Trench Stability in Cohesive Soil

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Trench stability in cohesive soilK. Gorska1

Wroclaw University of Technology, Poland

ABSTRACT

Trench is connected to very narrow and deep excavation filled with bentonite suspension. This paper presents estimation of itsstability in cohesive soil. The stability is assessed by two calculation methods. The first involves the equilibrium of forces acting

on the rigid wedge. The second one includes numerical calculations conducted in Plaxis 3D Foundations. A few examples

having different dimensions (length and depth) are analyzed in uniform soil conditions. Graphs defining the dependence of

length, depth and factor of safety are presented. It is found that for long trenches (L6m) the soil kinematics at failure coincideswith the literature data. Short trenches are under a large influence of the arching effect and cohesive forces. The limit

equilibrium method can be used under the condition of employing a factor, which reduces the value of the earth pressure.

Keywords: retaining wall, trench, safety, stability, numerical analysis, arching, failure.

1Wroclaw University of Technology, ul. Wybrzeze Wyspianskiego 27, 51-692 Wroclaw. [email protected]

1 INTRODUCTION

Trench excavation is widely used in geotechnicalworks. It is performed as the first stageof construction for diaphragm walls, barrettes orslurry walls [3, 12]. A deep vertical cut in theground is excavated under a slurry suspension.The first application of diaphragm walls was inthe early sixties [10] and now they are

continually used successfully supporting theconstruction of deep excavations or deep

foundations. Walls made from concrete and steelwork well against high values of internal forcesand permit the transfer of loads from leaningslabs. Another advantage is water resistance thatis only provided by a proper execution of paneljoints. Van Tol presents four cases of leakagethrough the diaphragm walls at stop end joints indeep excavations, which led to very serious

settlement behind the walls [8]. Different

variations in construction phases result in a hugerange of implementation possibilities, e.g. slurrywalls in which a suspension is mixed withcement that hardens [2].

The first stage of construction is critical forthe construction process. In the following phases,the stability of the surrounding soil is easier to

maintain. During the concreting process pressureinside the trench increases (slurry is beingcontinuously replaced by concrete) as does theexerted force on the faces and the toe of thetrench.

There is a widespread belief among

geoengineers that if the slurry level exceeds thewater level by more than 1 m and the slurry unitweight is greater than 10.5 kN/m

3trench stability

is guaranteed; although in this case the safetymargin is unknown. The primary question is inwhat situations special care or preparations


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during the trenching process should be taken.

Usually, major problems are not related to thetrenching process, but are connected to human

error.There are several theories for calculating

trench stability that have been implemented inpractice. The first group of theories concerns 2D

cases with long trenches. The wedge is triangularand the slip surface is inclined by the angle

cr= /4 + /2, as in the Coulomb criterion,where is friction angle. Initially onlyhomogeneous, perfectly cohesive soil conditions

without groundwater were analyzed by Nash andJohns [5]. Later, other forces such asgroundwater pressure and a varying slurry levelwere taken into consideration (Morgerstern and

AmirTahmasseb [4]). These solutions can beassumed for shallower rather than longer

trenches (trenches with dimensions ofL


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20.5C ctg H c=

cosx

C C =

sinz

C C =

/ sinRC L H c =

3 FINITE ELEMENT METHOD

The second method includes numericalcalculations conducted in Plaxis 3D Foundations.

This program uses a finiteelement method.During numerical calculations, displacementsand stress in the surrounding soil are determined.This enables estimating the shape and range ofthe wedge and to establish if the arching effectoccurs.

The trench excavation process in a block of

soil is taken into consideration and the advantageof two axes of symmetry is used. On verticalsurfaces of the resulting solid boundaryconditions allowing only vertical movement. Onthe basis of body movement is blocked in alldirections. Surface area remains free. At first the

sensitivity of the size of the modeled area inrelation to the mesh size was tested. Within threeblocks different in size eventually a block sizes14 16 20m was adopted, as to eliminate theeffect of the impact of sides, and at the sametime not unduly magnifying the size of the task.

Adopted regions of different mesh size reducecomputation time without significantly affectingthe accuracy of the results.

The excavation process is modeled as theremoval of 2 m thick layers of soil and theapplication of slurry pressure. For simplification,

the hydrostatic slurry pressure is assumed as

external stabilizing load. It increases linearlywith depth and is applied to all faces of thetrench, including the toe. The slurry level is keptunchanged at the ground surface.

To determine the shape of the wedge, the

standard procedure of tan/c reduction is used.Since no limit state i.e. no failure is observed,

the interpretation of the FS/displacement curve ismade. The FS/displacement curve indicates arapid change in the inclination. The factor ofsafety increases until soil displacements reach

values of 3 8 cm. The exact displacement valuedepends on which point is observed. Points nearthe toe have larger displacements. FS values for

displacements greater than 8 cm are constant. Inaddition, in examining the FS/step curve a pointof deflection is observed at the same step and thesame displacement.

4 EXAMPLES

A few examples having different dimensions(length and depth) are analyzed in uniform soilconditions. Graphs defining the dependence oflength, depth and factor of safety are presented.

4.1 Soil conditions

In the examples, uniform ground conditions withthe Coulomb criterion are analyzed. The materialparameters are presented in Table 1. No waterlevel is considered. The unit slurry weight is 10.5kN/m3. No filtration or improvement of the soil

conditions in the surrounding layer is considered.

Table 1. Parameters of the homogeneous soil.

Ka c E kN/m3 kPa MPa

Clay 20 0,66 12 15 28 0.35

4.2 Trench dimensions

An analysis of typical trench dimensions wasconducted for the following dimensions: lengthL

3 to 10 m, widthB1.0 m and depthH8 to15 m.

4.3 Results

During the excavation process, soil and slurrypressures acting on the trench sides are inequilibrium. No change in the stresses around thetrench is observed. At the toe of the trench, theforce exerted by the slurry causes a reduction of

the stress in the soil. This is also influenced bythe smaller unit weight of the suspension

compared to the soil. Figure 2 presents the totaldisplacements after excavation with the largestvalues of 5.5 mm occurring at the center of thetoe. This is typical for the rebound connected


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The general rule that factors of safety decrease

with an increase in length for the same depth oftrench is fulfilled (Figures 3, 4 and 5). For the

limit equilibrium method a surprisingphenomenon occurs, i.e. the tendency of higherFS values for very short trenches of the samedepth (Figure 4 and Table 2). This is not

observed for the finite element method (Figure 5and Table 3) and is caused by the formulation of

method solutions. Very short trenches shear andcohesion forces have a determinant influence onthe earth pressure value. This is also a result of

the arching effect. If the trench length increases,acting forces decrease and the arching effectdisappears.

Table 2. FS values limit equilibrium method.

L length of the trenchH depth of

the trench 10 6 5 4 3

8 2.13 2.58 2.84 3.16 3.7510 2.00 2.51 2.76 3.13 3.81

12 1.96 2.50 2.77 3.19 3.95

15 1.93 2.53 2.85 3.32 4.25

Table 3. FS values finite element method.

L length of the trenchH depth ofthe trench 6 5 4 3

8 2.32 2.52 2.71 3.00

10 2.18 2.36 2.56 2.83

12 2.09 2.29 2.47 2.7315 2.04 2.21 2.41 2.65

Figure 6. 3D total displacements for a 6 m long trench atfailure finite element method.

Figure 7. 3D total displacements for a 3 m long trench at

failure finite element method.

Results of this phenomenon are observed ondisplacement maps. In figure 6, the shape of the

wedge is easily recognized. It can be assumedthat the prism approximation used in the limitequilibrium method is acceptable. The shape and

the inclination of the failure surface (= 65) isclose to the simplified calculations. Washbourneassumption [11] for sides inclination angle ishighly underestimated. If the length of the trenchdecreases, the displacements at the higher partincrease very slowly during excavation progressafter it reaches 6m (see Figure 7). For a 3 m long

trench, no wedge is observed.

2

2,5

3

3,5

4

4,5

2 2,5 3 3,5 4 4,5

FS - finite element method

FS-limitequilibriu

mm

ethod

810

12

15

Figure 8. A set of points FS finite element vs. FS limitequilibrium method for the same geoengineering data.


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A comparison of results from both methods is

presented in Figure 8. Each concentration ofresults represents different lengths of the trench.

The bottom points represent a 6 m long trenchwhile the top points represent a 3 m trench. Thelimit equilibrium method produces higher factorof safety values than the finite element method.

In the figure, the dashed line shows a perfectcorrelation of results.

Although the limit equilibrium method givesresults in a very short time, one should take intoconsideration the overestimated values of FS.

Another advantage of this method is that aspecialized computation program is notnecessary.

For engineering purposes, the results obtainedfrom the limit equilibrium method can be takeninto consideration only when employing a factor,which would reduce the value of the earth

pressure. This kind of factor is used byPiaskowski and Kowalewski [6] and it is afunction of the length, the depth and the frictionangle.

5 CONCLUSIONS

Short trenches in cohesive soil are under alarge influence of the arching effect and cohesiveforces. The wedge is not observed in the finiteelement method. In the limit equilibrium method

factors of safety are greater for deeper trenches.Curves on the graph intersect.

The failure surface inclination decreases withthe length of the trench. The Coulomb criterionis the lower bound estimation.

FS values are between 2.5 and 4.5 and aremuch higher than expected to fulfill the stabilityconditions.

The limit equilibrium method can be usedunder the condition of employing a factor whichreduces the value of the earth pressure. This kindof factor is used by Piaskowski and Kowalewski[6] and it is a function of the length, the depthand the friction angle.

REFERENCES

[1] W. Brzakala, K. Gorska, On safety of slurrywalltrenches, Studia Geotechnica et Mechanica XXX

No.12 (2008), 199206.

[2] G.M. Filz, T. Adams, R.R. Davidson, Stability of longtrenches in sand supported by bentonitewater slurry,

Journal of Geotechnical and Goenviromental

Engineering130(9) (2006), 915921[3] I. Hanjal, J. Marton, Z. Regele, Construction of slurry

walls, Akad.Kiado, Budapest, 1984.[4] N.R. Morgenstern, J. Amir-Tahmasseb, The stability of

a slurry trench in cohesionless soils, Geotechnique15(4) (1965), 387395.

[5] J.K.T. Nash, G.K. Jones, The support of trenches usingfluid mud, Grouts and Drilling Muds in Engineering

Practice (1963), 177180.

[6] A. Piaskowski, Z. Kowalewski, Application oftixotropic clay suspensions for stability of vertical sides

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[7] E.G Prater, Die Gewlbewirkung der Schlitzwnde,Bauingenieur48 (1973), 125131

[8] A.F. van Tol, V. Veenbergen, J. Maertens, Diaphragmwalls, a reliable solution for deep excavations in urban

areas?. In s.n. (Ed.) DFI and EFFC, London, Deep

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[9] J.S. Tsai, J.C. Chang, Threedimensional stabilityanalysis for slurry trench wall in cohesionless soil,

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[10] C. Veder, Excavation of trenches in the presence ofbentonite suspension for the construction of

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Proceedings of Symposium on Grouts and DrillingMuds in Engineering Practic,. London, (1963), 181

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[11] J. Washbourne, The three dimensional stability analysisof diaphragm wall excavation, Ground Engineering

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Trench Stability in Cohesive Soil