Geosynthetics and yReinforced Soil Structures
Geosynthetic Reinforced Pile Platforms
Dr. K. Rajagopalf f CProfessor of Civil Engineering
IIT Madras, Chennai, Indiae-mail: [email protected]
Construction on Soft Foundation SoilProblems
(a) Slope instability (b) Unacceptable vertical settlements (From Lawson,2012)
(From Lawson,2012)
2(c) Localised differential settlements at
embankment surface(d) Difficulty to move the construction
equipment( Concept- Lawson,2012)
Methods of Ground Improvement
Soil Replacement Preloading Preloading Light Weight Fill
Preloading ith Vertical Drain Preloading with Vertical Drain Vacuum Preloading Stone Column-OSC,ESC Piled Raft Basal Reinforcement Piled Embankment Geosynthetic Reinforced Pile Supported
Embankment3
Geosynthetic Reinforced yPiled Embankments
Rail/Road embankmentRail/Road embankment
Soft clay
Pil I li dPiles Inclined Piles
Firm stratum
Advantages of Geosynthetic Reinforced Piled g yEmbankments
Faster construction-Loading rate not dependent on the rate of consolidation of soil
Eliminates differential settlements especially for large height embankments
Slope stability
Relatively small pile caps and no need for raking piles
Low long term maintenance costs
5
Embankment Piling
CFA (Continuous Flight Auger) piles
Load Transfer Platform at Second Severn Crossing13
Load Transfer Platform at Second Severn Crossing(Tensar, UK brochure)
Measured data fromMeasured data from Second Severn, UK(Tensar, UK brochures)
14
( )
Application areas
Bridge abutment approach roads (Buchanan 1984)(Buchanan,1984)
Airport runways (Hossain and Rao, 2005) Subgrade improvement (Han, 1975) Minimize differential settlements under storage
tanks (Alzamora et al. 2000) Segmental retaining wall (Alzamora et al. 2000) Widening of the existing roadway embankment
(Han and Gabr 2002) To construct confined embankment structures
(Lawson 2012)
15
Construction Sequence
Installing piles with certain grid formation in the soft soil up toa certain depth.
Geosynthetic material is laid on top of a thin layer (0.1 m) ofl t i lgranular material.
After placing the geosyntheticp g g ylayer, the embankment fill isconstructed to the requiredheight in stages.
Fi ll th t ti h Finally the construction such asrailway or road pavement isbuilt on top of the embankment
16
built on top of the embankment Geosynthetic Reinforced Piled Embankment System
Plan Layout of the Piles
L t ( ) S d (b) T i l(a) (b)
Layout (a) Square and (b) Triangular
Geosynthetic Layout
(a) (b)
17Optimal geosynthetic layout (a) direction of placing the layers and (b) direction of load
(Lawson,2012)
Load Transfer Mechanism
(b) Membrane action of geosynthetic(a) Soil Arching (b) Membrane action of geosynthetic(Russell and Pierpoint,1997)
(a) Soil Arching
( ) C t ti f t d th il d t th(c) Concentration of stresses around the pile due to thestiffness difference between the soft foundation soil and the rigid pile
18
soil and the rigid pile
Design Methods
(a) British Standard-BS8006:1995
This is the most widely used method and is very conservative This is the most widely used method and is very conservative.
Based on Marston’s (1913) formula for positive projectingased o a s o s ( 9 3) o u a o pos ve p ojec gconduits, Jones et al.(1990) developed an empirical relationshipfor the ratio of average vertical stress acting on the pile caps tothe average vertical stress acting across the base of theembankment .
where c cv
v
p C a HH
pc=Arched vertical stress on top of the pileσv=Average vertical stress on top of the pile
19
Cc= Arching Coefficient (Marston 1913)a = size of pile caps Positive Projecting Conduit
(Marston,1913)
BS8006 adopted Jones et al.(1990) for the design of piled b k tembankments.
BS8006 gives empirical equations for arching coefficient as follows
cCas follows
cEnd bearing piles,C 1.95 0.18Ha
cFriction piles,C 1.5 0.07
aHa
a
BS8006 considers two cases
1. Embankment height is below thecritical height of 1.4(s-a):
Arching is not fullydeveloped
Partial arching
20
Partial archingHere A= Load acting on the piles due to arching, B= Load taken by the geosynthetic andC= Load acting on the soft subsoil
For 0.7 1.4 ,s a H s a
2 22 2Load on the geosynthetic, fs q s c
Tv
s f H f w pW s as a
1Geosynthetic Tension T 1
v
TW s a
rGeosynthetic Tension, T 1
2 6where is the geosynthetic strain
f are the partial fact
a
f
ors used in the designfs f , are the partial factqf ors used in the design
b k h i h i b2. Embankment height is abovethe critical height of 1.4(s-a):
Full arching is developed
21 Full arching
Height of embankment above arching height plays no Height of embankment above arching height plays norole in the tension developed on the geosynthetic.
Same is the case with surcharge
For H>1 4 s a
For H>1.4 ,
1 4
s a
sf s a
2 22 2
1.4 fs cT
v
sf s a pW s as a
r
1Geosynthetic Tension, T 12 6
TW s a
2 6a
22
Horizontal force at the slope
,,
Horizontal force at the embankment slope after BS8006(Satibi,2009)
Geosynthetic tensile load needed to resist the horizontal f f th b k t i T
0.5 ( 2 )
hrs a fs qT K f H f q H
force of the embankment is Trs
a
where K Active lateral earth pressure coefficient
, = partial factors used in the designfs qf f
23
, p gfs qf f
(b) Hewlett and Randolph Method(1988)
This theory is based on limit state of soil inhemispherical domed region over piles.p g p
The stability of arch at the crown and at the pile top ofh h i h i l d d d i h ithe hemispherical dome formed defines the entire
stability.
24Hemispherical domes (Hewlett & Randolph, 1997)
Stress Reduction Ratio ( S3D ) defined as the ratio of the Stress Reduction Ratio ( S3D ) defined as the ratio of theaverage vertical stress acting on the reinforcement tothe overburden pressure due to the embankment fill was
1
used to check the stability.
3 1 2
2
1 at the crown of the arch2
1 1 1 11
pD k
pp
Sk a a a ak
k
2 1k
21 ppk s s s s
2 1
3
2 1 2 1 at the pile top 1 1
2 2 3 2 2 3
pkp p
Dp p
s k s a kaSs H k H k
- Largest value is the critical S3D
25
(c) The new German Method (EBGEO 2004)
In the old German approach the arching modeldeveloped by Hewlett and Randolph (1988) was used tocalculate the stresses generated due to arching.
EBGEO 2004 adopts the m lti shell arching theor EBGEO 2004 adopts the multi-shell arching theorybased on the work of Zaeske (2001).
26Multi shell arching theory adopted in New German Method
(Kempfert,2004)
3-dimensional soil element is considered and theequilibrium of forces about the radial direction is usedto calculate the vertical stress coming onto the soil,zo k
2
22 2, 1 1 2 1 1 24
gzo k g g g
hp h h h hh
, 4g g gh
where
2 2
221 21
2
where1 1 2 ( ), K=tan 45 , ,
2 8 8crit ka K s a s as a
s
In the second step the vertical stress acting on the top of,zo k
p g pthe subsoil is used to calculate the vertical load Fk onthe geosynthetic.
27
Load distribution on the geosynthetic for rectangular pile layout (Kempfert,2004)
2
x,k ,1 , F2 2 180
yLx x y Lx zo k
x
saA s s atn As
2
y,k ,1 , F2 2 180
xLy x y Ly zo k
y
saA s s atn As
28
y
The maximum strain k is obtained from thedimensionless design graphs (EBGEO 2004)
kJwLErsb
dimensionless design graphs (EBGEO, 2004).
Here,J t il tiff f thJk= tensile stiffness of the
geosynthetic (kN/m)Lw= (s-a)= pile clear spacingbErs= width of support
29(EBGEO,2004)
Horizontal force at the slope
The additional horizontal force in the reinforcement The additional horizontal force in the reinforcementbeneath the embankment slope is given by
1E E h k P h k ,
ah
2where K Active earth pressure coefficient
k ah k k k ahE E h k P h z k
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(d) The Dutch Method (CUR 226)
Introduced in 2009.
Adopts major parts of the German EBGEO 2004.
Flat terrain-thin embankments are constructed andtherefore the EBGEO method was modified to suit thetherefore the EBGEO method was modified to suit therequirements. (Eekelen et al.2010)
Main difference from EBGEO-Different set of load-and-resistance factors were adopted in the DutchG id liGuideline.
31
(e) Guido Method
Guido et al. (1987) observed that the inclusion of stiffbiaxial geogrid within a granular fill improved thebearing capacity of the foundation soilbearing capacity of the foundation soil.
Concluded that the angle of load spread through a Concluded that the angle of load spread through agranular fill reinforced with geogrid would be at anangle of 45 degrees.
The approach is mainly for asingle layer of geosynthetic atthe base of the embankmentfill.
3Stress Reduction Ratio= D
s aS
32
3Stress Reduction Ratio3 2DS
H
(f) The Swedish Method
Carlsson (1987) considered a wedge of soil with aninternal angle at the apex of the wedge equal to 30º.g p g q
Valid in two-dimensional model.
Carlsson adopted a critical height of 1.87(s-a).
Miriam and George (2003) presented the expression for p pS3D for this model as per Hewlett & Randolph (1997)
3
26 tan15D
s a s aS
H
33
6 tan15s a HTwo dimensional model by Carlsson,1987
Rogbeck et al. (1998) modified this model into a 3D form which is an inverted truncated pyramid
h di i l d l b b k l 1998
Modified form of this 3D arching model was adopted by
Three dimensional model by Rogbeck et al. ,1998(Lawson,2012)g p y
Nordic authorities (Svanø et al.2000).
I N di d i th hi l id d t i l d34
In Nordic design the arching angle was widened to includean angle of arching between 68º-75º.
A i i U i C ll (R ll d Pi i 1997 H
Numerical Analyses-Different approaches Axisymmetric Unit Cell (Russell and Pierpoint 1997, Han
and Gabr 2002, Yoo and Kim 2009)
3D Column (Yoo and Kim 2009, Jenck et al. 2009)
Full three dimensional analyses (Huang et al.2005,Liu etal.2007)
3DColumnPile
Full Embankment
35 Axisymmetric unit cell
Major Numerical Work-3D Column Russell and Pierpoint (1997) carried out a numerical study using Russell and Pierpoint (1997) carried out a numerical study using
FLAC3D to compare the different analytical methods.-Terzaghi (1943), Hewlett and Randolph (1988) and BS 8006g ( ), p ( )
Two cases were considered-The A13 piled embankment(heavily reinforced) and the Second Severn Crossingembankment (minimal reinforcement).
Design methods predicted differently for different embankmentgeometriesgeometries
Tension force calculated by different design methods Design Methods A13 Embankment
(Reinforcement Tension,
kN/ )
Second Crossing
(Reinforcement Tension,
kN/ )kN/m) kN/m)
BS8006 73 491
Ter aghi 104 297
36
Terzaghi 104 297
Hewlett & Randolph 104 280
Han and Gabr (2002)Major Numerical Work-Axisymmetric unit cell
Han and Gabr (2002)investigated the influenceof the tensile stiffness ofthe geosynthetic, the heightof the fill, and the elasticmodulus of the pilematerial.
One layer of geosyntheticwas used and a full bondwas assumed between thegeosynthetic and the soil.
Major findings are givenbelow
Pile Layout and the axisymmetric model considered for the analysis (Han and Gabr,2002)
37
below.
(a) (b)Effect of (a) pile modulus and (b) geosynthetic stiffness on the maximum settlements
(Han and Gabr,2002)
The influence of geosynthetic tensile stiffness becomes lessimportant when the stiffness exceeds 4,000 kN/m.
( , )
For a pile of elastic modulus of 30,000 MPa, the maximumttl t f th i f d d d b 20% f
38
settlement for the reinforced case was reduced by 20% fromthat for the unreinforced case.
(a) (b)Effect of geosynthetic (a) Stress Concentration Ratio(b) Tensile force distribution
(Han and Gabr,2002)
(a) (b)
The inclusion of geosynthetic reinforcement enhances thestress transfer from the soil to the piles.
Tension is not uniform along the geosynthetic and themaximum tension occurs at the edge of the pile.
39
maximum tension occurs at the edge of the pile.
Major Numerical Work-Full three dimensional
Geogrid Reinforced Pile supported highway embankment Geogrid Reinforced Pile supported highway embankmentlocated in Shanghai China-Liu et al. (2007)
Case history back analyzed by 3D fully coupled finite-element analysis.
Instrumented cross section of the embankment40
Instrumented cross section of the embankment (Liu et al.,2007)
Full three dimensional model developed (Li l 2007)(Liu et al.,2007)
Significant load transfer from the soil to the piles due to soilg parching-contact pressure acting on the pile was 14 timeshigher than that acting on the soil located between the piles.
Lateral displacements considerably reduced- stability of theembankment increased significantly
41
embankment increased significantly.
Design of Geosynthetic Reinforced Piled Embankment - Example
Pulverized fly ash filled embankment
9 m = 14kN/m3Pile caps
(1.1 m square)( q )
Soft clay(Without piles
settlement = 700 mm)
4 m
)
Embankment Details
Reinforcement detailsReinforcement details
Low creep reinforcement Tensile safety factor = 3.0 Peak extension at failure = 12%
Geotextiles Longitudinal Strength (kN/m)
Transverse Strength (kN/m)g ( ) g ( )
A 1000 50A 1000 50
Circular arc Deformation analysis
Aa = 4-1.1 = 2.9 m
A
2Geosynthetic
RG
Assuming
b = 0.2 0.7= 0.14 m
TT
b 0.2 0.7 0.14 m
From the geometryb TTTT
212
b tana
a11 03
2
.
a R sin
2
7 58
G
G
a R sin
R . m
1
2T R bGWeight of the fill , W
52 08W . kN mT
Considering the reaction force asg
0 15 18.9 kN mBW . h
The tension in the geosynthetic,
251.5 kN/m
Consider a single layer of geosynthetic (Optimal)
T G T BT R W W
Consider a single layer of geosynthetic (Optimal),
total strength = 1050 kN/m
The strain in the ge G251 5otextile, 12 2 871050
. % . %
GFrom the geometry 90 0 6GR a . %
As εG < the predictedG p
Try with b = 0.19 m
14 93º = 14.93º
RG = 5.63 m
WT = 38.08 kN/m
TT = 108 kN/mT
For this the strain εG, from the load deformation data = 1.23%
F h 1 2%From the geometry, εG = 1.2%
As these two are compatible the tension in the geosynthetic TT = 108 kN/m.
εG = 1.2 %G
Catenary Deformation analysis
From the Equation of the catenary, the tension in the geosynthetic is given byg y g y
21 1 aT WT WB
2
2 2
1 1 162
1 16 4 161 1 1
TaT WT WB a b
b a b bl
2 21 16 4 161 1 182G e
b a b blogb aa a
1 69Loading coefficient 0 12cc
. hC .B c
1D Arching: Pressure ratio = C B /h1D Arching: Pressure ratio = Cc Bc/h
2D Arching: Pressure ratio = (Cc B /h)22D Arching: Pressure ratio = (Cc Bc/h)2
Loading Coefficient,1 69 0 12 13.71c
c
. hC .B
Pressure ratio – (1D) = CcBc/h = 1.676Pressure ratio – (2D) = (1.676)2 = 2.809In any 4 square piles,
o Pile area = 1.21 m2
T l 16 2o Total area = 16 m2
o Soil area = 14.79 m2
Total load = 16149 = 2016 kN Total load = 16149 = 2016 kN Load on the pile = 1.211492.809 = 428 kN Load on soil = 2016 428 = 1588 kN = 107 4 kN/m2 Load on soil = 2016-428 = 1588 kN = 107.4 kN/m2
W 107 4 kN/WT = 107.4 kN/m
WB = 0.15 h = 18.9 kN/m
As per the equations shown earlier
T = 309 8 kN/mTT = 309.8 kN/m
From load-extension data εG = (309.8/1050)12 = 3.5 %
Using the equation for ‘1+εG’ as shown earlier, εG = 3.4 %
As the two values are in close agreement further iteration is notAs the two values are in close agreement further iteration is not necessary.
BS 8006-1995 Method
According to BS8006, the minimum height of embankmentrequired is 0.7 (s-a) and for full arching to develop the height of theembankment should be greater than 1.4 (s-a)
In the present case, 0.7(4 – 1.1) = 2.03 m < 9 m and1.4(4-1.1)=4.06 m < 9 m- Full arching develops in this case
The Arching coefficient (considering end bearing pile).1 95 0 18. HCc .a
The vertical stress on the pile cap = 15.77
2 215 77 1 1C a 215 77 1 114 9 468 1 kN/m9c
c vC a . .p .H
For H > 1.4(s-a), The distributed load carried by the geosynthetic reinforcement
1 4s s a 2 22 2
1 4
176 85 kN/
cT
v
. s s a pW s as a
(Serviceability condition, partial factors in the equations are given a value of 1) = 176.85 kN/m
Tension in the reinforcement (BS8006-Design strain is 5%) 11 486 2 kN/mTW s a
T
Tension due to lateral thrust,
1 486.2 kN/m2 6rT a
0 5 170 1 kN/mLT . Ka H .
Total tension = 656.3 kN/m
7 /L
Results of Design
By Circular arc methodTT = 108 kN/m; εG = 1.2 %; WT = 38.08 kN/mT ; G ; T
By Catenary deformation methody yTT = 310 kN/m; εG = 3.4 %; WT = 107.4 kN/m
By BS 8006 1995 methodTT = 656.3 kN/m; εG = 5 %; WT = 176.85 kN/mT 656.3 N/ ; εG 5 %; WT 76.85 N/
54
References1. Alzamora, D., M. H. Wayne and J. Han (2000) Performance of SRW supported by geogrids and jet
grout columns Proc., ASCE Specialty Conf. on Performance Confirmation of Constructed Geotechnical Facilities, Geotechnical Special Publication, 94, 456–466.
2 British Standards BS8006: 1995 Code of practice for strengthened/Reinforced soilsand other fills2. British Standards BS8006: 1995 Code of practice for strengthened/Reinforced soilsand other fills. Section 8.3.3 British Standard Institution.
3. Carlsson, B. Reinforced soil, principles for calculation, Terratema AB, Linköping (in Swedish), 1987.4. CUR 226 2010(2010) Dutch CUR design guideline for piled embankments. ISBN 978 –90–376-0518-1.( ) g g f p5. EBGEO (2004): Bewehrte ErdkÖrper auf punkt - und linienfÖrmigen Traggliedern, Entwurf Kapitel
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earth slabs” Proceedings of Geosynthetics 87 Conference, New Orleans, 216-225.7. Han, R. (1975) Piled Embankment Supported by Single Pile Caps. Proceedings of the Conference on
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platforms over soft soil. Journal of Geotechnical and Geoenvironmental Engineering, ASCE,128(1), 44-53.
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10. Hossain, S. and K.N. Rao (2006) Performance Evaluation and Numerical Modeling of Embankment over Soft Clayey Soil Improved with Chemico-Pile. Transportation research record, USA, Issue Number: 1952, 80-89.,267–274.
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References
11. Huang, J., J.G. Collin, and J. Han (2005) “3D Numerical Modelling of a Geosynthetic –Reinforced Pile-Supported Embankment- Stress and Displacement Analysis”16th International Conference on Soil Mechanics and Geotechnical Engineering, Osaka, Japan, 12-16.Confe ence on Soil echanics and Geotechnical nginee ing, Osa a, Japa , 6.
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15. Liu, H.L., W. W. Charles, and K. Fei (2007) Performance of a geogrid-reinforced and pile-supported highway embankment over soft clays-Case study. Journal of Geotechnical and G i t l E i i ASCE 133(12) 1483 1493Geoenvironmental Engineering, ASCE, 133(12), 1483-1493.
16. Marston, A. and A.O. Anderson (1913) The theory of loads on pipes in ditches and tests of cement and clay drain tile and sewer pipe. Engineering experiment station, Bulletin No.31.
17 Miriam E S and M F George (2003) Influence of Clay Compressibility on Geosynthetic Loads17. Miriam, E.S., and M.F. George (2003) Influence of Clay Compressibility on Geosynthetic Loads in Bridging Layers for Column-Supported Embankments.Geotechnical Special Publication, no 130-142, 447-460.
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References
18. Reid, W. M. and N. W. Buchanan(1984)Bridge approach support piling. Piling and Ground Treatment, Thomas Telford Ltd., LondonTreatment, Thomas Telford Ltd., London
19. Rogbeck, Y., S. Gustavsson, I. Sodergren and D. Lindquist(1998) Reinforced Piled Embankments in Sweden-Design Aspects. Proceedings of the Sixth International Conference on Geosynthetics, 2, 755-762.
20. Russell, D. and N. Pierpoint (1997) An assessment of design methods for piled embankments. Ground Engineering, 30(11), 39-44.
21. Satibi, S. (2009) Numerical analysis and design criteria of embankments on floating piles. A h h b d h f S S GPhD thesis submitted to the Universität of Stuttgart, Stuttgart, Germany.
22. Yoo, C. and S.B. Kim (2009) Numerical modeling of geosynthetic-encased stone column-reinforced ground. Geosynthetics International, 16(3), 116-126.
23 Z k D (2001) Z Wi k i b h t d23. Zaeske, D. (2001). ZurWirkungsweise von unbewehrten und bewehrtenmineralischenTragschichtenu berpfahlartigenGrundungsetementen. SchriftenreiheGeotechnik,University of Kassel, Germany, Heft 10, February.
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