This article was downloaded by: [University of Guelph] On: 25 August 2012, At: 03:10 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Geomechanics and Geoengineering: An International Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tgeo20 Effect of nail characteristics on slope stability based on limit equilibrium and numerical methods Mahdi Rasouli Maleki a & Mohammad Mahyar b a Engineering Geology & Rock Mechanic Department, Tunnel Consulting Engineers, Tehran, Iran b Mining Engineering, Tunnel Consulting Engineers, Tehran, Iran Version of record first published: 31 Jan 2012 To cite this article: Mahdi Rasouli Maleki & Mohammad Mahyar (2012): Effect of nail characteristics on slope stability based on limit equilibrium and numerical methods, Geomechanics and Geoengineering: An International Journal, 7:3, 197-207 To link to this article: http://dx.doi.org/10.1080/17486025.2011.631037 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
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This article was downloaded by: [University of Guelph]On: 25 August 2012, At: 03:10Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK
Geomechanics and Geoengineering: An InternationalJournalPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tgeo20
Effect of nail characteristics on slope stability based onlimit equilibrium and numerical methodsMahdi Rasouli Maleki a & Mohammad Mahyar ba Engineering Geology & Rock Mechanic Department, Tunnel Consulting Engineers, Tehran,Iranb Mining Engineering, Tunnel Consulting Engineers, Tehran, Iran
Version of record first published: 31 Jan 2012
To cite this article: Mahdi Rasouli Maleki & Mohammad Mahyar (2012): Effect of nail characteristics on slope stability basedon limit equilibrium and numerical methods, Geomechanics and Geoengineering: An International Journal, 7:3, 197-207
To link to this article: http://dx.doi.org/10.1080/17486025.2011.631037
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form toanyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses shouldbe independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly inconnection with or arising out of the use of this material.
(Received 23 November 2010; final version received 7 October 2011)
One of the most important phases in tunnel excavation in urban areas is to design and make a place for assembling and establishing excavationmachines into the earth. Since providing stable trench walls in loose earth environment is very important, this paper tries to use a nailing methodto make trench walls stable in the entrance of the 2nd subway line in Tabriz. To do so, the effect of each of the nailing designing parameters inimproving and stabilizing of entrance trench walls of earth pressure balance digging machines (EPB) of the 2nd line in the Tabriz subway wereanalyzed by limit equilibrium and numerical methods using two pieces of software, Plaxis 3D and Slide 5.0. Of all the parameters, nail length, nailaxis incline angle with horizon level, distance from centre to centre of nails, and nail capacity are the most important ones affecting the amplitudereliance coefficient so that in this paper the researcher attempts to determine and provide the optimum condition of all mentioned parameters to geta desirable safety factor (1.4). Also, to confirm and compare the relationship resulting from these parameters, limit equilibrium method was used fordifferent lengths of nails through numerical method by Plaxis 3D software. The paper tries to compare the relationship between the displacementamount and the strain percent established in amplitude due to excavation for each of the conditions in nail installing.
Keywords: stability analysis nailing; limit equilibrium and numerical methods; slide 5.0 software; Plaxis 3D software
1. Introduction
With an area of 118 square kilometres and 1.7 million residents,Tabriz is one of the big cities in north-western Iran. The dailycommute of people throughout the city leads to traffic prob-lems and noise pollution and problems in breathing, also thegeneral view of the city is undesirable from a tourism pointof view. Therefore, through several studies, it was approved toreplace the public transportation and personal vehicles with anunderground transportation system and urban trains to solve theproblems.
2. Background of project
The Tabriz subway project is studied and executed with fourlines of a total length of 48 km (Figure 1). In order to developthe Tabriz subway, its second line in an east-west directionis the longest line of this project from Azad University toGharamalek with 22 km of length and excavation diameter of9.3 m that is going to be excavated by a total-section excava-tion machine of Earth Pressure Balanced (EPB). To do so, it isjudged that a trench of at least 15 m in depth and 22 m in width
(floor) at the western end is required to establish and assemblethe excavation machine.
According to the problems and limitation of the facilities andequipment, the trench wall stabilization is going to be under-taken by nailing method. This research, hence, tried to study theeffect and behaviour of nailing on the stability of the entrancetrench walls of the EPB excavation machine by limit equilib-rium and numerical methods. Then a comparison was madeof the results by numerical methods after determining the bestcondition of nail installation (by limit equilibrium method).
3. Geology of the site
Tabriz is surrounded by the Own-ebn-Ali mountain range to thenorth and by high hills to the south that are composed of hard-ened alluvial deposits and conglomerates. Geological studies inthe region indicate that deposits and stones around Tabriz arenot developed in terms of time so that their composing partsbelong to Cenozoic and Quaternary eras. Also development ofincompact fresh alluvial deposits of the Quaternary era in thewestern part of Tabriz, mainly due to erosion of high landsaround Tabriz plain and carrying them by surface waters and
Figure 1. Tabriz subway lines network (approved by Traffic Excellence Association of the country).
alluvial fans, has meant major parts of the tunnel excavation ofTabriz subway’s 2nd line will be performed in these materials(Rasouli Maleki and Mahyar 2011).
4. Geotechnical data and underground survey ofexcavation area
In order to recognize the underground conditions around thewestern trench of the second line of the Tabriz subway, threemechanic boreholes (SH-1, SH-2, SH-3) were excavated by
rotary drilling machine. Figure 2 shows the position of exca-vated boreholes around the western trench and Figure 3 showsthe area’s geological profile.
According to surveys performed on the obtained samples andfield observations of the western trench, it was cleared that mostof the materials found in the entrance of excavation machinewere fine soils, so that due to weathering processes and erosionon the ground, alluvial fine materials are of clay-silt kind withlow plastic index which from upper parts of the trench. In lowerparts; however, alluvial materials are coarse and it is possibleto see sand and gravel. Through some surveys, in terms of
Figure 2. The location of the excavated borehole around the western trench.
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Table 1. Type of the earth composing the westerntrench area according to SH-1and SH-2 boreholes data
geo-hydrology, it was cleared that average underground waterin the area is located 16 m deep. Generally speaking, afterstudying and combining the excavated boreholes logs, one candefine the kind and border of the earth layers of the trench areaaccording to Table 1 and Figure 3 (Rasouli Maleki and Mahyar2011).
5. Field and laboratory data
Determination of the site’s geotechnical characteristics by lab-oratory experiments and field observations is one of the mostimportant stages of the studies before designing and construct-ing a tunnel. For the second line of the Tabriz subway project,several in-situ experiments on boreholes and wells were carriedout. Table 2 indicates the most important engineering char-acteristics resulted for alluvial materials around the entrancetrench.
6. The project requirements
In this study, and because of the project’s needs, the heightand slope of the excavation walls were considered 8 m and
Table 2. The engineering characteristics of the soil in western trench area
vertical in all analysis. Therefore, one can say that the variableparameter to get a desirable and appropriate safety factor isfeatures and conditions for nailing surveyed in this research.Figure 4 shows the geometry and dimensions of the Tabrizsubway entrance.
One of the negative factors in declining stability and safetyfactor of slope is the presence of external forces applied onthe slope (Grasso et al. 2003). According to previous stud-ies, one can claim that most of the external forces appliedon the entrance trench slope are due to 1) vehicle traffic loadand 2) excavation materials depot on the amplitude. In thisproject, in all analysis the acceleration amount due to vehiclestraffic is equal to 0.08 g and the force due to depot of exca-vation materials on the slope is equal to 2 t/m2 for temporarycondition.
7. Slope stability analysis
During the studies performed in this research, two methods:limit equilibrium and numerical methods were used to investi-gate the effect of nailing on the slope stability by the help of thepowerful Slide 5.0 and Plaxis 3D software. In addition, in orderto do correct engineering surveys and analysis, several condi-tions and methods of nailing for stabilizing the entrance trenchwall were studied.
Figure 3. The Geological profile resulted from SH-1 and SH-2 excavated boreholes.
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Figure 4. Geometrical model and dimensions of Tabriz subway entrance trench.
Figure 5. Nail length effect on slope factor of safety (FOS).
7.1 Use of the limit equilibrium method
Limit equilibrium is one of the most efficient and the simplestways of analysing the stability of rock and earth soil slopes.In this analytic method, all analyses are based on the com-parison of stable and unstable forces and momentums of theamplitude. Parameters such as shear strength of the rock mass(cohesion and friction angle) are the most important factorsbeing applied in this method.
Soil nailing technique has been applied in civil engineer-ing projects in Mexico City back to 1960s and has gainedpopularity in Europe since 1970 (Liew 2005). This techniquehas been used for excavation to a depth of 20 meters inCanada (Edmonton Light Rail Transit) and up to 30 meterselsewhere in the world (Juran 1987). The theory of usingthe nailing method is on the basis of reinforcing the soilthrough combining soil mass by nail steel tensile moor-ings in close proximity to each other. Increasing the shear
Figure 6. Nail slope angle effect on slope safety factor.
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Figure 7. Nail slope angle effect on slope safety factor.
Figure 8. Slope stability safety factor decrease when increase of nails spacing.
strength of soil mass and limiting and controlling soil dis-placements due to increasing shear strength in sliding surfaceare the most important advantages of this method (RasouliMaleki 2008).
Nail length, slope angle of nail axis with horizon level, dis-tance from centre to centre of nails, and type and capacity of
Table 3. Nails optimum distance computed per different lengths of nails
nails are some significant factors having an effect on increas-ing the slope safety factor by nailing method. In this study, theresearcher tried to provide the effect of each of these parame-ters on slope safety factor and finally concluded an appropriateoptimum state for stabilizing the western trench walls in Tabrizsubway.
7.1.1 Nail length effect on slope stabilityNail length is one of the most important characteristics of nailsthat increases slope safety factor. Nail length increase leads tofailed soil mass over shearing surface joined to the rough part.In other words, when nail length increases, resistant forces areincreased in the failure surface. Figure 5 shows the effect of thenail length on slope safety factor of the entrance trench in theTabriz subway. From this figure one can discover that the limit
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Figure 9. Nails optimum distance per 1.4 stability factor.
equilibrium condition on that slope will be obtained whenevernail length is 2.7 m. That is, if the selected nail’s length for thestabilizing slope is less than 2.7 m a failure in the slope willoccur.
7.1.2 Nail slope angle effect on slope stabilityIn the theory of using nailing method to resist against soil massnailing angle, it means the angle between the nail axis with thehorizon surface is one of several important and effective factorsin increasing resistant forces and safety factor against slopefailure. In this survey, in order to determine the effect of thenail’s slope effect on slope safety factor of the entrance trenchthrough limit equilibrium method, a safety factor amount forthe 0–90◦ angles was analyzed for three different lengthsof nails.
The results of these studies indicate that the nail’s slopeangle can increase slope safety factor a little, so that in highslopes, safety factor is declined due to resistant force (Fy)highlight obtained from nail tensile force decomposition. Theeffect of nail slope on slope safety factor of entrance trench
in Tabriz subway is shown in Figure 6. Results on nail slopeeffect show that the highest safety factor and optimum angleof nailing will be obtained when the nail’s slope angle is in20–40◦ degrees range. Also the figure indicates that a suddendecline in slope safety factor can be seen in angles more than 40degrees.
7.1.3 The effect of adjacent nail’s distance on slope stabilityStudies done by several researchers show that distance betweennails is one of the factors which effect slope stability by nail-ing method. Increasing the number of nails in slope lengthimproves slope material and then increases slope safety factor.Here in order to study the effect of this parameter on stabil-ity and also to determine the optimum nail distance for theentrance trench in Tabriz subway, surveys were performed bylimit equilibrium method. The result of the nail’s length effecton slope safety factor is shown in Figure 7.
Also, in this research several studies carried out on nails withdifferent lengths to indicate the effect of the distance between
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Table 4. Optimum distance of nails computed per different lengths of nails
Nail length (m) 5 7.5 10
Parameters Spacing of nails (m) Safety Factor Spacing of nails (m) Safety Factor Spacing of nails (m) Safety Factor
Figure 10. Safety factor resulted from nailing with different characteristics.
nails and slope angle effect on slope stability. The results showthat for the favourite angles of nails to slope (20–40 degree),the highest slope safety factor will be obtained when distancebetween nails is decreased (Figure 8). In other words the nailing
slope angle shows a contrary relationship with the safety factorof slope.
Another important factor performed in this study is to deter-mine the best nail distance to get 1.4 slope safety factor under
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Figure 11. Nail tensile capacity effect on slope safety factor.
the condition of optimum nailing slope angle (20–40 degrees).The results of this study are indicated for slopes of 20, 30, and40 degrees in Table 3 and Figure 9.
Also, in order to confirm and be sure of the results presentedin Table 3, Slide 5.0 software was used to calculate, model,and analyse safety factor per length, angle slope, and nails dis-tance suggested for the entrance trench walls in Tabriz subway(Table 4). According to Table 4, one can deduce that 1.4 safetyfactors can be obtained for all conditions. Figure 10 indicatesthe outcome of Slide 5.0 software for different conditions ofnailing suggested in Table 4 for three different states.
Table 5. Effect of tensile capacity and nail length on factor of safety(FOS)
Nail angle (degree) Tensile capacity, KN Safety Factor
20 80 1.3230 90 1.3840 100 1.41
7.1.4 Nail tensile capacity effect on slope stabilityIn empirical analysis and calculations, the nail’s tensile capac-ity is another effective factor in increasing the slope safetyfactor. As there are different type of nails (in terms ofresistance).
It is necessary to determine the type of the nails applied inslope stability analysis to get a safety factor. In this study, theresearcher tried to investigate the suggested nail’s behaviourin terms of tensile capacity in order to determine the effectof this parameter of nails on slope stability of the entrancetrench of Tabriz subway. All these were done after modellingthe nails by Slide 5.0 software and limit equilibrium method(using the optimum conditions resulting from the previousphases).
The results show that whenever the nailing angle increasedtowards horizon, slope safety factor for different lengths ofnails will be the same and equal in higher levels of tensilecapacity (Figure 11). In other words, nailing angle in highertensile capacities increase, the effect of nail length on slopesafety factor will be the same (Table 5).
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Figure 12. A schematic of modeling, meshing, and nailing of the project area.
Table 6. Excavation steps in analyzing the stability by Plaxis 3D program
Excavation Steps Description
1 Geometry modeling of trench2 Excavation of first bench and run program3 Excavation of second bench with steps 1 m to 1 m4 Installing of recommended nails by limit equilibrium
and run program
7.2 Using of numerical methods
Plaxis 3D Tunnel is a finite element and three-dimensionalprogram for geotechnical applications and applications inunderground construction in which models are used to simulatethe soil behaviour. The program code and soil modelshave been developed with great care. The simulation ofgeotechnical problems by means of the finite element methodimplicitly involves some inevitable numerical and modellingerrors.
This program offers a variety of soil models in addition tothe Mohr-Coulomb model. As a general second-order model,an elastoplastic type of hyperbolic model is available, whichis called the Hardening Soil model. To analyse accuratelythe time-dependent and logarithmic compression behaviour of
normally consolidated soft soils, a Creep model is available,which is referred to as the Soft Soil Creep Model. In additionto these soil models, a special model is available to analyse theanisotropic behaviour of jointed rock.
In this study, first of all nailing optimum conditions (length,install angle, and nails distance) were determined by limitequilibrium method; afterwards, the researcher tried to deter-mine displacement amount, stress and strains in excavatedfills; and finally general comparison was done by Plaxis 3D’spowerful finite element software (Plaxis 3D Tunnel 2001).
Numerical methods such as the Finite Element Method(FEM) and the Discrete Element Method (DEM) have beensuccessfully applied to slope stability analysis (Plaxis BV2006). In this software first slope geometry was drawn, thenengineering characteristics of each of layers and the region’sground water level were defined, then mesh analysis wasperformed (Figure 12). Also, analysing the stability by thisprogram, according to Table 6, excavation steps were defined.Analysis was done in several phases for three different lengthsof nails (according to Table 3). Finally, vertical and horizon-tal displacements also main stress and strain amounts werecalculated for a slope with an excavation depth of 8 m and slopeof 1:1 (horizontal: vertical). Table 7 includes the most importantdata output by the Plaxis 3D software.
The results obtained by the numerical method indicate thatdisplacement, stress, and strain percents of the slope are thesame for length, slope angle, and distance between centre tocentre of nails as defined in Table 3, so that horizontal displace-ment amount and slope strain will be in range of 16.2–18.3mmand 0.07–0.13 percent respectively for three different states,which in turn confirms the results and analysis obtained by thelimit equilibrium method (that is Table 3). Table 8 shows themost important parameters got from Plaxis 3D finite elementsoftware. Figure 13 indicates the outcome model of this soft-ware for the condition of nailing with 7.5 m of length, angle of30 degree, and distance of 1.1 m.
8. Conclusion
Results obtained from the present study show that the maindeposits in the entrance trench location for the excavationmachine in Tabriz subway are of alluvial fine sediments of clayand sand types that, due to having a low plasticity index in termsof walls stability, will create problems. Results of analysis by
Table 7. The most important input data in Plaxis 3D software
ID Name Type γunsat [kN/m3] γsat [kN/m3] v [-] Eref [kN/m2] cref [kN/m2] ϕ [◦] ψ [◦] Eincr [kN/m3]
Figure 13. Plaxis 3D software result model for nailing condition with 7.5 m of length, angle of 30 degree and spacing of 1.1 m.
limit equilibrium method on different condition of nailing indi-cated that length, nailing angle, and distance between centreto centre of nails are the most important effective parametersin slope safety factor of the excavation trench. So the highestsafety factor will result whenever the optimum angle of nailsin the slope is in the 20–40 degrees range. Also, these resultsshow that for a safety factor of 1.4, nailing dip has a directrelationship with nailing distance so that when nailing dip is
increased from 20 to 40 degrees, the distance among nails willbe increased too. Also, for this slope, the amount of displace-ment and strain resulting from numerical analysis shows thatfor optimum nailing condition (resulted from limit equilibriummethod) the displacement and strain amounts established forthree different lengths of nails are the same and equal, whichin turn confirm the relationship among nail characteristics sug-gested on the basis of limit equilibrium method. Therefore,
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according to the results obtained from limit equilibrium andnumerical methods, it is suggested that to get a 1.4 safety factorone should install nails with 7.5 m length, 30 degrees of instal-lation angle, and 1.1 m of spacing (or 20 degree of installationangle and 1.0 m of spacing).
References
Grasso, P., Xu, Sh., Fedele, M., Russo, G. and Ch, E., 2003. Particularfailure mechanisms of weathered granite observed during con-struction of metro tunnels by TBM. In: Proceedings of the WorldTunnel Congress, Amsterdam, 2, 497–503.
Juran, I., 1987. Nailed-soil retaining structures: design and practice.Transportation Research Record 1119, 139–150.
Liew, S., 2005. Soil nailing for slope strengthening. Penang, Malaysia:Seminar on Geotechnical Engineering (IEM).
Plaxis 3D Tunnel computer software, 2001. Technical University ofDelft.
Plaxis BV, 2006. Manual of Plaxis V8 – 2D finite element programfor geotechnical analysis.
Rasouli Maleki, M., 2008. Study of engineering geology of tunneldiversion site of the Garmi Chay dam with emphasis on the damimpounding effect on the tunnel behaviour. Thesis (MSc). Bu AliSina University, Hamedan, Iran.
Rasouli Maleki, M., and Mahyar, M., 2011. Stability analysisand determining of stable excavation depth of inlet shaftof the EPB machine. In: 4th International Conference onGeotechnical Engineering and Soil Mechanics (ICGESM),Iran.
