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Technical Note
A comprehensive method for analyzing the effect of geotextile layerson embankment stability
A. Tolooiyan a,*, I. Abustan a, M.R. Selamat a, Sh. Ghaffari b
a School of Civil Engineering, Universiti Sains Malaysia (USM), Engineering Campus, Pulau Pinang, Malaysiab Soil and Water Engineering, P.O. Box 3185838143, Karaj, Tehran, Iran
a r t i c l e i n f o
Article history:
Received 28 March 2008
Received in revised form
20 November 2008
Accepted 21 November 2008
Available online 20 February 2009
Keywords:
Embankment
Water condition
Geotextile
Mathematical model
Stability
Finite element method
a b s t r a c t
Commercial software is used widely in slope stability analyses of reinforced embankments. Almost all of
these programs consider the tensile strength of geotextiles and soilgeotextile interface friction.
However, currently available commercial software generally does not consider the drainage function of
nonwoven geotextile reinforcement. In this paper, a reinforced channel embankment reinforced by
a nonwoven geotextile is analyzed using two methods. The first method only considers the tensile
strength and soilgeotextile interface friction. The second method also considers the drainage function.
In both cases, the reinforced embankment is modeled in rapid drawdown condition since this is one of
the most important conditions with regard to stability of channel embankments. It is shown that for this
type of application, modeling a nonwoven geotextile reinforced embankment using commercial software
which neglects the drainage function of the geotextile may be unrealistic.
2009 Elsevier Ltd. All rights reserved.
1. Introduction
When it happens, embankment collapse can be disastrouscausing serious loss of life, money and time. Reconstructingcollapsed embankments can be very costly and from a purely
economic standpoint, it would be more beneficial to reinforce theembankment so that it does not fail rather than reconstruct.Nowadays advances in technology in material science have
produced geosynthetic materials for usage in various aspects ofcivil engineering.
Geosynthetic materials that are used widely in embankments toincrease stability (Bergado and Teerawattanasuk, 2008; Brianon
and Villard, 2008; Chen et al., 2008; Li and Rowe, 2008; Rowe and
Taechakumthorn, 2008; Sarsby, 2007). Geotextile layers increasethe embankment stability by virtue of two primary functions:
tensile reinforcement (as in the cases cited above) and as a drainageelement reducing pore pressures.
Most analyses of geotextile reinforced embankments considerthe effect of the tensile stiffness of the geotextiles but generally
do not consider the drainage function of nonwoven geotextilereinforcement. While this is suitable for most applications, in thecase of nonwoven geotextile reinforced channel embankments this
may represent a significant oversight. Thus the objective of thispaper is to examine the effect of ignoring and considering thedrainage function for a channel embankment subject to rapid
drawdown.
2. Current numerical procedure
Lemonnier et al. (1998), analyzed the effect of geotextile rein-forcement on the stability of embankments by a mathematicaldisplacement method presented by Gourc et al. (1986), where byonly the tensile strength of the geotextile was taken into consid-
eration in the analysis. Sharma and Bolton (2001), Bergado et al.(2002), and Hinchberger and Rowe (2003), utilized different
commercial and non-commercial FEM models to analyze thestability of geotextile reinforced embankments. In all of theseinvestigations, tensile strength and soilgeotextile interface frictionwere taken into consideration, while ignoring the geotextiledrainage ability. Nagahara et al. (2004) used FEM to analyze the
effect of the drainage ability of geotextiles on stability of embank-ments. Nagahara and colleagues reported that the measured hori-zontal deformation of the case study embankment was much
smaller than estimated by FEM due to the neglecting soilgeo-textile interface friction in their FEM analysis. Iryo and Rowe (2005)firstly used FEM to model the drainage ability of geotextile, then,after estimating the water surface in embankment, they used
* Corresponding author.
E-mail address: [email protected] (A. Tolooiyan).
Contents lists available at ScienceDirect
Geotextiles and Geomembranes
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / g e o t e x m e m
0266-1144/$ see front matter 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.geotexmem.2008.11.013
Geotextiles and Geomembranes 27 (2009) 399405
mailto:[email protected]://www.sciencedirect.com/science/journal/02661144http://www.sciencedirect.com/science/journal/02661144mailto:[email protected]7/28/2019 Geotextiles 5
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a limit equilibrium method to consider the tensile strength of thegeotextile.
The FEM can be used to compute stresses and displacements inearth structures and soil masses. The method is particularly usefulfor soilstructure interaction problems, in which structuralmembers interact with a soil mass (USACE, 1995). In complex
conditions, it is often difficult to anticipate failure modes, particu-larly if reinforcement or structural members such as geotextiles,concrete retaining walls, or sheet piles are included (USACE, 2003).Another important input to the stability analyses for reinforced
slopes is the load in the reinforcement and FEM can provide usefulguidance for establishing the load that will be used ( USACE, 2003).The capabilities of FEM, led to it being used by many researchers toinvestigate the behavior of reinforced embankments e.g. Rowe
(1982, 1984), Rowe and Soderman (1984), Humphrey and Holtz(1989), Hird and Kwok (1989), Rowe and Mylleville (1990), Bergadoet al. (2002), etc. However, Rowe and Mylleville (1994) explainedthat careful consideration must be given to the type of FEM and
constitutive relationships that will be used to model the discretecomponents of the reinforced embankments.
2.1. Serious limitations of mathematical models in modeling
geotextile
While commercial software such as Plaxis Ver.7.2 (1998), MStabVer.9.8 (2004), Geostab 2004 (2004), Stedwin Ver.2.6 (1999) andPcstabl Ver.6 (1999) are widely used to analyze geotextile rein-
forced embankment, those just consider the geotextile tensilestrength and/or soilgeotextile interface friction. In some cases ofreinforced embankment analysis, the results may be unrealisticbecause the drainage function of geotextile is ignored by the
previously mentioned commercial software.
2.2. Equations to evaluate geotextile tensile strength and soil
geotextile interface friction
To evaluate the soilgeotextile interface in FE analysis theMohrCoulomb equation can be used as it is widelyemployed in FEmodeling of soilstructure interfaces. This equation is able toconsider both cohesion and friction angle of interfaces.
The equation for considering geotextile tensile strength is
usually strain energy equation that is shown in Eq. (1).
Pa
ZL0
EA
2
du
dx0
2dx0 (1)
wherePa is the strain energy, EA is the axial rigidity, L is the lengthof geotextile, u is the axial displacement along the geotextile, andx0
is the distance along the geotextile.
2.3. Necessity of equations associated with water in reinforcedembankment components
In analyzing the embankment, one of the most important issues
is pore water pressure since pore water pressure whether positiveor negative has a direct effect on the stability and safety factor ofembankment. Therefore realistic pore water pressure conditionsneed to be considered explicitly in the analysis.
2.4. Water flow in reinforced embankment components
Richards (1931) derived the governing equation for transient
water flow within an unsaturated material from Darcys law andContinuity. For the two dimensional homogeneous anisotropic
material, the equation is as Eq. (2).
kxv
2h
vx2 ky
v2h
vy2
vQ
vt mwgw
vh
vt(2)
where h is the total hydraulic head, kx, ky are the unsaturatedhydraulic conductivities for the x- and y-directions, mw is the slope
of the water volume characteristic curve, gw is the unit weight ofwater, Q is the volumetric water content, and t is the time.
2.5. Water storage in reinforced embankment components
Both the soil and geotextile consists of a collection of solidparticles and interstitial voids. The pore spaces or voids could befilled either with water or air, or with a combination of both. In
saturated materials (soil and/or geotextile), all the voids are filledwith water and the volumetric water content of the materials isequal to the porosity of the soil according to Eq. (3).
Q nS (3)
where Q is the volumetric water content, n is the porosity, and Sisthe degree of saturation (in saturated materials equal to 1.0 or100%)
In unsaturated materials, the volume of water stored withinthe voids depends on the negative water pressure (suction). Thewater content is not constant and therefore so a function isrequired to describe how the water contents changes with
different stresses in the materials. The volumetric water contentfunction describes the capability of materials to store water underchanges in pore water pressures (Krahn, 2004). A typical functionof volumetric water content and pore water pressure is shown in
Fig. 1.The volumetric water content function describes what portion
(or volume) of the voids remains water-filled as the materials
drains. The three main features that characterize the volumetricwater content function are the air-entry value (AEV), the slope ofthe function for both the positive and negative pore water pressure
(mw), and the residual water content (Qr). The air-entry value (AEV)corresponds to the value of negative pore water pressure when thelargest voids or pores begin to drain freely. It is a function of themaximum pore size in a soil and is also influenced by the pore-sizedistribution within a soil. Soils with large and uniformly shaped
pores have relatively low AEVs (Krahn, 2004). Another key featureof the volumetric water content function is the residual volumetricwater content (Qr), which represents the volumetric water contentof a soil where a further increase in negative pore water pressure
does not produce significant changes in water content (Krahn,2004).
Fig. 1. Volumetric water content (storage) function (Krahn, 2004).
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As shown in Fig. 1, the slope of the volumetric water contentversus pore water pressure has different slopes (mw) in positive and
negative pore water pressure regions. The value of the slope in thepositive pressure range is the coefficient of volume compressibility,and in physical terms, it describes how much a saturated soilvolumewill swell or shrinkfor a given change in pore pressure. This
coefficient can be back-calculated from consolidation test data(Krahn, 2004).
mw 1
E(4)
where E is the elasticity modulus.
Van Genuchtens (1980) predictive method for measurement ofa volumetric water content function in negative pore water pres-sure has been used in many studies and its validity has beenexamined for a wide range of soils. Iryo and Rowe (2004) also used
this model to investigate infiltration into a soil column containinga nonwoven geotextile layer and found that it works well formodeling the unsaturated reaction of the nonwoven geotextile.Therefore, this method could be used in modeling to describe the
hydraulic properties of both soil and the geotextile.
Q Qr Qs Qrh
1 Ja
nim (5)
where Q is the volumetric water content, Qr is the residual volu-metric water content,Qs is the saturated volumetric water content,
J is the negative pore water pressure, and a, n, m are the curvefitting parameters.
Rawls et al. (1982), and Carsel and Parrish (1988) conductedsubstantial experimental work and obtained the Van Genuchten
model parameters for different soil textural groups according to theUSDA soil classification system. As a result of this work, laboratoryparticle size analysis can be directly related to the modeledparameters. Further, for geotextile, the Van Genuchten model
parameters could be taken from the typical values evaluated frompublished data compiled by Iryo and Rowe (2003).
2.6. Hydraulic conductivity in reinforced embankment components
In a saturated soil, allthe pore spacesbetween the solid particlesare filled with water. Once the air-entry value is exceeded, airenters the largest pores and the air-filled pores become non-
conductive conduits to flow and increase the tortuosity of the flowpath. As a result, the ability of the soil and geotextile to transportwater (the hydraulic conductivity) decreases. As pore water pres-sures become increasingly more negative, more pores become air-
filled and the hydraulic conductivity decreases further. By thisdescription, it is clear that the ability of water to flow througha profile depends on how much water is present in the soil, which is
represented by the volumetric water content function (Krahn,2004).
The hydraulic conductivity function for an unsaturated soil canbe developed using Van Genuchten method. Van Genuchten (1980)offered the following closed form equation to describe the
hydraulic conductivity of soil as a function of suction, as seen inEq. (6).
kw ks
h1
aJn1
1
aJn
mi21 aJn
m2
(6)
where ks is the saturated hydraulic conductivity, a, n, m are thecurve fitting parameters, J is the required suction range, and n is
equal to 1/(1m).
From Eq. (6), the hydraulic conductivity function of soil orgeotextile could be estimated once the saturated conductivity and
the two curve fitting parameters, a and m are known.Van Genuchten showed that the curve fitting parameters could
be estimated graphically based on the volumetric water contentfunction of the soil and suggested that the best point to evaluate
these parameters is the halfway point between the residual andsaturated water content of the volumetric watercontent function. IfQp be the volumetric water content at the halfway point of thevolumetric water content function, and Jp be the suction at thesame point, then the slope Sp of the function could be calculated as
Eq. (7) (Krahn, 2004).
Sp 1
Qs Qr
dQpdlog Jp (7)
Van Genuchten proposed Eqs. (8) and (9) to estimate the parame-ters m and a, when Sp is calculated.
m 1 exp0:8Sp
(8)
where Sp is between 0 and 1.
m 1 0:5755
Sp
0:1
S2p
0:025
S3p(9)
where Sp > 1.
After calculating m, a could be estimated by Eq. (10).
a 1
J
2
1m 1
1m(10)
2.7. Analysis associated with drainage ability of geotextile
By applying Eqs. (2)(10) to both the soil and geotextile, it ispossible to obtain water storage and hydraulic conductivity inembankment components and subsequently estimate the mannerof water flow in reinforced embankments. It follows that the effect
of geotextiles as a drain layer could be taken into consideration inanalysis of reinforced embankment. In this research, the finiteelement computer program SEEP/W Ver. 5.18 (GEO-SLOPE Inter-
national Ltd., 2002a) was used to solve Eqs. (2)(10).
Table 1
Specifications of nonwoven geotextile.
Material property Value
Thickness (mm) 2.5
Unit weight (kN/m3) 1.11
Hydraulic conductivity in plane direction (m/s) 2.72E3
Hydra uli c conduc ti vi ty in c ross plan e dir ection (m/s) 7E2
Elasticity modulus (kPa) 33,000
Maximum tensile strength (kN/m) 21
Table 2
Soil characteristics and soilgeotextile interface specifications.
Specification Value
Water content (percentage) 32
Dry unit weight (g/cm3) 1.43
Poisson Ratio 0.38
Specific gravity 2.71
Hydraulic conductivity (m/s) 4.29E7
Elasticity modulus (kPa) 12,500
Soil cohesion in saturated condition (kPa) 10.571
Soil f riction angle in saturat ed condition (degree) 50.06
Soilgeotextile interface cohesion in saturated condition (kPa) 15.491
Soilgeotextile interface friction angle in saturated condition (degree) 52.21
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2.8. Analysis associated with geotextile tensile strength
and soilgeotextile interface friction
In this research, Eq. (1) was used in a linear-elastic mode tomodel the effect of tensile strength of geotextile. The finite elementcomputer program SIGMA/W Ver. 5.18 (GEO-SLOPE InternationalLtd., 2002b) was used to solve Eq. (1). To estimate the elasticity
modulus of a particular kind of nonwoven geotextile, appropriatetests were done according to ASTM D4632 (2003) in the compositematerial laboratory of USM. The specifications of this particular
nonwoven geotextile are mentioned in Table 1.To analyze the strain, stress and shape change in embankment
components, an elasticplastic model was used by utilizing SIGMA/W Ver. 518. Also, this model can consider the soilgeotextile
Fig. 2. Situation of the geotextile layers in reinforced channel embankment.
Fig. 3. Slip surface of reinforced embankment during rapid drawdown, analyzed by proposed method.
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interface friction by very fine meshes in the soilgeotextile inter-face. The friction angle and cohesion between the soil and geo-textile were determined using direct shear tests in the geotechnical
laboratory of USM. The geotextile layers were cut to square piecesof 100 mm by 100 mm and then it was glued using epoxy glue tothe top of a piece of hard wood having the same dimensions(100 mm by 100 mm). This procedure was used previously by
Mahmood et al. (2002). The soil used was classified as Silt Loamand Lean Clay with Sand in USDA and Unified Soil ClassificationSystem, respectively. Soilgeotextile interface specifications andcharacteristics of the soil are mentioned in Table 2.
3. Modeling reinforced channel embankment
Different size of three node triangular meshes with three inte-gration points was employed to model the embankment, althoughground surface and the geotextile layers were formed by very finefour node quadrilateral meshes with four integration points. In
seepage analysis, the left and right boundaries were modeled by
infinite elements however in stressstrain analysis the embank-ment bounded with zero displacement along edges. As a multi-
joined analysis, the stressstrain distribution FEM analysis was
conducted by SIGMA/W. In parallel, the FEM analysis was con-ducted by SEEP/W to model pore water pressure distribution in theembankment material.Finally, the FE results of SIGMA/Wand SEEP/
W were jointly imported into the SLOPE/W (GEO-SLOPE Interna-tional Ltd., 2002c) to analyze the embankment stability and safetyfactor.
To estimate the effect of reinforcement and to compare between
the conventional analysis and the proposed analysis method,a channel embankment was simulated using three methods duringrapid drawdown condition. Firstly, the channel embankment wasanalyzed without reinforcement (non-reinforced embankment).
Secondly, with the same rapid drawdown condition, a geotextile
reinforced embankment was analyzed by using the conventional
method and the geotextile tensile strength and soilgeotextileinterface friction were considered together. Thirdly, with the samerapid drawdown condition and reinforcing method, the reinforcedembankment was analyzed by using the proposed method that
considered geotextile tensile strength, soilgeotextile interfacefriction, and the drainage ability of the geotextile, together. Beforerapid drawdown, the water level in the channel was 3 m. In 8 h thewater level was dropped down about 2.5 m and the water level in
the channel reached 0.5 m. In all of the three mentioned analyses,
Fig. 4. Water table at 3 h and 8 h after start of rapid drawdown, analyzed by conventional method (a) and proposed method (b).
Fig. 5. Maximum effective stress at 3 h after start of rapid drawdown, analyzed by
conventional method (a) and proposed method (b).
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embankment stability was calculated at 3 h after the start of rapiddrawdown. USACE (2003) emphasize that the minimum required
safety factor of earth slope should be 1.30 during rapid drawdowncondition.
3.1. Stability safety factor of reinforced and non-reinforced
embankment
Analysis of the non-reinforced embankment gives a stabilitysafety factor equal to 1.26. According to USACE (2003), thisembankment might be unstable during rapid drawdown condition,indicating reinforcement is needed to prevent instability.
To reinforce the embankment, as shown in Fig. 2, three layers ofneedle-punched nonwoven geotextile with 1.5 m length and 1 mdistance in between were laid inside the embankment. The
reinforced embankment was analyzed by the aforementionedconventional method under the rapid drawdown condition.Analysis of the reinforced embankment gave a stabilitysafety factorequal to1.29. The increase in safety factor in the reinforcedembankment is due to the effect of tensile strength and soilgeo-
textile interface friction.To find the more complete effect of geotextile, the reinforced
embankment was analyzed by the proposed method with the samerapid drawdown condition. A safety factor of 1.33 was obtained
from the analysis which considered the drainage property of thegeotextile. This factorof safety meets the USACE (2003) guideline inembankment stability which requires a minimum value of 1.3 forrapid drawdown conditions. The slip surface of the reinforced
embankment, analyzed by the proposed method is shown in Fig. 3.
Fig. 4(a) and (b) show the water table at 3 h and 8 h after start ofrapid drawdown analyzed by conventional and proposed method,respectively. Comparing the water table in Fig. 4(a) and (b), shows
that geotextile layers drain the embankment internal water duringthe rapid drawdown and the embankment becoming lighter. Fig. 5shows the result of stressstrain analysis at 3 h after start of rapid
drawdown. Different effective stress at the right side of embank-ment is due to the different pore water pressure condition analyzedby the conventional and proposed method.
Fig. 6 shows the safety factor of non-reinforced embankment
and reinforced embankment analyzed by conventional andproposed method. Although all the reinforcing and rapid draw-down conditions are exactly the same in both analyses, theproposed complete method offered the highest safety factor. This
highlights the benefit of accurately modeling both the
reinforcement and drainage functions of needle-punchednonwoven geotextile as demonstrated in the proposed method.
4. Conclusion and results
Conventionalanalysesof a needle-punched nonwoven geotextile
reinforced channel embankment which only considers the effect ofthe tensile stiffness and strength of the geotextile on embankmentstabilityunderestimated the stabilitycompared to analysesthat alsoconsidered the drainage function of the geotextile. The proposed
analysis method which considers both functions provides a morerealistic methods of assessing the stability of needle-punchednonwoven geotextile reinforced channel embankments subjectedto
rapid drawdown.
Acknowledgements
The first author would like to express his sincere appreciation toDavid Igoe, Tom Doyle and Paul Doherty PhD researchers fromUniversity College Dublin for their help in reviewing this paper.
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Nomenclature
Pa: strain energyE: elasticity modulusA: cross section areaEA: axial rigidityL: length of geotextileu: axial displacement along the geotextile
x0: distance along the geotextileh: total hydraulic headkw: unsaturated hydraulic conductivitykx: unsaturated hydraulic conductivities in x-directionky: unsaturated hydraulic conductivities in y-directionmw: slope of the water volume characteristic curvegw: unit weight of waterQ: volumetric water contentt: timen: porosityS: degree of saturation
AEV: air-entry valueQr: residual water contentQs: saturated volumetric water contentJ: negative pore water pressurea, n, m: curve fitting parametersks: saturated hydraulic conductivityQp: volumetric water content at the halfway point of the volumetric water content
function
Jp: suction at the halfway point of the volumetric water content functionSp: slope of volumetric water content function at the halfway point
A. Tolooiyan et al. / Geotextiles and Geomembranes 27 (2009) 399405 405
http://www.ejge.com/2000/Ppr0013/Ppr0013.htmhttp://www.ejge.com/2000/Ppr0013/Ppr0013.htm