation (34) into equation (40) yields thefollowing equation
Figure 10 Simplified calculation model considering seepage force
10 Advances in Civil Engineering
If the ground subsidence S1 and S2 corresponding to thedewatered and saturated zones are separately figured outthen the total ground subsidence is obtained and illustratedin the following equation
S S1 + S2 (42)
where the symbols are illustrated in Section 41
5 Numerical Simulation
With fast development of urban construction various in-frastructures such as subway station high-speed railwaytunnel underground parking lot and basement are beingbuilt especially the construction of deep foundation pits-e geological condition varies in the foundation pit and thesurrounding environment is becoming more and morecomplex after construction -eoretical analysis and em-pirical calculation methods are no longer suitable forpractical projects Nowadays actual projects are generallybuilt ahead of theoretical research and calculation methodsof numerical simulation provide an effective approach forthe design and construction of practical foundation pitAlthough numerical simulation method has not been pro-posed for very long time it has become the most commonmethod used in structural analysis and calculation so farMany scholars [24ndash31] have also used FLAC3D to conductfluid-solid coupling analysis of dewatering in the foundationpit Finite difference software FLAC3D is also employed inthis paper to carry out three-dimensional numerical sim-ulation of the whole process of the environment changecaused by dewatering in the open cut foundation pit of ametro station on Chengdu Metro Line 6 -e calculationresults are compared with the one obtained from theoreticalcalculation and on-site monitoring which are used to makefurther study on the surrounding ground subsidence due todewatering in the open cut foundation pit
51 Physical and Mechanical Parameters According to thegeological properties of the actual project the calculationparameters are obtained and shown in Table 2
52 Establishment of 3D Model and Initial Stress BalanceBased on the engineering geological and hydrologicalconditions a three-dimensional stratigraphic model isestablished From the geological section of the site (Fig-ures 11 and 12) it can be seen that each of soil layers is nearlyhorizontal and almost parallel to each other So in order toestablish the model and conduct the calculation conve-niently [26] the geological layers in the model are simplified-e foundation pit and wells are set up in the stratigraphicmodel according to dewatering scheme According to theprinciple of Saint Venant in elastic mechanics in order toeliminate the boundary influence of the foundation pit oncalculation results the calculation model is extended to 3sim5times of the depth of the pit from the periphery of thefoundation pit on the plane -e depth is set to the im-pervious interface and the entire calculation domain is inthe volume of 420mtimes 198mtimes 72m Referring to
geotechnical mapping and geological properties the modelis divided into 9 strata and the generalized permeabilitycoefficient and stratum thickness are shown in Table 1
According to stratum distribution and initial conditionsthe calculation domain is divided into grids by consideringthe monitoring points the dewatering wells and the dia-phragm wall considering the geological survey In themeshing the grid of the calculation domain is locally refinedin addition the grids around the foundation pit are alsorefined but grids far away from the pit are sparsely meshedconsidering Saint Venantrsquos principle [26] So the entiremodel is then divided into 42 layers 78 rows 300 columnsand a total of 982800 elements and the established three-dimensional model is shown in Figure 13
After the 3D model is set up the initial stress balance isneeded to be applied firstly and the corresponding seepageand displacement boundary conditions are also applied -enormal displacement and the bottom displacement of thefour sidewalls and bottom of the model are restricted andthe horizontal displacement of the wall of the dewateringwell is also constrained -e bottom filter pipe of thedewatering well belongs to the seepage boundary which isachieved by applying the pore water pressure After nu-merical simulation the pore water pressure of initialequilibrium is shown in Figure 14 -e pore water pressurein the initial state is evenly applied on the strata
53 Numerical Simulation of Fluid-Solid Coupling of GroundSubsidence Caused by Dewatering in Foundation PitAfter the initial equilibrium of the numerical model theeffect of single well and group wells were firstly carried outseparately -e stepwise dewatering was conducted underthe conditions of single well and group wells separatelywithout considering excavation -e calculated isograms ofthe dewatering at all levels and the subsidence of themonitoring points are analyzed
531 Numerical Simulation Analysis of Single WellDewatering Considering the symmetry of the well pointthe well point at the center of the calculation domain isselected when carrying out single well simulation -e po-sition of the single well on the model is shown in Figure 15-e isograms showing subsidence and pore water pressure atvarious drawdowns are also shown in Figures 16ndash23
It is seen from Figures 16ndash23 that the influence range ofdewatering well increases with the increase of drawdownAfter the well is dewatered the pore water pressure formsthe dewatering funnel surface -e bigger the drawdown isthe deeper the surface becomes Moreover due to the wa-terproofing effect of the diaphragm wall the stratum insidethe pit bulges but the stratum outside the pit subsides -isis because the groundwater flows through the bottom of thediaphragm wall into the pit -e increase of the pore waterpressure in the pit causes the additional stress to increasewhich leads to the upheaval of the soil layer in the pit -edecrease of the pore water pressure outside the pit causes theeffective stress of the outer stratum to decrease resulting inthe formation of downward additional stress in the outer
Advances in Civil Engineering 11
stratum of the pit which eventually leads to the consoli-dation and settlement of the outer stratum
532 Numerical Simulation Analysis of Group WellsDewatering When conducting the effect of group wells onground subsidence the calculation is based on the well pointlayout diagram designed in Section 3 -e layout of thegroup wells in the model is shown in Figure 24-e isogramsshowing ground subsidence and pore water pressure cal-culated using FLAC3D are shown in Figures 25ndash32
It is seen from the isograms shown in Figures 25ndash32 thatthe deeper the groundwater drawdown the greater theimpact on the surrounding ground is and the lower thefunnel surface formed by the pore water pressure Comparedwith the isogram under the effect of single well in Section 52the influence of dewatering by group wells is much largerGroup wells dewatering has a great impact on the pore waterpressure and strata deformation inside the foundation pit
that is the influence of group wells on ground subsidenceinside the pit is obvious and should not be ignored -esubsidence and pore water pressure isogram under differentdrawdowns are symmetrically distributed As shown in thesingle well effect the strata inside the foundation pit bulgeduring the dewatering of group wells but the strata outsidethe foundation pit subside-is is mainly due to the result ofthe movement of groundwater outside the pit to the dew-atering well inside the pit
533 Effect of Group Wells According to the results ob-tained from 3D simulation the subsidence of the sixmonitoring points on the sides of the foundation pit due todifferent drawdowns caused by the single well and the groupwells are shown in Figure 33
Grade I drawdown including Grade II Grade III andGrade IV drawdown means that the dewatering depth ofgroundwater level is in 6m 11m 16m and 22m respectively
Table 2 Physical and mechanical parameters of strata
Name of stratum Modulus of deformation E (MPa) Poissonrsquos ratio Density (gcm3) Permeability coefficient (md)Miscellaneous fill 2 035 18 125Silty clay 4 029 196 005Clayey silt 4 030 194 015Fine sand 4 028 185 3Medium sand 40 026 19 10Loose pebble layer 18 025 2 22Slightly condensed pebble layer 20 023 21 22Medium condensed pebble layer 28 020 22 22Compacted pebble layer 38 017 23 22
Miscellaneous fill Silty clayClayey siltFine sandMedium sand
12345
Figure 11 Geological section of the territory
12 Advances in Civil Engineering
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
(3) When the soil layer numbered as i is below thegroundwater level after dewatering then the effectivestress is
Δσziprime λicwsi (32)
-e soil compression caused by dewatering is as follows
s ψw 1113944Δσziprime Δhi
Esi
(33)
where cw means the bulk density of water kNm3 z denotesthe vertical distance from the midpoint of the soil layer i tothe initial groundwater level m and λi represents the cal-culation coefficient it should be based on the analysis ofgroundwater seepage If the analysis data are not availablethen its value should be based on local engineering expe-rience si refers to the depth of lowering groundwater levelcorresponding to the calculation profile m s denotes theground compression in the calculation profile m and ψw
means the empirical coefficient of subsidence calculationbased on local engineering experience If no experience isavailable then the value is set to be 1 Δσzi
prime denotes the meanadditional stress of the soil layer i under the ground surfacecaused by dewatering kPa Δhi means the thickness of thesoil layer i m and Esi
denotes the compressive modulus ofthe soil layer i kPa
42 Calculation of Ground Subsidence Caused by Dewateringin Foundation Pit under Seepage Force -e pumping anddrainage of the dewatering well will cause the change ofthe underground seepage field which will generate a newseepage field and lead to the variation of the stress fieldaround the well -erefore the seepage force is the maincause of soil consolidation and settlement -e seepage ofgroundwater causes the dissipation of pore water pres-sure resulting in an increase in effective stress -at isadditional stress is generated in the soil the direction ofwhich is vertically downward in addition it produces a
horizontal component Deformation can be caused by theimpact of seepage force -e additional stress namely thevertical component of the seepage force will cause theground subsidence [21] -e horizontal component of theseepage force will cause the lateral deformation of the soilAccording to reference [22] the seepage direction of anywater head at any point is tangent to the phreatic surfaceat that point pointing to the well axis as shown inFigure 10
Wu and Zhu [22] performed related research andproposed a new algorithm of ground subsidence caused byseepage force Yang and Zhao [23] also used this method tocalculate ground subsidence -is section draws lessonsfrom their research to discuss the dewatering in open cutfoundation pit of a subway station on Chengdu Metro Line6
-e stratum is divided into three parts dry soil zonedewatered zone and saturated zone -e dry soil zone isalways above the groundwater level before and afterdewatering which does not contain groundwater andseepage force during dewatering -us additional stressdoes not appear in this zone and the subsidence valuecaused by dewatering is 0 -e other two layers are S1 andS2 respectively as shown in Figure 10 -e soil layer S1 inthe dewatering process is drained -e zone S2 is alwaysbelow the groundwater level and is saturated In thissection the soil subsidence in the dewatered and saturatedzones is calculated separately -e seepage forces in thesetwo areas possess horizontal components as shown inFigure 10
Assuming that the water head that keeps a distance of x0from itself to the well axis is z0 as shown in Figure 10according to the dewatering funnel curve equation (29) inSection 41 the height of the falling funnel curve is obtained-e direction of the seepage force is actually in the straightline vector on the curve which goes through the point(x0 z0) and is tangent to the falling funnel curve and pointsto the well In order to find out the slope of the straight linethe following equation is obtained by performing the de-rivative of equation (29) at point x x0
R
H
h 0
x
z
r0 l
Ground
Impermeable stratum
Well point
Groundwater level
Figure 9 Dewatering funnel curve for the phreatic incomplete well point
-e equation of the line that passes through the point(x0 z0) and is tangential to the curve after the dewateringbecomes stable is assumed to be
y x tan α + b (35)
-e slope of the equation is
tan α zprime x0( 1113857 (36)
According to the trigonometric function conversion
sin α tan α
1 + tan2 α
radic (37)
-en the following equation is obtained
sin α zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (38)
-erefore the vertical component of additional pressureof the surrounding stratum caused by dewatering in thefoundation pit is expressed as
ΔPy ΔP sin α ΔPzprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (39)
where ΔPx denotes the horizontal component of the seepageforce ΔPy denotes the vertical component of the seepageforce and ΔP is the seepage force
-e angle α in equation (39) denotes the one existsbetween the horizontal component of the additional pres-sure and the additional pressure caused by dewatering in thefoundation pit
Based on equations (30) to (33) the additional stresses inthe dewatering zone and the saturation zone are calculatedseparately -e rewriting is carried out on the basis ofequation (34) which derives the ground subsidence causedby dewatering in the foundation pit considering the action ofseepage force -e formula after rewriting is as follows
s ψw 1113944Δσziprime Δhi
Esi
zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (40)
Substituting equation (34) into equation (40) yields thefollowing equation
Figure 10 Simplified calculation model considering seepage force
10 Advances in Civil Engineering
If the ground subsidence S1 and S2 corresponding to thedewatered and saturated zones are separately figured outthen the total ground subsidence is obtained and illustratedin the following equation
S S1 + S2 (42)
where the symbols are illustrated in Section 41
5 Numerical Simulation
With fast development of urban construction various in-frastructures such as subway station high-speed railwaytunnel underground parking lot and basement are beingbuilt especially the construction of deep foundation pits-e geological condition varies in the foundation pit and thesurrounding environment is becoming more and morecomplex after construction -eoretical analysis and em-pirical calculation methods are no longer suitable forpractical projects Nowadays actual projects are generallybuilt ahead of theoretical research and calculation methodsof numerical simulation provide an effective approach forthe design and construction of practical foundation pitAlthough numerical simulation method has not been pro-posed for very long time it has become the most commonmethod used in structural analysis and calculation so farMany scholars [24ndash31] have also used FLAC3D to conductfluid-solid coupling analysis of dewatering in the foundationpit Finite difference software FLAC3D is also employed inthis paper to carry out three-dimensional numerical sim-ulation of the whole process of the environment changecaused by dewatering in the open cut foundation pit of ametro station on Chengdu Metro Line 6 -e calculationresults are compared with the one obtained from theoreticalcalculation and on-site monitoring which are used to makefurther study on the surrounding ground subsidence due todewatering in the open cut foundation pit
51 Physical and Mechanical Parameters According to thegeological properties of the actual project the calculationparameters are obtained and shown in Table 2
52 Establishment of 3D Model and Initial Stress BalanceBased on the engineering geological and hydrologicalconditions a three-dimensional stratigraphic model isestablished From the geological section of the site (Fig-ures 11 and 12) it can be seen that each of soil layers is nearlyhorizontal and almost parallel to each other So in order toestablish the model and conduct the calculation conve-niently [26] the geological layers in the model are simplified-e foundation pit and wells are set up in the stratigraphicmodel according to dewatering scheme According to theprinciple of Saint Venant in elastic mechanics in order toeliminate the boundary influence of the foundation pit oncalculation results the calculation model is extended to 3sim5times of the depth of the pit from the periphery of thefoundation pit on the plane -e depth is set to the im-pervious interface and the entire calculation domain is inthe volume of 420mtimes 198mtimes 72m Referring to
geotechnical mapping and geological properties the modelis divided into 9 strata and the generalized permeabilitycoefficient and stratum thickness are shown in Table 1
According to stratum distribution and initial conditionsthe calculation domain is divided into grids by consideringthe monitoring points the dewatering wells and the dia-phragm wall considering the geological survey In themeshing the grid of the calculation domain is locally refinedin addition the grids around the foundation pit are alsorefined but grids far away from the pit are sparsely meshedconsidering Saint Venantrsquos principle [26] So the entiremodel is then divided into 42 layers 78 rows 300 columnsand a total of 982800 elements and the established three-dimensional model is shown in Figure 13
After the 3D model is set up the initial stress balance isneeded to be applied firstly and the corresponding seepageand displacement boundary conditions are also applied -enormal displacement and the bottom displacement of thefour sidewalls and bottom of the model are restricted andthe horizontal displacement of the wall of the dewateringwell is also constrained -e bottom filter pipe of thedewatering well belongs to the seepage boundary which isachieved by applying the pore water pressure After nu-merical simulation the pore water pressure of initialequilibrium is shown in Figure 14 -e pore water pressurein the initial state is evenly applied on the strata
53 Numerical Simulation of Fluid-Solid Coupling of GroundSubsidence Caused by Dewatering in Foundation PitAfter the initial equilibrium of the numerical model theeffect of single well and group wells were firstly carried outseparately -e stepwise dewatering was conducted underthe conditions of single well and group wells separatelywithout considering excavation -e calculated isograms ofthe dewatering at all levels and the subsidence of themonitoring points are analyzed
531 Numerical Simulation Analysis of Single WellDewatering Considering the symmetry of the well pointthe well point at the center of the calculation domain isselected when carrying out single well simulation -e po-sition of the single well on the model is shown in Figure 15-e isograms showing subsidence and pore water pressure atvarious drawdowns are also shown in Figures 16ndash23
It is seen from Figures 16ndash23 that the influence range ofdewatering well increases with the increase of drawdownAfter the well is dewatered the pore water pressure formsthe dewatering funnel surface -e bigger the drawdown isthe deeper the surface becomes Moreover due to the wa-terproofing effect of the diaphragm wall the stratum insidethe pit bulges but the stratum outside the pit subsides -isis because the groundwater flows through the bottom of thediaphragm wall into the pit -e increase of the pore waterpressure in the pit causes the additional stress to increasewhich leads to the upheaval of the soil layer in the pit -edecrease of the pore water pressure outside the pit causes theeffective stress of the outer stratum to decrease resulting inthe formation of downward additional stress in the outer
Advances in Civil Engineering 11
stratum of the pit which eventually leads to the consoli-dation and settlement of the outer stratum
532 Numerical Simulation Analysis of Group WellsDewatering When conducting the effect of group wells onground subsidence the calculation is based on the well pointlayout diagram designed in Section 3 -e layout of thegroup wells in the model is shown in Figure 24-e isogramsshowing ground subsidence and pore water pressure cal-culated using FLAC3D are shown in Figures 25ndash32
It is seen from the isograms shown in Figures 25ndash32 thatthe deeper the groundwater drawdown the greater theimpact on the surrounding ground is and the lower thefunnel surface formed by the pore water pressure Comparedwith the isogram under the effect of single well in Section 52the influence of dewatering by group wells is much largerGroup wells dewatering has a great impact on the pore waterpressure and strata deformation inside the foundation pit
that is the influence of group wells on ground subsidenceinside the pit is obvious and should not be ignored -esubsidence and pore water pressure isogram under differentdrawdowns are symmetrically distributed As shown in thesingle well effect the strata inside the foundation pit bulgeduring the dewatering of group wells but the strata outsidethe foundation pit subside-is is mainly due to the result ofthe movement of groundwater outside the pit to the dew-atering well inside the pit
533 Effect of Group Wells According to the results ob-tained from 3D simulation the subsidence of the sixmonitoring points on the sides of the foundation pit due todifferent drawdowns caused by the single well and the groupwells are shown in Figure 33
Grade I drawdown including Grade II Grade III andGrade IV drawdown means that the dewatering depth ofgroundwater level is in 6m 11m 16m and 22m respectively
Table 2 Physical and mechanical parameters of strata
Name of stratum Modulus of deformation E (MPa) Poissonrsquos ratio Density (gcm3) Permeability coefficient (md)Miscellaneous fill 2 035 18 125Silty clay 4 029 196 005Clayey silt 4 030 194 015Fine sand 4 028 185 3Medium sand 40 026 19 10Loose pebble layer 18 025 2 22Slightly condensed pebble layer 20 023 21 22Medium condensed pebble layer 28 020 22 22Compacted pebble layer 38 017 23 22
Miscellaneous fill Silty clayClayey siltFine sandMedium sand
12345
Figure 11 Geological section of the territory
12 Advances in Civil Engineering
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
-e equation of the line that passes through the point(x0 z0) and is tangential to the curve after the dewateringbecomes stable is assumed to be
y x tan α + b (35)
-e slope of the equation is
tan α zprime x0( 1113857 (36)
According to the trigonometric function conversion
sin α tan α
1 + tan2 α
radic (37)
-en the following equation is obtained
sin α zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (38)
-erefore the vertical component of additional pressureof the surrounding stratum caused by dewatering in thefoundation pit is expressed as
ΔPy ΔP sin α ΔPzprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (39)
where ΔPx denotes the horizontal component of the seepageforce ΔPy denotes the vertical component of the seepageforce and ΔP is the seepage force
-e angle α in equation (39) denotes the one existsbetween the horizontal component of the additional pres-sure and the additional pressure caused by dewatering in thefoundation pit
Based on equations (30) to (33) the additional stresses inthe dewatering zone and the saturation zone are calculatedseparately -e rewriting is carried out on the basis ofequation (34) which derives the ground subsidence causedby dewatering in the foundation pit considering the action ofseepage force -e formula after rewriting is as follows
s ψw 1113944Δσziprime Δhi
Esi
zprime x0( 1113857
1 + zprime x0( 11138571113858 11138592
1113969 (40)
Substituting equation (34) into equation (40) yields thefollowing equation
Figure 10 Simplified calculation model considering seepage force
10 Advances in Civil Engineering
If the ground subsidence S1 and S2 corresponding to thedewatered and saturated zones are separately figured outthen the total ground subsidence is obtained and illustratedin the following equation
S S1 + S2 (42)
where the symbols are illustrated in Section 41
5 Numerical Simulation
With fast development of urban construction various in-frastructures such as subway station high-speed railwaytunnel underground parking lot and basement are beingbuilt especially the construction of deep foundation pits-e geological condition varies in the foundation pit and thesurrounding environment is becoming more and morecomplex after construction -eoretical analysis and em-pirical calculation methods are no longer suitable forpractical projects Nowadays actual projects are generallybuilt ahead of theoretical research and calculation methodsof numerical simulation provide an effective approach forthe design and construction of practical foundation pitAlthough numerical simulation method has not been pro-posed for very long time it has become the most commonmethod used in structural analysis and calculation so farMany scholars [24ndash31] have also used FLAC3D to conductfluid-solid coupling analysis of dewatering in the foundationpit Finite difference software FLAC3D is also employed inthis paper to carry out three-dimensional numerical sim-ulation of the whole process of the environment changecaused by dewatering in the open cut foundation pit of ametro station on Chengdu Metro Line 6 -e calculationresults are compared with the one obtained from theoreticalcalculation and on-site monitoring which are used to makefurther study on the surrounding ground subsidence due todewatering in the open cut foundation pit
51 Physical and Mechanical Parameters According to thegeological properties of the actual project the calculationparameters are obtained and shown in Table 2
52 Establishment of 3D Model and Initial Stress BalanceBased on the engineering geological and hydrologicalconditions a three-dimensional stratigraphic model isestablished From the geological section of the site (Fig-ures 11 and 12) it can be seen that each of soil layers is nearlyhorizontal and almost parallel to each other So in order toestablish the model and conduct the calculation conve-niently [26] the geological layers in the model are simplified-e foundation pit and wells are set up in the stratigraphicmodel according to dewatering scheme According to theprinciple of Saint Venant in elastic mechanics in order toeliminate the boundary influence of the foundation pit oncalculation results the calculation model is extended to 3sim5times of the depth of the pit from the periphery of thefoundation pit on the plane -e depth is set to the im-pervious interface and the entire calculation domain is inthe volume of 420mtimes 198mtimes 72m Referring to
geotechnical mapping and geological properties the modelis divided into 9 strata and the generalized permeabilitycoefficient and stratum thickness are shown in Table 1
According to stratum distribution and initial conditionsthe calculation domain is divided into grids by consideringthe monitoring points the dewatering wells and the dia-phragm wall considering the geological survey In themeshing the grid of the calculation domain is locally refinedin addition the grids around the foundation pit are alsorefined but grids far away from the pit are sparsely meshedconsidering Saint Venantrsquos principle [26] So the entiremodel is then divided into 42 layers 78 rows 300 columnsand a total of 982800 elements and the established three-dimensional model is shown in Figure 13
After the 3D model is set up the initial stress balance isneeded to be applied firstly and the corresponding seepageand displacement boundary conditions are also applied -enormal displacement and the bottom displacement of thefour sidewalls and bottom of the model are restricted andthe horizontal displacement of the wall of the dewateringwell is also constrained -e bottom filter pipe of thedewatering well belongs to the seepage boundary which isachieved by applying the pore water pressure After nu-merical simulation the pore water pressure of initialequilibrium is shown in Figure 14 -e pore water pressurein the initial state is evenly applied on the strata
53 Numerical Simulation of Fluid-Solid Coupling of GroundSubsidence Caused by Dewatering in Foundation PitAfter the initial equilibrium of the numerical model theeffect of single well and group wells were firstly carried outseparately -e stepwise dewatering was conducted underthe conditions of single well and group wells separatelywithout considering excavation -e calculated isograms ofthe dewatering at all levels and the subsidence of themonitoring points are analyzed
531 Numerical Simulation Analysis of Single WellDewatering Considering the symmetry of the well pointthe well point at the center of the calculation domain isselected when carrying out single well simulation -e po-sition of the single well on the model is shown in Figure 15-e isograms showing subsidence and pore water pressure atvarious drawdowns are also shown in Figures 16ndash23
It is seen from Figures 16ndash23 that the influence range ofdewatering well increases with the increase of drawdownAfter the well is dewatered the pore water pressure formsthe dewatering funnel surface -e bigger the drawdown isthe deeper the surface becomes Moreover due to the wa-terproofing effect of the diaphragm wall the stratum insidethe pit bulges but the stratum outside the pit subsides -isis because the groundwater flows through the bottom of thediaphragm wall into the pit -e increase of the pore waterpressure in the pit causes the additional stress to increasewhich leads to the upheaval of the soil layer in the pit -edecrease of the pore water pressure outside the pit causes theeffective stress of the outer stratum to decrease resulting inthe formation of downward additional stress in the outer
Advances in Civil Engineering 11
stratum of the pit which eventually leads to the consoli-dation and settlement of the outer stratum
532 Numerical Simulation Analysis of Group WellsDewatering When conducting the effect of group wells onground subsidence the calculation is based on the well pointlayout diagram designed in Section 3 -e layout of thegroup wells in the model is shown in Figure 24-e isogramsshowing ground subsidence and pore water pressure cal-culated using FLAC3D are shown in Figures 25ndash32
It is seen from the isograms shown in Figures 25ndash32 thatthe deeper the groundwater drawdown the greater theimpact on the surrounding ground is and the lower thefunnel surface formed by the pore water pressure Comparedwith the isogram under the effect of single well in Section 52the influence of dewatering by group wells is much largerGroup wells dewatering has a great impact on the pore waterpressure and strata deformation inside the foundation pit
that is the influence of group wells on ground subsidenceinside the pit is obvious and should not be ignored -esubsidence and pore water pressure isogram under differentdrawdowns are symmetrically distributed As shown in thesingle well effect the strata inside the foundation pit bulgeduring the dewatering of group wells but the strata outsidethe foundation pit subside-is is mainly due to the result ofthe movement of groundwater outside the pit to the dew-atering well inside the pit
533 Effect of Group Wells According to the results ob-tained from 3D simulation the subsidence of the sixmonitoring points on the sides of the foundation pit due todifferent drawdowns caused by the single well and the groupwells are shown in Figure 33
Grade I drawdown including Grade II Grade III andGrade IV drawdown means that the dewatering depth ofgroundwater level is in 6m 11m 16m and 22m respectively
Table 2 Physical and mechanical parameters of strata
Name of stratum Modulus of deformation E (MPa) Poissonrsquos ratio Density (gcm3) Permeability coefficient (md)Miscellaneous fill 2 035 18 125Silty clay 4 029 196 005Clayey silt 4 030 194 015Fine sand 4 028 185 3Medium sand 40 026 19 10Loose pebble layer 18 025 2 22Slightly condensed pebble layer 20 023 21 22Medium condensed pebble layer 28 020 22 22Compacted pebble layer 38 017 23 22
Miscellaneous fill Silty clayClayey siltFine sandMedium sand
12345
Figure 11 Geological section of the territory
12 Advances in Civil Engineering
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
If the ground subsidence S1 and S2 corresponding to thedewatered and saturated zones are separately figured outthen the total ground subsidence is obtained and illustratedin the following equation
S S1 + S2 (42)
where the symbols are illustrated in Section 41
5 Numerical Simulation
With fast development of urban construction various in-frastructures such as subway station high-speed railwaytunnel underground parking lot and basement are beingbuilt especially the construction of deep foundation pits-e geological condition varies in the foundation pit and thesurrounding environment is becoming more and morecomplex after construction -eoretical analysis and em-pirical calculation methods are no longer suitable forpractical projects Nowadays actual projects are generallybuilt ahead of theoretical research and calculation methodsof numerical simulation provide an effective approach forthe design and construction of practical foundation pitAlthough numerical simulation method has not been pro-posed for very long time it has become the most commonmethod used in structural analysis and calculation so farMany scholars [24ndash31] have also used FLAC3D to conductfluid-solid coupling analysis of dewatering in the foundationpit Finite difference software FLAC3D is also employed inthis paper to carry out three-dimensional numerical sim-ulation of the whole process of the environment changecaused by dewatering in the open cut foundation pit of ametro station on Chengdu Metro Line 6 -e calculationresults are compared with the one obtained from theoreticalcalculation and on-site monitoring which are used to makefurther study on the surrounding ground subsidence due todewatering in the open cut foundation pit
51 Physical and Mechanical Parameters According to thegeological properties of the actual project the calculationparameters are obtained and shown in Table 2
52 Establishment of 3D Model and Initial Stress BalanceBased on the engineering geological and hydrologicalconditions a three-dimensional stratigraphic model isestablished From the geological section of the site (Fig-ures 11 and 12) it can be seen that each of soil layers is nearlyhorizontal and almost parallel to each other So in order toestablish the model and conduct the calculation conve-niently [26] the geological layers in the model are simplified-e foundation pit and wells are set up in the stratigraphicmodel according to dewatering scheme According to theprinciple of Saint Venant in elastic mechanics in order toeliminate the boundary influence of the foundation pit oncalculation results the calculation model is extended to 3sim5times of the depth of the pit from the periphery of thefoundation pit on the plane -e depth is set to the im-pervious interface and the entire calculation domain is inthe volume of 420mtimes 198mtimes 72m Referring to
geotechnical mapping and geological properties the modelis divided into 9 strata and the generalized permeabilitycoefficient and stratum thickness are shown in Table 1
According to stratum distribution and initial conditionsthe calculation domain is divided into grids by consideringthe monitoring points the dewatering wells and the dia-phragm wall considering the geological survey In themeshing the grid of the calculation domain is locally refinedin addition the grids around the foundation pit are alsorefined but grids far away from the pit are sparsely meshedconsidering Saint Venantrsquos principle [26] So the entiremodel is then divided into 42 layers 78 rows 300 columnsand a total of 982800 elements and the established three-dimensional model is shown in Figure 13
After the 3D model is set up the initial stress balance isneeded to be applied firstly and the corresponding seepageand displacement boundary conditions are also applied -enormal displacement and the bottom displacement of thefour sidewalls and bottom of the model are restricted andthe horizontal displacement of the wall of the dewateringwell is also constrained -e bottom filter pipe of thedewatering well belongs to the seepage boundary which isachieved by applying the pore water pressure After nu-merical simulation the pore water pressure of initialequilibrium is shown in Figure 14 -e pore water pressurein the initial state is evenly applied on the strata
53 Numerical Simulation of Fluid-Solid Coupling of GroundSubsidence Caused by Dewatering in Foundation PitAfter the initial equilibrium of the numerical model theeffect of single well and group wells were firstly carried outseparately -e stepwise dewatering was conducted underthe conditions of single well and group wells separatelywithout considering excavation -e calculated isograms ofthe dewatering at all levels and the subsidence of themonitoring points are analyzed
531 Numerical Simulation Analysis of Single WellDewatering Considering the symmetry of the well pointthe well point at the center of the calculation domain isselected when carrying out single well simulation -e po-sition of the single well on the model is shown in Figure 15-e isograms showing subsidence and pore water pressure atvarious drawdowns are also shown in Figures 16ndash23
It is seen from Figures 16ndash23 that the influence range ofdewatering well increases with the increase of drawdownAfter the well is dewatered the pore water pressure formsthe dewatering funnel surface -e bigger the drawdown isthe deeper the surface becomes Moreover due to the wa-terproofing effect of the diaphragm wall the stratum insidethe pit bulges but the stratum outside the pit subsides -isis because the groundwater flows through the bottom of thediaphragm wall into the pit -e increase of the pore waterpressure in the pit causes the additional stress to increasewhich leads to the upheaval of the soil layer in the pit -edecrease of the pore water pressure outside the pit causes theeffective stress of the outer stratum to decrease resulting inthe formation of downward additional stress in the outer
Advances in Civil Engineering 11
stratum of the pit which eventually leads to the consoli-dation and settlement of the outer stratum
532 Numerical Simulation Analysis of Group WellsDewatering When conducting the effect of group wells onground subsidence the calculation is based on the well pointlayout diagram designed in Section 3 -e layout of thegroup wells in the model is shown in Figure 24-e isogramsshowing ground subsidence and pore water pressure cal-culated using FLAC3D are shown in Figures 25ndash32
It is seen from the isograms shown in Figures 25ndash32 thatthe deeper the groundwater drawdown the greater theimpact on the surrounding ground is and the lower thefunnel surface formed by the pore water pressure Comparedwith the isogram under the effect of single well in Section 52the influence of dewatering by group wells is much largerGroup wells dewatering has a great impact on the pore waterpressure and strata deformation inside the foundation pit
that is the influence of group wells on ground subsidenceinside the pit is obvious and should not be ignored -esubsidence and pore water pressure isogram under differentdrawdowns are symmetrically distributed As shown in thesingle well effect the strata inside the foundation pit bulgeduring the dewatering of group wells but the strata outsidethe foundation pit subside-is is mainly due to the result ofthe movement of groundwater outside the pit to the dew-atering well inside the pit
533 Effect of Group Wells According to the results ob-tained from 3D simulation the subsidence of the sixmonitoring points on the sides of the foundation pit due todifferent drawdowns caused by the single well and the groupwells are shown in Figure 33
Grade I drawdown including Grade II Grade III andGrade IV drawdown means that the dewatering depth ofgroundwater level is in 6m 11m 16m and 22m respectively
Table 2 Physical and mechanical parameters of strata
Name of stratum Modulus of deformation E (MPa) Poissonrsquos ratio Density (gcm3) Permeability coefficient (md)Miscellaneous fill 2 035 18 125Silty clay 4 029 196 005Clayey silt 4 030 194 015Fine sand 4 028 185 3Medium sand 40 026 19 10Loose pebble layer 18 025 2 22Slightly condensed pebble layer 20 023 21 22Medium condensed pebble layer 28 020 22 22Compacted pebble layer 38 017 23 22
Miscellaneous fill Silty clayClayey siltFine sandMedium sand
12345
Figure 11 Geological section of the territory
12 Advances in Civil Engineering
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
stratum of the pit which eventually leads to the consoli-dation and settlement of the outer stratum
532 Numerical Simulation Analysis of Group WellsDewatering When conducting the effect of group wells onground subsidence the calculation is based on the well pointlayout diagram designed in Section 3 -e layout of thegroup wells in the model is shown in Figure 24-e isogramsshowing ground subsidence and pore water pressure cal-culated using FLAC3D are shown in Figures 25ndash32
It is seen from the isograms shown in Figures 25ndash32 thatthe deeper the groundwater drawdown the greater theimpact on the surrounding ground is and the lower thefunnel surface formed by the pore water pressure Comparedwith the isogram under the effect of single well in Section 52the influence of dewatering by group wells is much largerGroup wells dewatering has a great impact on the pore waterpressure and strata deformation inside the foundation pit
that is the influence of group wells on ground subsidenceinside the pit is obvious and should not be ignored -esubsidence and pore water pressure isogram under differentdrawdowns are symmetrically distributed As shown in thesingle well effect the strata inside the foundation pit bulgeduring the dewatering of group wells but the strata outsidethe foundation pit subside-is is mainly due to the result ofthe movement of groundwater outside the pit to the dew-atering well inside the pit
533 Effect of Group Wells According to the results ob-tained from 3D simulation the subsidence of the sixmonitoring points on the sides of the foundation pit due todifferent drawdowns caused by the single well and the groupwells are shown in Figure 33
Grade I drawdown including Grade II Grade III andGrade IV drawdown means that the dewatering depth ofgroundwater level is in 6m 11m 16m and 22m respectively
Table 2 Physical and mechanical parameters of strata
Name of stratum Modulus of deformation E (MPa) Poissonrsquos ratio Density (gcm3) Permeability coefficient (md)Miscellaneous fill 2 035 18 125Silty clay 4 029 196 005Clayey silt 4 030 194 015Fine sand 4 028 185 3Medium sand 40 026 19 10Loose pebble layer 18 025 2 22Slightly condensed pebble layer 20 023 21 22Medium condensed pebble layer 28 020 22 22Compacted pebble layer 38 017 23 22
Miscellaneous fill Silty clayClayey siltFine sandMedium sand
12345
Figure 11 Geological section of the territory
12 Advances in Civil Engineering
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
-e curves of both single well and group wells under differentdrawdowns are similar and the farther the monitoring point isaway from the well axis in foundation pit the smaller the
ground subsidence becomes -e greater the depth ofgroundwater is lowered at the same monitoring point thegreater the ground subsides -e maximum settlement after
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Figure 16 Single well subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Figure 18 Single well subsidence nephogram after the second drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 17 Single well pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 19 Single well pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 15
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
single well dewatering is about 446mm and the maximumsettlement after group wells dewatering reaches 48mm -esubsidence of the monitoring points that are far away from thewell axis are basically the same this occurs under single welland group wells Generally ground subsidence around the wellafter group wells dewatering is slightly larger than that aftersingle well dewatering but the increment is very smallMoreover as the distance from the well axis is farther the
increase effect is less obvious It is also seen that the groundsubsidence caused by group wells dewatering inside thefoundation pit is not obvious especially when the distancefrom the foundation pit is faraway or the drawdown is notlarge -erefore in order to facilitate research the group wellseffect can sometimes be simplified into single well effect ingeotechnical engineering -erefore through FLAC3D nu-merical simulation it is found that the group well effect on
Figure 20 Single well subsidence nephogram after the third drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 21 Single well pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
16 Advances in Civil Engineering
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
ground subsidence is not obvious so for the convenience ofcalculation the group well effect can be ignored while de-signing dewatering wells for open cut foundation pit
54 Comparative Analysis of Subsidence due to DewateringBased on the improved settlement calculations under the actionof seepage force three calculation methods including theconventional algorithm proposed in the Chinese Specification
the on-site monitoring and the numerical simulation are usedin the paper and the ground subsidence around the foundationpit under different drawdowns in the context of the project isobtained -e comparison of ground subsidence due to dif-ferent drawdowns is illustrated in Figure 34
It is seen from Figure 34 that the overall trend underdifferent conditions fits the on-site monitoring results -eresults obtained from theoretical analysis considering theeffect of seepage force and numerical simulation are much
Figure 22 Single well subsidence nephogram when the drawdown is stable (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 23 Single well pore water pressure nephogram when the drawdown is stable (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Figure 25 Group wells subsidence nephogram after the first drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 26 Group wells pore water pressure nephogram after the first drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
18 Advances in Civil Engineering
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
closer to the ones of the field monitoring Moreover thetheoretical calculation and numerical simulation values aremuch closer to the on-site monitoring values but there is alarge gap between the normative calculation and the on-sitemonitoring values -e greater the depth of groundwaterdewatering is the larger the groundwater jumps and theactual stable dewatering curve is higher than Dupuitrsquos fallingfunnel curve However the group wells effect of groundsubsidence around the foundation pit is obvious at highdrawdown -erefore as the depth of groundwater ex-traction is larger the effect of group wells and the depth of
dewatering should be considered in the calculation ofground subsidence -e farther the point is away from thefoundation pit the smaller the difference is in settlementvalues calculated by each calculation method Consideringthe deformation of the soil layer caused by dewatering is notonly the vertical deformation but also the lateral defor-mation when the vertical deformation is only consideredthe calculation results may reach the on-site monitoringvalues By considering the calculation under the action ofseepage force the settlements of the ground around thefoundation pit caused by dewatering at different drawdowns
Figure 27 Group wells subsidence nephogram after the second drawdown (unit m) (a) -ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 28 Group wells pore water pressure nephogram after the second drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
Advances in Civil Engineering 19
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
can be quickly estimated -erefore the calculation resultscan provide effective guidance for groundwater extraction inthe related foundation pit
6 Curve Fitting for Ground SubsidenceCaused by Dewatering in Foundation Pit
-e variation law of ground subsidence after stabilization ofgroundwater drawdown is observed and the correspondingsettlement curve is proposed -e three curves obtainedfrom field monitoring values the calculated values under the
effect of seepage force and the numerical simulation are allconsistent with each other as shown in Figure 35
It is clearly seen from Figure 35 that the ground sub-sidence obtained by using the normative algorithm areconservative and the ground subsidence obtained by nu-merical simulation and seepage force are close to the on-sitemonitoring values Moreover the ground subsidence cal-culated by using the algorithm under the action of seepageforce fits the on-site monitoring values among thesemethods which shows that the calculation method of theground subsidence under the seepage force is more accurate
Figure 29 Group wells subsidence nephogram after the third drawdown (unit m) (a)-ree-dimensional diagram (b) Cross section of thewell midpoint (c) Longitudinal section of the well midpoint
Figure 30 Group wells pore water pressure nephogram after the third drawdown (unit Pa) (a) Cross section of the well midpoint(b) Longitudinal section of the well midpoint
20 Advances in Civil Engineering
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
-e ground subsidence curve that is fitted by the theoreticalcalculation can be better used to simulate the variation ofground subsidence Referring to the research results inreference [30] the ground subsidence Z(x) is easily obtainedas expressed below
Z(x) minus beminus (xc)
xgt 0 (43)
where x denotes the distance from the monitoring point tothe foundation pit m b and c indicate coefficients to bedetermined respectively and Z(x) represents the groundsubsidence mm
Substituting numerical values into equation (43) andafter calculation the ground subsidence-fitting curve isobtained as shown below
Z(x) minus 971eminus (x1211)
xgt 0 (44)
Since this equation is obtained based on the actualproject it has provided specific guidance for the assessmentof ground settlement due to the dewatering in foundation pitof the subway station on Chengdu metro line 6 And it canalso be used to calculate ground subsidence due to similarfoundation pit dewatering in sandy pebble soil strata
Figure 31 Group wells subsidence nephogram when the drawdown is stable (unit m) (a)-ree-dimensional diagram (b) Cross section ofthe well midpoint (c) Longitudinal section of the well midpoint
Figure 32 Group wells pore water pressure nephogram when the drawdown is stable (unit Pa) (a) -ree-dimensional diagram (b) Crosssection of the well midpoint
Advances in Civil Engineering 21
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(a)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(b)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(c)
Gro
und
subs
iden
ce v
alue
s (m
m)
Single wellGroup wells
ndash6
ndash5
ndash4
ndash3
ndash2
ndash1
04 8 12 16 20 24 28
Distance from the well axis (m)
(d)
Figure 33 Comparison of ground subsidence around single well and group wells under different drawdowns (a) Grade I drawdown(b) Grade II drawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown stability)
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
Calibration was accomplished by applying a set of hy-draulic parameters boundary conditions and stresses thatproduce computer-generated simulated pressure heads thatmatch actual field measurement within an acceptable rangeof error Model calibration was performed manually (trialand error) and automatically -e model is calibrated byapplying the permeability coefficient and the ground sub-sidence around the foundation pit under different draw-downs as well as limiting the normal displacement and thebottom displacement of the four sidewalls and bottom of the
model while limiting the horizontal displacement of thedewatering wells -e bottom part of the filter pipe of thedewatering wells belongs to the seepage boundary Based onthe on-site monitoring the improved settlement calculationsunder the action of seepage force and the conventionalalgorithm proposed in the Chinese Specification the groundsubsidence around the foundation pit under differentdrawdowns is obtained -e comparison of ground subsi-dence due to different drawdowns is used to calibrate themodel
Figure 34 Comparison of settlements at different distances from well axis under different drawdowns (a) Grade I drawdown (b) Grade IIdrawdown (c) Grade III drawdown (d) Grade IV drawdown (drawdown becomes stable)
Figure 35 Curve fitting for ground subsidence around the foundation pit
Advances in Civil Engineering 23
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
7 Conclusions
(1) According to the assumption of seepage field at thebottom of the enclosure structure in the open cutfoundation pit the influence range of the dewateringwell inside open cut foundation pit and the calcu-lation method of the water inflow are presented byusing the principle of equality of water inflow insidethe pit and outside the pit -e corresponding designof the foundation pit is also carried out in combi-nation with the actual project
(2) -e dewatering funnel curve of the dewatering wellin the phreatic aquifer is derived using Dupuitrsquosassumption and the groundwater levels at differentlocations under different drawdowns are obtained-rough the comparison of the results obtained fromthe Chinese Specification the algorithm consideringthe seepage force the numerical simulation and theon-site monitoring of the ground settlement theoverall trend of ground settlement in these four casesis basically consistent and the ground subsidencearound the foundation pit increases with the gradualproceeding of dewatering -e farther the moni-toring point is away from the well axis the smallerthe ground subsidence becomes Moreover thecurrent normative calculation method recom-mended in the Chinese Specification is more con-servative However the calculation methodconsidering the action of seepage force is more ac-curate and it can provide a theoretical basis for theestimation of ground subsidence caused by dew-atering in the foundation pit in actual engineering
(3) -e three-dimensional fluid-solid coupling numer-ical simulation of the ground subsidence caused bydewatering inside the foundation pit is carried outusing the finite difference method -e simulation ofdewatering in the foundation pit under the singlewell and the group wells effect was carried outseparately and the group wells effect of the groundsubsidence around the foundation pit was analyzed-e results show that the dewatering in the foun-dation pit will cause the upheaval of the strata insidethe foundation pit and the ground subsidence out-side the pit -e variation trend of ground subsi-dence around the foundation pit caused by the singlewell and group wells dewatering at each drawdownsis the same -e group wells effect on the groundsubsidence after dewatering inside the foundation pitis not obvious especially when it is far away from thefoundation pit or the drawdown is not large
(4) By fitting the ground subsidence value calculatedusing the algorithm considering the effect of seepageforce on a metro station on Chengdu Metro Line 6the final ground subsidence curve is obtained asshown in equation (44) -e curve can be used toaccurately simulate the ground settlement trendinduced by dewatering inside the foundation pit inthe actual project which provides effective guidance
for the similar dewatering project inside open cutfoundation pit in sandy pebble stratum
(5) A practical project in sandy pebble soil strata is takenas an example to study the ground subsidence causedby dewatering in the foundation pit in this paperSimilarly for other geological conditions corre-sponding conclusions can also be obtained using thesame way in the paper
Data Availability
All data and models employed to support the findings of thiswork are available from the corresponding author uponrequest
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e authors are grateful to the support of Research andDevelopment Program of the 21st China Railway Con-struction Corporation Ltd
References
[1] Y Yihdego ldquoEvaluation of flow reduction due to hydraulicbarrier engineering structure case of urban area floodcontamination and pollution risk assessmentrdquo Geotechnicaland Geological Engineering vol 34 no 5 pp 1643ndash16542016
[2] X Zhang X Ou J Yang and J Fu ldquoDeformation response ofan existing tunnel to upper excavation of foundation pit andassociated dewateringrdquo International Journal of Geo-mechanics vol 17 no 4 Article ID 04016112 2017
[3] J Wang Y Wu X Liu T Yang H Wang and Y Zhu ldquoArealsubsidence under pumping wellndashcurtain interaction in sub-way foundation pit dewatering conceptual model and nu-merical simulationsrdquo Environmental Earth Sciences vol 75no 3 p 198 2016
[4] Y Xu H Nawu B Z Wang and T Yang ldquoDewateringinduced subsidenceduring excavation in a Shanghai softdepositrdquo Environmental Earth Sciences vol 76 no 9 p 3512017
[5] J Wang X Liu S Liu Y Zhu W Pan and J Zhou ldquoPhysicalmodel test of transparent soil on coupling effect of cut-off walland pumping wells during foundation pit dewateringrdquo ActaGeotechnica vol 14 no 1 pp 141ndash162 2019
[6] E Pujades S De Simone J Carrera E Vazquez-Suntildee andA Jurado ldquoSettlements around pumping wells analysis ofinfluential factors and a simple calculation procedurerdquoJournal of Hydrology vol 548 pp 225ndash236 2017
[7] J Wang B Feng H Yu T Guo G Yang and J TangldquoNumerical study of dewatering in a large deep foundationpitrdquo Environmental Earth Sciences vol 69 no 3 pp 863ndash8722013
[8] J Wang X Liu Y Wu et al ldquoField experiment and numericalsimulation of coupling non-darcy flow caused by curtain andpumping well in foundation pit dewateringrdquo Journal of Hy-drology vol 549 pp 277ndash293 2017
24 Advances in Civil Engineering
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
Advances in Civil Engineering 25
[9] Y X Wu H Min Lyu J Han and S Shen ldquoDewateringinduced building settlement around a deep excavation in softdeposit in Tianjin Chinardquo Journal of Geotechnical andGeoenvironmental Engineering vol 145 no 5 Article ID05019003 2019
[10] D-d Zhang C-y Song and L-z Chen ldquoNumerical evalu-ation of land subsidence induced by dewatering in deepfoundation pitrdquo Journal of Shanghai Jiaotong University(Science) vol 18 no 3 pp 278ndash283 2013
[11] Q Liu J Liu P Tan Y Li and Z Lin ldquoCalculation of waterinflow of foundation pits considering water insulation effectof retaining structuresrdquo Tunnel Construction vol 33 no 2pp 142ndash146 2013 in Chinese
[12] S J Kollet and R M Maxwell ldquoCapturing the Influence ofgroundwater dynamics on land-surface processes using anintegrated distributed watershed modelrdquo Water ResourcesResearch vol 44 2008
[13] Z-j Luo Y-y Zhang and Y-x Wu ldquoFinite element nu-merical simulation of three-dimensional seepage control fordeep foundation pit dewateringrdquo Journal of Hydrodynamicsvol 20 no 5 pp 596ndash602 2008
[14] W Dragoni ldquoSome considerations regarding the radius ofinfluence of a pumping wellrdquo Hydrogeology no 3 pp 21ndash261998
[15] Y Yihdego ldquoEngineering and enviro-management value ofradius of influence estimate frommining excavationrdquo Journalof Applied Water Engineering and Research vol 6 no 4pp 329ndash337 2018
[16] Y Yihdego and L Drury ldquoMine dewatering and impact as-sessment in an arid area case of Gulf regionrdquo EnvironmentalMonitoring and Assessment vol 188 no 11 2016
[17] Y Yihdego and A Paffard ldquoPredicting open pit mine inflowand recovery depth in the Durvuljin soum Zavkhan provinceMongoliardquo Mine Water and the Environment vol 36 no 1pp 114ndash123 2017
[18] JGJ 120-2012 Technical Specification for Retaining and Pro-tection of Building Foundation Excavations China BuildingIndustry Press Beijing China 2012 in Chinese
[19] JGJ 111-2016 Technical Code for Groundwater Control inBuilding and Municipal Engineering China Building IndustryPress Beijing China 2016 in Chinese
[20] C Hu Z Zhou Y Li and Li Yuan ldquoAnalysis of the groundsubsidence caused by dewatering in deep foundation pitrdquo inProceedings of the International Conference of the AssociatedResearch Centers for Urban Underground Space ShenzenChina November 2009
[21] J Bear Hydraulics of Groundwater Courier CorporationChelmsford MA USA 2012
[22] Y Wu and Y Zhu ldquo-e simplified calculation and well-pumping test of settlement on the dewatering of foundationpits location in phreatic aquiferrdquo Journal of Civil Architec-tural and Environmental Engineering vol 37 no S2pp 168ndash177 2015 in Chinese
[23] Q Yang and B Zhao ldquoExperimental and theoretical study onthe surface subsidence by dewatering of foundation pit inphreatic aquiferrdquo Chinese Journal of Rock Mechanics andEngineering vol 37 no 6 pp 1506ndash1519 2018 in Chinese
[24] S-L Shen and Y-S Xu ldquoNumerical evaluation of landsubsidence induced by groundwater pumping in ShanghairdquoCanadian Geotechnical Journal vol 48 no 9 pp 1378ndash13922011
[25] D D Zhang ldquoCoupled numerical simulation research ondewatering and land subsidence in deep foundation pitrdquo
Applied Mechanics and Materials vol 275ndash277 pp 1549ndash1552 2013
[26] N Zhou P A Vermeer R Lou Y Tang and S JiangldquoNumerical simulation of deep foundation pit dewatering andoptimization of controlling land subsidencerdquo EngineeringGeology vol 114 no 3-4 pp 251ndash260 2010
[27] Y-Q Zhang J-H Wang J-J Chen and M-G Li ldquoNu-merical study on the responses of groundwater and strata topumping and recharge in a deep confined aquiferrdquo Journal ofHydrology vol 548 pp 342ndash352 2017
[28] J Wang B Feng Y Liu et al ldquoControlling subsidence causedby de-watering in a deep foundation pitrdquo Bulletin of Engi-neering Geology and the Environment vol 71 no 3pp 545ndash555 2012
[29] M N Houhou F Emeriault and A Belounar ldquo-ree-di-mensional numerical back-analysis of a monitored deep ex-cavation retained by strutted diaphragm wallsrdquo Tunnellingand Underground Space Technology vol 83 pp 153ndash1642019
[30] F Li and G Chen ldquoStudy of ground surface settlement offoundation pit with suspended waterproof curtain in Yangtzeriver flood plainrdquo Tunnel Construction vol 38 no 1pp 33ndash40 2018 in Chinese
[31] X Zhou H Hu B Jiang Y Zhou and Y Zhu ldquoNumericalanalysis on stability of express railway tunnel portalrdquoStructural Engineering and Mechanics vol 57 no 1 pp 1ndash202016
