Infiltration into an embankment reinforced by nonwoven ...

Infiltration into an embankment reinforced bynonwoven geotextiles

T. Iryo and R.K. Rowe

Abstract: The hydraulic behaviour of permeable geosynthetics within unsaturated embankments subjected to infiltrationis examined using the finite element method. The van Genuchten – Mualem model is employed to evaluate unsaturatedhydraulic characteristics for both the soil and nonwoven geotextile. Using pore-water pressures obtained from the finiteelement analysis, stability analyses are conducted for the embankments, and the contribution of the nonwoven geo-textile to stability is evaluated with reference to the observed performance of instrumented embankments. A numericalstudy is also conducted to examine the effect of geotextile configuration on the performance of reinforced embank-ments subject to infiltration. This study shows that nonwoven geotextiles may retard water flow in situations where thepore pressure is negative, whereas they act as a drainage material in situations where pore pressures are positive. It isalso shown that the contribution of the nonwoven geotextile to the stability of the embankment as a drainage materialis much less substantial than its role as a reinforcing material.

Key words: nonwoven geotextile, drainage, unsaturated, embankment, stability.

Résumé : On examine au moyen de la méthode des éléments finis le comportement hydraulique des géosynthétiquesperméables installés à l’intérieur de remblais non saturés assujettis à l’infiltration. Le modéle de van Genuchten –Mualem est utilisé pour évaluer les caractéristiques hydrauliques non saturées tant pour le sol que pour le géotextilenon tissé. En utilisant les pressions interstitielles obtenues par l’analyse en éléments finis, on a réalisé des analyses destabilité pour les remblais, et évalué la contribution à la stabilité du géotextile non tissé par rapport à la performancedes remblais instrumentés. On a aussi réalisé une étude numérique pour examiner l’effet de la configuration du géotex-tile sur la performance des remblais armés sujets à l’infiltration. Cette étude montre que les géotextiles non tissés peu-vent retarder l’infiltration d’eau dans les situations où les pressions interstitielles sont positives. On montre égalementque, comme matériau de drainage, la contribution du géotextile non tissé à la stabilité du remblai est beaucoup moinssubstantielle que son rôle comme matériau d’armature.

Mots clés : géotextile non tissé, drainage, non saturé, remblai, stabilité.

[Traduit par la Rédaction] Iryo and Rowe 1159

Introduction

Nonwoven geotextiles have been used to replace conven-tional granular material to provide drainage within soilstructures. In this application they are expected to contributeto the stability of the soil structure by facilitating the dissi-pation of excess pore pressures during construction and min-imizing the build up of pore pressure caused by subsequentinfiltration events. There have been a number of full scalestudies on the effectiveness of geosynthetic drainage layersin embankments, as described in the following paragraphs.

Mitchell and Zornberg (1995) reviewed 11 field cases ofsoil structures where poorly draining fill material was rein-forced with nonwoven geotextiles. It was noted that the per-meable geosynthetics increased the stability of soilstructures by providing both mechanical reinforcing and in-creasing soil strength by facilitating dissipation of excess

pore-water pressures in the fill material. Miki (1997) exam-ined the performance of nonwoven geotextiles as a drainagematerial within three embankments. The first embankmentwas constructed using volcanic ash clay, the second used thesame volcanic ash clay mixed with sandy silt, and the thirdgravelly clay and sand soil containing 30–50% fines content.Several layers of nonwoven geotextiles were installed withinthe fill material. For these cases, pore pressure dissipationoccurred during the construction period, and consolidationsettlement of fill material was completed in a relatively shortperiod of time. It was inferred that the nonwoven geotextilesworked effectively as a drainage material within the em-bankment.

Kamon et al. (2001) examined three full-scale clayey soilembankments reinforced with a variety of geosynthetic hori-zontal drains (GHDs) including a composite nonwovengeotextile reinforced with a high tensile strength yarn. It was

Can. Geotech. J. 42: 1145–1159 (2005) doi: 10.1139/T05-035 © 2005 NRC Canada

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Received 6 May 2004. Accepted 1 March 2005. Published on the NRC Research Press Web site at http://cgj.nrc.ca on 2 September2005.

T. Iryo. SNC-Lavalin Engineers & Constructors Inc., 2200 Lake Shore Blvd. West, Toronto, ON M8V 1A4, Canada.R.K. Rowe.1 GeoEngineering Centre at Queen’s-RMC, Department of Civil Engineering, Queen’s University, Kingston,ON K7L 3N6, Canada.

1Corresponding author (e-mail: [email protected]).

found that two embankments remained stable for severalyears after the completion of construction, while one em-bankment failed under surcharge loading. GHDs were foundto be effective at promoting consolidation and increasing theshear strength of the fill material.

Tan et al. (2001) conducted a laboratory test on a 2D soil–geosynthetics layered system. A horizontal layer of non-woven geotextile reinforced with a grid network of polyesteryarn was laid between two layers of residual soil and a sur-charge loading was applied to the top of the upper layer ofsoil. It was found that the nonwoven geotextile effectivelyreduced excess pore-water pressures caused by the loading.The dissipation of pore-water pressure was evaluated usingTerzhaghi’s consolidation theory.

The studies described above related to the hydraulic be-haviour of permeable geosynthetics within soil experiencingpositive pore-water pressures. However, in many practicalapplications the water content of the fill material is relativelylow and pore-water pressures are negative during and fol-lowing construction. Thus it is also necessary to examine thehydraulic behaviour of permeable geosynthetics within un-saturated soil when subjected to infiltration from a watersource such as rainfall. Nishigaki et al. (1993) conducted ex-periments to examine the hydraulic behaviour of geotextilesunder infiltration loading. A geotextile layer was placed atan inclination of 12° within a fine-grained soil, and artificialrainfall was applied to part of the top surface. The infiltratedwater accumulated above the geotextile layer, and the wet-ting front advanced along the geotextile layer, but it did notcross the geotextile layer, until the wetting front reached theend of the geotextile layer. It was concluded that seepagewater did not drain from the geotextile layer but, rather,from the soil immediately above it. This is very similar tothe hydraulic behaviour reported by Miyazaki (1988) of asandy loam containing a thin layer of gravel. Several differ-ent angles of inclination of the gravel layer were examined,and Miyazaki (1988) found that the advancement of the wet-ting front was stopped at the interface between the uppersandy loam and the gravel layer, and that water then flowedalong the interface.

Iryo and Rowe (2003) compiled published data on unsatu-rated hydraulic functions of nonwoven geotextiles (watercharacteristic curve and hydraulic conductivity function) andreported that these geotextiles had characteristics similar tothose of coarse materials, such as gravel. This suggests asimilarity in behaviour for infiltration into soil–geosyntheticslayered systems to that in fine–coarse soil layered systems,and implies that the study of the hydraulic behaviour of lay-ered soil systems may be helpful in understanding thehydraulic behaviour of soil–geosynthetics layered systems.Iryo and Rowe (2004) numerically examined infiltration intoa 1D soil column containing a nonwoven geotextile layer. Itwas reported that advancement of the wetting front from thetop of the column was temporally halted, and a higher watercontent developed above the geotextile layer during theperiod that the wetting front was halted. This raises the pos-sibility that geotextiles may retard water flow within unsatu-rated soil.

In a cooperative research effort the Public Works Re-search Institute of Japan (PWRI et al. 1988) examined theeffect of artificial rainfall on the performance of one unrein-

forced embankment and three embankments reinforced witha variety of nonwoven geotextile configurations. It was re-ported that failure of the reinforced embankments startedfrom erosion of the slope surface caused by higher watercontent in the vicinity of the geotextiles. However, thesestudies did not clearly define the role of the geotextile layersin the failure of these sand embankments.

The objective of this paper is to examine the hydraulic be-haviour of permeable geosynthetics, particularly nonwovengeotextiles, within unsaturated embankments subjected to in-filtration. Firstly, the infiltration into reinforced embank-ments conducted in PWRI et al. (1988) is numericallysimulated using the finite element method. The vanGenuchten – Mualem model (van Genuchten 1980) is em-ployed to evaluate unsaturated hydraulic characteristics forboth the soil and nonwoven geotextile. The suitability ofsuch a numerical procedure for simulating infiltration into a2D soil-nonwoven geotextile layered system is examined,and the hydraulic behaviour of the embankments is dis-cussed. Secondly, using pore-water pressures obtained fromthe finite element analysis, stability analyses are conductedfor the embankments, and the contribution of the nonwovengeotextile to stability is evaluated. Finally, a numerical studyexamines the effect of geotextile configuration on the perfor-mance of reinforced embankments subject to infiltration.The results of the numerical investigation are then used toprovide insight into the use of geotextile layers.

Numerical simulation of infiltration intoembankment reinforced with nonwovengeotextile

Experiment outlineTo examine hydraulic behaviour under rainfall loading

one unreinforced embankment (embankment 1) and three re-inforced embankments (embankments 2, 3, and 4) were con-structed by PWRI et al. (1988) and artificial rainfall wasapplied until all embankments failed. The embankmentswere 3 m high, 6 m long, and 4 m wide with a slope of0.7H:1V. Sandy soil (Tables 1 and 2) was used as a fill mate-rial. Spunbond nonwoven geotextiles (Table 3) were used asa reinforcing drainage material. Three layers of nonwoven

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1146 Can. Geotech. J. Vol. 42, 2005

Gravel content (d ≥ 2 mm), (%) 1Sand content (2 > d ≥ 0.074 mm), (%) 91Fine content (d < 0.074 mm), (%) 8Gravimetric water content, w (%) 24Unit weight of soil, γ t (kN/m3) 17.5a

Porosity, np 0.465a

Note: d, soil particle diameter.aDeduced from PWRI et al. (1988).

Table 1. Basic properties of sand (modified from PWRI et al.1988).

Saturated hydraulic conductivity, ksat (m/s) 1.3×10–5

Friction angle, φ′ (°) 39

Cohesion in terms of effective stress, c′ (kPa) 0

Table 2. Parameters used in the analysis of the embankment.

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Iryo and Rowe 1147

geotextiles were placed with a vertical spacing of 0.75 mand configurations as shown in Fig. 1. A 1.5 m long geo-textile layer was also placed at the toe of each embankmentto avoid the development of excess pore pressures at thebase. As shown in Fig. 1, embankment 2 was reinforcedwith geotextiles of equal length. Embankment 3 was rein-forced with a 4 m long geotextile in the upper layer and 1 mlong geotextiles in the middle and lower layers. Embank-ment 4 was reinforced with 1 m long geotextiles in the upperand middle layers and a 4 m long geotextile in the lowerlayer. After the embankments were constructed, artificialrainfall with an intensity of 12.7 mm/h was applied to theslope and the top of the embankments. The rain was halted

for about 1 h after the failure of embankment 1 and forabout 13 h after the failure of embankment 3.

The cumulative rainfall, R was reported to be 90 mm atfailure for embankment 1, 132 mm for embankment 3,217 mm for embankment 4, and 231 mm for embankment 2.The failure of embankment 1 occurred suddenly over the en-tire slope surface, while the other embankment failures de-veloped from partial erosion on the slope surface.

Outline of numerical simulation

Numerical procedureRichards (1931) derived the governing equation for tran-

Mass per unit area, ma (g/m2) 310

Thickness of geotextile, tgeotextile (mm) 3

Porosity, np 0.92a

Saturated hydraulic conductivity in plane direction, ksat geotextile plane (m/s) 2.3×10–2

Saturated hydraulic conductivity in cross plane direction, ksat geotextile cross (m/s) 3.5×10–3

Tensile strength in machine direction (kN/m) 21.6Tensile strength in cross to machine direction (kN/m) 17.2

aEstimated value assuming unit weight of polyester γpolyester = 12.75 kN/m3.

Table 3. Basic properties of geotextile (PWRI et al. 1988).

Fig. 1. Outline of infiltration experiments modelled.

sient water flow within an unsaturated material fromDarcy’s law and continuity. For the 2D homogeneousanisotropic material, the equation becomes

[1] kh

xk

hy t

mht

x y∂∂

∂∂

∂θ∂

γ ∂∂

2

2

2

2+ = = w w

where h is the total hydraulic head; kx and ky are the unsatu-rated hydraulic conductivities for the x- and y-directions, re-spectively; mw is the coefficient of water volume change(slope of the water characteristic curve); γw is the unitweight of water; θ is the volumetric water content; and t isthe time.

The parameters, mw and kx, ky in eq. [1] are material spe-cific and are functions of pore pressure. Thus the functionsfor hydraulic properties, water characteristic curve (volumetricwater content versus pore pressure) and hydraulic conductiv-ity function (hydraulic conductivity versus pore pressure)must be specified to solve eq. [1]. The van Genuchten –Mualem model (van Genuchten 1980) has been employed inmany studies, and its validity has been examined for a widerange of soils. Iryo and Rowe (2004) also used this model tostudy infiltration into a 1D soil column containing a non-woven geotextile layer and found that it worked well formodelling the unsaturated response of the geotextile. Thusthis model was used in the present study to characterize thehydraulic properties of both the soil and the geotextiles.Using the van Genuchten – Mualem model, the water char-acteristic curve is given by eq. [2], and the hydraulic con-ductivity function is given by eq. [3]. Based on vanGenuchten (1980), m is assumed to be 1 – 1/n.

[2] Θ = −−

θ θθ θ

αγ

r

s r

w

=

+⎛

⎝⎜

⎞

⎠⎟

⎡

⎣

⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥

1

1s

n

m

where Θ is the normalized water content; θ is the water con-tent at a given suction; θs is the saturated water content; θr isthe residual water content; s is the suction; and α, n, and mare the fitting parameters.

[3] kkk

m mr

sat

( )( )

[ ( ) ]/ /Θ Θ Θ Θ= = − −1 2 1 21 1

where kr is the relative hydraulic conductivity; k(Θ) is thehydraulic conductivity at a given Θ; and ksat is the saturatedhydraulic conductivity.

The finite element computer program SEEP/W Ver. 5(GEO-SLOPE International Ltd. 2001a) was used to solveeq. [1]. Ju and Kung (1997) have shown that to obtain a sta-ble numerical solution when solving Richard’s equation us-ing linear finite elements, it is necessary to employ acapacitance matrix established with a lumped formulation.This approach was adopted (using SEEP/W Ver.5), and theembankments were modeled using 20 524 three noded trian-gular elements arranged as shown in Fig. 1. A series of nu-merical experiments with different mesh configurations andtime steps were used to established a suitable numericalscheme. Based on this study, the 3 mm thick nonwoven

geotextile was modeled using 6 three noded triangularelements across the thickness of the geotextile. The time in-crement was automatically adjusted between 0.1 s and 100 sas needed to achieve convergence. The reported intensity ofrainfall, 12.7 mm/h per horizontal m (per m width in this 2Danalysis), was applied to the top and side slope of the em-bankment as a specified flux boundary condition. To allowsimulation of seepage from the embankment, once the porepressure became positive at any nodes on the top surface andside slope surface the boundary condition was changed fromspecified flux to specified pressure head, hp = 0 conditionand the simulation was repeated until all boundary nodeshad a pore pressure less than or equal to 0 kPa. The embank-ments were constructed on a solid concrete slab (PWRI et al.1988) and so the base was taken to be a zero flux boundary.It is noted that the infiltration pattern around the toe and theeffectiveness of the geotextile there might be different if thefoundation were a permeable material. However, for the pur-poses of modelling the PWRI embankments, this boundaryis considered to be the most realistic. PWRI et al. (1988) re-ported that about 10 cm of geotextile extended out from theslope surfaces. Thus, unlike most parts of the slope surface,the slope surface 10 cm below each geotextile layer wasshielded from infiltration, and along these segments theboundary was treated as a no flux condition while pore pres-sure was negative and hp = 0 when the pore pressure becamepositive (Fig. 2).

The soil was placed with a controlled water content and itwas considered reasonable to assume that pore-water pres-sure was relatively uniform (with some statistical variability)at the end of construction. Thus a uniform distribution ofpore-water pressure of –3.5 kPa was assumed based on theaverage value reported for all of the embankments.

Material parametersSince PWRI et al. (1988) did not provide any information

regarding the unsaturated hydraulic properties of the soil, thevan Genuchten – Mualem model parameters for the soilwere obtained based on consideration of typical publishedvalues combined with a parametric study of the infiltrationinto unreinforced embankment 1. The values used for thisstudy were: n = 1.5, 2.0, 2.5; α α γ′ = =( / )w 0.25, 0.4,0.8 (1/kPa); ksat = 1.0, 1.3, 2.0, 4.0, 6.0, 10.0 × 10–5 (m/s);and Sr max = 90, 92, 95, and 100%. Different combinationsresulted in quite different behaviour (e.g., the shape of thewetting front, the initial water content, and the rate of ad-vance of the wetting front varied). The most significant pa-rameters were n and α. A suitable combination was selectedbased on a comparison of the behaviour and volumetric wa-ter content profiles to the observed values. The saturated wa-ter content, θs was evaluated from the reported properties.The water characteristic curve and the hydraulic conductiv-ity function for the soil deduced from this study are shownin Figs. 3a and 3b, respectively. Profiles showing the degreeof saturation for the soil, Sr at the failure of embankment 1are shown in Fig. 4. The water content within the embank-ments was measured using the radio isotope method alongthree vertical observation lines. The left vertical axis of eachgraph in Fig. 4a is located at the line where the measure-ments were made. There was a fair agreement between thenumerical and the experimental results and adjustment of pa-

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rameters did not provide any improved fit between the two.Thus the parameters in Figs. 3a and 3b were used for furtherstudy.

The saturated hydraulic conductivity of the nonwovengeotextile, ksat geotextile was reported and its θs was estimatedfrom the other reported physical properties of the geotextile.The other van Genuchten – Mualem model parameters forthe nonwoven geotextile were chosen from the typical valuesevaluated from published data compiled by Iryo and Rowe(2003). The parameters and the functions obtained withthese parameters are shown in Figs. 3a and 3b. Both the wa-ter characteristics curve and the hydraulic conductivity func-tion of the nonwoven geotextile are steeper than those of thesoil, and the hydraulic conductivity functions of the soil andthe nonwoven geotextile cross at a suction of about 1 kPa.This implies that the nonwoven geotextile acts as a less per-meable material than the soil for suctions greater than 1 kPa.

Comparison between experimental result and numericalsimulation

The advancement of the infiltration front into embank-ment 2 is shown in Fig. 5. The change of water content ofthe soil is illustrated in terms of the degree of saturation, Srat critical locations. The distribution of Sr over the entireembankment obtained from the numerical simulation is alsoshown by contour lines. The profiles and the distribution ofthe degree of saturation after cumulative rainfall R = 90 mm(at the point where failure of embankment 1 occurred) areshown in Fig. 5a, and those after 132 mm (failure of em-

bankment 3) are shown in Fig. 5b. The experimental resultsindicate that a higher Sr area occurred near the slope faceand developed inward with increasing cumulative rainfall R.The numerical results showed the same trend, and they werein fair agreement with the experimental results. The Sr pro-file from the numerical simulation showed that the geo-textiles caused a discontinuity in water content distribution.This was most pronounced at locations not reached by thewetting front (e.g., the middle and lower layers at R = 90and 132 mm) where the water content of the soil increasedimmediately above the geotextile layer and decreasedslightly below it (from the initial value of about 63%). Thusthe nonwoven geotextile acted as an impermeable layer.This occurred because the hydraulic conductivity of the geo-textile, kgeotextile was less than that of the soil, ksoil at the neg-ative pore pressure of soil around the geotextile layers. Evenat locations where the wetting front had already passed,there was slight discontinuity in the Sr of the soil around thegeotextile (e.g., the upper layer at R = 132 mm). This is be-cause the specified boundary flux, q was smaller than ksat soil,thus the pore pressure of the soil remained negative, and

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Iryo and Rowe 1149

Fig. 2. Boundary conditions on slope. Fig. 3. Hydraulic properties for sand and geotextile. (a) Watercharacteristic curves. (b) Hydraulic conductivity functions.

kgeotextile was still smaller than ksoil at this negative value.Thus the geotextile acted as a less permeable material thanthe soil even after the wetting front had passed the geotextilelayer. A similar trend was observed and discussed for infil-tration into a 1D soil column containing a nonwoven geo-textile layer (Iryo and Rowe 2004). While the numericalsimulation showed discontinuities in Sr profiles around thegeotextile layers, the experimental results showed less varia-tion in Sr. It is reported that the radio isotope method (whichis based on neutron-scattering and thermalization process(Gardner 1986)) works on the basis of volume averaging andhas limited resolution in water content measurements. Thismay be the reason why the experimental results did not ex-hibit discontinuities of Sr. More frequent measurement ofwater content with a technique that provides finer resolutionof the data is recommended for further investigation of thehydraulic interaction between soil and geotextiles.

Referring to the contour lines shown in Fig. 5a-ii and 5b-ii, the Sr = 70% line developed above the geotextile along itslength and a 90% saturation zone appeared close to the slope

surface. On the other hand, immediately below the geo-textile layer there was little to no increase in water content,and the distribution of Sr was discontinuous at each geo-textile layer. Infiltration from the slope surface developedabove the geotextile layer. The hydraulic behaviour of thisreinforced embankment can be explained by the differenceof the hydraulic conductivity functions for the soil and thegeotextile. At the beginning of infiltration, kgeotextile is sev-eral orders of magnitude smaller than ksoil. Thus the geo-textile acts as an impermeable material. When waterinfiltrating through the slope surface reaches the geotextilelayer, a portion of the infiltrated water flows inward alongthe geotextile layer at the interface between the soil and thegeotextile. As a consequence, a higher Sr develops above thegeotextile layer. As infiltration continues, water migratesinto the geotextile and kgeotextile increases. Eventually waterstarts flowing across the geotextile and the Sr of the soil im-mediately below the geotextile begins to increase. Such anincrease of Sr was found near the slope for each geotextilelayer. This behaviour was also observed for embankments 3

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Fig. 4. Advancing of infiltration for embankment 1 at failure (observed values by PWRI et al. (1988) and numerical results from thepresent analysis). (a) Profiles of Sr. (b) Distribution of Sr.

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Iryo and Rowe 1151

and 4, as shown in Figs. 6 and 7, respectively. This mecha-nism resulted in increased water content (higher Sr) along al-most the entire length of the upper geotextile layer forembankment 3.

It was reported that the failures of the reinforced embank-ments started from slope surface erosion in the vicinity ofthe geotextiles (PWRI et al. 1988). The results of the numer-ical simulation showing higher Sr above the geotextile layerare consistent with this observation. Thus, consistent withthe finding of Nishigaki et al. (1993), it appears that thenonwoven geotextile did not provide drainage as originallyexpected in this experiment. This suggests that proper treat-ment of the water infiltrating from the slope surface is nec-essary to avoid such a failure.

Stability analysis considering infiltration

In this section, the contribution of nonwoven geotextilesto the stability of the embankments studied in the previoussection is examined. The stability analysis is conducted us-ing the limit equilibrium method considering the tensileforce developed in nonwoven geotextiles and the pore pres-sure distribution within the soil, including negative values.First, shear strength parameters of the soil were obtainedfrom a back analysis for embankment 1. Then the stabilityof reinforced embankments is studied and the effectivenessof nonwoven geotextiles is discussed.

Method and strength parametersThe limit equilibrium method employed in this study was

the general limit equilibrium method (GLE) from Fredlundand Krahn (1977), wherein the factor of safety (F.S.) withrespect to horizontal interslice forces is equal to that with re-spect to moment equilibrium. The computer programSLOPE/W Ver.5 (GEO-SLOPE International Ltd. 2001b)was used for an effective stress analysis (considering nega-tive pore pressures). The shear strength of unsaturated soil isgiven by Fredlund et al. (1978):

[4] τ = c′ + (σ – ua) tan φ′ + (ua – uw) tan φb

where τ is the shear strength; c′ is the effective cohesion pa-rameter; σ is the total normal stress; ua is the pore-air pres-sure; uw is the pore-water pressure; φ′ is the friction anglewith respect to changes in (σ – uw) when (ua – uw) is heldconstant; and φb is the friction angle with respect to changesin (ua – uw) when (σ – ua) is held constant.

It has been reported that φb is equal to φ′ at saturation andthen decreases with an increase in suction (Fredlund et al.1996). Vanapalli et al. (1996) noted that tan φb in eq. [4]may be approximated by Θ tan φ′. In this study, the shearstrength of the fill material was estimated using this relation-ship with a certain value of Θ. As a result, the required shearstrength parameters were reduced to c′ and φ′ for the satu-rated soil. The value of Θ used to estimate the shear strengthin this study is discussed later in this section.

The shear strength of the soil was estimated at the base ofeach soil slice, and the pore pressure was obtained from thetransient water flow analysis discussed in the previous sec-tion. As noted earlier, the shear strength parameters c′ and φ′were obtained from a back analysis for the failure of em-

Fig. 5. Advancing of infiltration for embankment 2. (a) Infiltration of R = 90 mm (failure of embankment 1 occurred); (i) profiles ofSr; (ii) distribution of Sr (numerical result). (b) Infiltration of R = 132 mm (failure of embankment 3 occurred); (i) profiles of Sr;(ii) distribution of Sr (numerical result).

bankment 1. First, combinations of c′ and φ′ giving F.S. =1.0 were obtained with the analysis along the observed fail-ure approximated with a circular slip surface (step I). Then,the analysis of possible slip surfaces specified by a grid ofthe centre and tangential lines of circular slip surfaces wasconducted with the combinations of c′ and φ′ obtained ear-lier (step II). A suitable combination of c′ and φ′ was obtainedfrom the case of the slip surface closest to the approximatedobserved failure surface (Wesley and Leelaratnam 2001).

The observed failure surface for embankment 1 passedthrough the area of Sr from 70 to 90% for the numerical re-sult discussed in the previous section. Thus the back analysisfor step I was conducted with a series of values of Θ (0.7,0.8, and 0.9) to obtain shear strength parameters that consid-ered the effect of Θ. The combinations of c′ and φ′ obtainedfrom the step I analysis are shown in Fig. 8. It was observedthat c′ decreased linearly with an increase in φ′ for all Θ, andc′ showed slightly lower values with higher Θ. Using thesecombinations, the back analysis for step II was conducted.From the step II analysis, the failure circles closest to theobserved failure with F.S. = 1 were obtained at c′ = 0 kPafor all Θ. Thus, in this study, the values of φ′ and c′ chosenfor the average value of Θ = 0.8 (i.e., φ′ = 39.3° (φb = 33°)and c′ = 0 kPa) were used for further study of the reinforcedembankments.

The elongations of the nonwoven geotextiles were mea-sured for all reinforced embankments at each failure. The re-lationship between stress and strain on the geotextiles wasalso reported. It has been reported that the strain rate used intypical index tensile tests is several orders of magnitude

higher than the loading rate in an actual soil structure(Walters et al. 2002). As a result, the stiffness of geotextilesobtained from a typical index tensile strength test tends to belarger than that for the geotextiles within actual structures.On the other hand, the stiffness of nonwoven geotextiles in-creases with an increase in confining pressure. As a result, atypical index tensile test, which is without any confiningpressure, underestimates the stiffness (Walters et al. 2002).In this study, the effects of high elongation rate and no con-fining pressure of the index tensile strength test are assumedto be compensating. Thus, the tensile forces employed forthe stability analysis for the reinforced embankments were es-timated from the measured elongations of the geotextiles andreported stress–strain relationship. For embankment 2, thiscorresponded to a mobilized tensile force of about 0.5 kN/mfor all geotextiles. For embankments 3 and 4, 0.7 kN/m wasmobilized for the longer geotextiles, and 0.2 kN/m was mo-bilized for the shorter geotextiles (Figs. 6 and 7).Michalowski (1998) considered the reaction force on the re-inforcement caused by small displacements at the failurezone and noted that the reinforcing force should be appliedparallel to the original direction of placement. Thus, in thisstudy the reinforcing forces were applied in the horizontaldirection.

The F.S. for reinforced embankments was evaluated intwo separate ways in this study: analysis 1 and 2. For analy-sis 1, both the pore pressure distribution from transient waterflow analysis and the tensile force acting on the nonwovengeotextiles were taken into account to include the contribu-tion from both the reinforcing and drainage functions of

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nonwoven geotextiles. For analysis 2, the tensile resistancewas neglected and only the pore pressure distribution wasconsidered to examine the contribution of the drainage func-tions of the geotextile in isolation. The analysis is conducted

with the pore pressure distribution at R = 0 (initially), 90(failure of embankment 1), and 132 mm (failure of embank-ment 3) for all embankments. Although embankment 1failed at R = 90 mm, its F.S. was calculated at R = 132 mmfor comparison with the other calculated results.

Results of stability analysis for reinforced embankmentand discussion

The slip surfaces obtained from analysis 1 and 2 for em-bankment 3 at R = 132 mm are shown in Fig. 9. The ob-served slip surface for embankment 3 at R = 132 mm andthat for embankment 1 at R = 90 mm are also shown.Changes of F.S. for all embankments with increasing rain-fall, R, are shown in Fig. 10.

While embankment 3 failed at R = 132 mm, the F.S. ob-tained from the stability analysis (analysis 1) was 1.025, andthe calculated slip surface was deeper than that observed. Itwas reported that the failure of embankment 3 started from asurface erosion, (i.e., it failed because of a different mecha-nism than that reproduced in the stability analysis). This im-plies that embankment 3 could have been stable, if there hadbeen the proper treatment of the slope surface for the surfaceerosion. The F.S. obtained from analysis 2 was 0.956. Thisimplies that the reinforcing function of the geotextile was re-quired to maintain stability and that the geotextile did notprovide the drainage that would have been required to main-tain embankment stability without the reinforcement func-tion.

The F.S. from analysis 1 for the reinforced embankments(embankments 2, 3, and 4) reduced with an increase in

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Fig. 8. Combination of c′ and φ ′ obtained from back analysis(step I).

cumulative rainfall R. Thus, for the design of reinforced em-bankments, it is important to exam stability when the area issubjected to high infiltration.

As noted earlier, the difference in F.S. between embank-ment 1 and that from analysis 2 for embankments 2, 3, and 4represents the contribution of the geotextile as a drainagematerial to the stability of embankments, and the differencein F.S. between analysis 1 and 2 for embankments 2, 3, and4 represents the contribution of the geotextile as a reinforc-ing material. It appears that the contribution as reinforce-ment is considerably larger than that as a drainage material.Thus, it is concluded that the nonwoven geotextile contrib-uted to the stability of embankments as a reinforcing mate-rial rather than as a drainage material.

While the geotextiles worked mainly as a reinforcing ma-terial for the embankments discussed above, they are also

expected to work as a drainage material within soil struc-tures in some cases. Therefore further study of the hydraulicbehaviour of geotextiles within soil is necessary to examineunder what conditions they provide drainage and under whatconditions they act as a barrier to drainage. An examinationof this issue is presented in the following section.

Numerical experiments for infiltration intoreinforced embankment

The effectiveness of nonwoven geotextiles as a drainagematerial subjected to infiltration is studied in this sectionthrough numerical experiments. The embankments were as-sumed to have the same geometry and material properties asthe PWRI embankment examined earlier. The effects of geo-textile layout and boundary conditions are examined.

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Fig. 9. Failure surfaces for embankment 3.

Fig. 10. Stability of embankments and cumulative rainfall.

Outline of numerical experimentsEight cases (see Table 4) were examined with four differ-

ent geotextile layouts and two types of boundary condition.Cases 1 and 2 did not have any geotextile. Cases 3 and 4 hada 1.5 m long geotextile at the toe. Cases 5 and 6 had a 1.5 mgeotextile layer at the toe and three 2 m long geotextiles inthe fill. While the reinforcing materials were placed at an in-clination of several percent in some cases (e.g., Miki 1997),there has been no published study of the effect of geotextileinclination on its performance as a drainage layer. Thus, tostudy the effectiveness of sloped geotextiles, cases 7 and 8had a 1.5 m long geotextile at the toe and three layers ofgeotextiles in the fill with a 10% inclination. The odd-numbered cases had a specified flux boundary conditionboth on the slope and the top of the embankments. The flux,q, was 12.7 mm/h, which is the same value used for the pre-vious numerical simulation of the PWRI embankments.These cases represent relatively light rainfall that did notcause ponding. The even-numbered cases had a specifiedpressure head boundary condition both on the slope and thetop of the embankments. The pressure head, hp, applied tothe top was 0.1 m and that to the slope was 0.0 m. This cor-responds to heavy rainfall with ponding on the top of the

slopes and water running down the slope. The initial condi-tion was a uniform distribution of the pore pressure of− 3.5 kPa. Rain was applied continuously for the duration ofthe numerical experiments.

Results and discussionThe results of this numerical experiment are discussed in

terms of the pore pressure distribution and the Darcy flux(Darcy velocity) within the geotextiles at the time when thewetting front had advanced from the top to the middle of theembankment. For the no ponding cases (cases 1, 3, 5, and7 – specified flux), this occurred at 6.0 × 104 s (16.7 h) andthe results are shown in Fig. 11. For the ponding cases(cases 2, 4, 6, and 8 – specified head boundary conditioncases), it occurred at 2.0 × 104 s (5.6 h) and the results areshown in Fig. 12.

Effect of geotextile layer at toeWhile positive pore pressure developed around the toe for

case 1, there was no positive pressure developed around thetoe for case 3 (Figs. 11a and 11b). Additionally, outwardflow was predicted within the geotextile at the toe for case 3.Thus, the geotextile at the toe effectively worked as a drain-

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Boundary condition

Top Slope Geotextile layout

Case 1

Case 2

q = 12.7 mm/h

hp = 0.1 m

q = 12.7 cos δ mm/h

hp = 0 m

No geotextile

Case 3

Case 4

q = 12.7 mm/h

hp = 0.1 m


hp = 0 m

1 geotextile layer attoe

Case 5

Case 6

q = 12.7 mm/h

hp = 0.1 m


hp = 0 m

1 layer at toe and 3layers in fill

Case 7

Case 8

q = 12.7 mm/h

hp = 0.1 m


hp = 0 m

1 layer at toe and 3layers in fill with10% slope

Table 4. Numerical experiment cases.

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Fig. 11. Darcy flux in geotextiles and pore pressure distribution in the embankment for cases with infiltration caused by q < ksat soil

(t = 6.0 × 104 s): (a) case 1, (b) case 3, (c) case 5, (d) case 7.

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Fig. 12. Darcy flux in geotextiles and pore pressure distribution in the embankment for cases with infiltration caused by hp = 0.1 m ontop (t = 2.0 × 104 s): (a) case 2, (b) case 4, (c) case 6, (d) case 8.

age material and prevented the pore pressure from becomingpositive at the toe. The effectiveness of the geotextile at thetoe was also observed for the ponding cases (compare cases2 and 4). For case 4 (with a geotextile at the toe), the porepressure around the toe became positive, but it was muchsmaller than that of case 2 (without a geotextile) (Figs. 12aand 12b). Outward flow was also observed in the geotextile.Thus, whether ponding occurs or not, the geotextile at thetoe worked effectively as a drainage material and reducedthe pore pressure development around the toe.

Effect of boundary condition and slope of geotextileFor the no ponding cases, there was no flow in the geo-

textiles in the fill regardless of their inclinations (Figs. 11cand 11d). This arose because even after passage of the wet-ting front, the pore pressure remained less than –1.0 kPa(since the infiltration q was smaller than ksat soil) and at thesepore pressures the geotextile was less permeable than thesoil (kgeotextile < ksoil, for suctions greater than 1 kPa, asshown in Fig. 3b). Thus the geotextiles did not drain any in-filtrated water outwards, and there was little difference in thepore pressure distributions between cases 5 and 7.

For the ponding cases (cases 6 and 8), after passage of thewetting front the pore pressure exceeded –1.0 kPa and oftenbecame positive (Figs. 12c and 12d). Thus, after the wettingfront had passed the geotextile became saturated andkgeotextile became greater than ksoil, and flow occurred withinthe geotextiles under some conditions. For case 6, outwardflow occurred in the upper layer, no flow occurred at themiddle layer, and inward flow occurred at the lower layer.The outward flow at the upper layer was caused by the pres-sure gradient along the geotextile from about 0.1 mH2O ontop of the geotextile to zero pressure on the slope face. Al-though the wetting front advancing from the top had passedthe middle layer, the pressure along the geotextile was rela-tively constant at around 0 kPa. Thus, the hydraulic gradientwas not large enough to cause flow within the horizontalgeotextile layer. For the lower layer, the geotextile close tothe slope became permeable as the wetting front advancedinward from the slope. As a result, the geotextile induced in-ward water flow because of the pressure gradient along it.On the other hand, outward flow occurred in all the inclinedgeotextiles for case 8. The flow in the geotextiles for case 8resulted in the 0 kPa contour line remaining above the topgeotextile layer, (i.e., the pore pressure within the fill mate-rial between the geotextiles remained negative), while the0 kPa contour line passed the top geotextile layer for case 6,(i.e., the pore pressure within the fill material between thegeotextiles became positive).

Thus, it is concluded that a geotextile acted as a drainagematerial at locations where pore pressure in the fill exceededthe threshold at which kgeotextile becomes larger than ksoil(–1 kPa in this study). It was also found that placing geo-textiles with a 10% inclination made them more effective indraining the embankment under conditions of high infiltration.

Practical implications

The numerical results and discussion in the previous sec-tions have a number of practical implications. With the de-velopment of pore pressure beyond threshold within anembankment (e.g., because of ponding on the top of the

structure when the infiltration rate is larger than ksat soil), thegeotextile will become more permeable than the soil andwork as a drainage layer that will help in reducing the posi-tive pore pressures and hence help maintain embankmentstability. However, under other circumstances (e.g., whenthe infiltration rate is smaller than ksat soil), the pore pressuremay increase but not exceed threshold, and the geotextile isless permeable than the soil and retards water flow. Thus,water content of the soil increases above the geotextile. As aresult, the soil in the vicinity of the geotextile and the inter-face between the soil and the geotextile may weaken andcause undesired deformation or even failure. This could hap-pen not only when the geotextile is used for drainage butalso when it is used for other functions such as filtration,separation, or reinforcement. This could give rise to surfaceerosion as a consequence of wet soil around the geotextile,as observed in the experiments conducted by PWRI et al.(1988). Thus, embankments need adequate surface treatmentto avoid erosion in cases such as this. The numerical studyindicates that for cases where the geotextiles are expected todrain infiltrated water, they would be expected to performbest if installed at an angle, as is done for other subsurfacedrainage systems.

Conclusions

The hydraulic behaviour of a series of test embankmentswas simulated using the finite element method and their sta-bility was evaluated. Numerical experiments were conductedto examine the effect of geotextile arrangement and infiltra-tion conditions as well to provide insight into the effective-ness of nonwoven geotextiles as drainage material. From thefindings of this study, it is concluded that:(1) The finite element method, employing the van

Genuchten – Mualem model for hydraulic functions ofunsaturated soil and nonwoven geotextiles, reasonablysimulated infiltration into the embankments reinforcedwith nonwoven geotextiles.

(2) A numerical simulation showed that higher degrees ofsaturation developed above nonwoven geotextile layers.This is consistent with the higher water content in thevicinity of nonwoven geotextiles observed in the experi-ments.

(3) A stability analysis using pore pressure distributions ob-tained from a finite element analysis showed that thecontribution of the nonwoven geotextile as a drainagematerial was much less substantial than its role as a re-inforcing material.

(4) Nonwoven geotextiles placed at the toe of an embank-ment were effective at reducing pore pressure develop-ment regardless of rainfall intensity.

(5) Nonwoven geotextiles acted as a drainage materialwhere pore pressure exceeds the threshold at whichgeotextiles become more permeable than soil, and theirperformance was improved when they were installedwith an inclination.

Many studies have shown that nonwoven geotextiles actas a drainage material in situations where pore pressures arepositive. However, it is not so well recognized that thesesame materials may retard water flow and cause undesirablehigh water contents in their vicinity in situations where the

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pore pressure is negative. Thus, in the design of these earthstructures, it is necessary to consider the hydraulic behaviorof a soil–geotextile layered system under unsaturated as wellas saturated conditions.

Acknowledgement

The authors gratefully acknowledge financial support pro-vided by the Natural Sciences and Engineering ResearchCouncil of Canada (NSERC).

References

Fredlund, D.G., and Krahn, J. 1977. Comparison of slope stabilitymethods of analysis. Canadian Geotechnical Journal, 14: 429–439.

Fredlund, D.G., Morgenstern, N.R., and Widger, R.A. 1978. Theshear strength of unsaturated soils. Canadian Geotechnical Jour-nal, 15: 313–321.

Fredlund, D.G, Xing, A., Fredlund, M.D., and Barbour, S.L. 1996.The relationship of the unsaturated soil shear strength to thesoil-water characteristic curve. Canadian Geotechnical Journal,33: 440–448.

Gardner, W.H. 1986. Water content. In Methods of soil analysis,Part 1, Agronomy monograph No. 9, 2nd ed. Edited by A.Klute. American Society of Agronomy – Soil Science of Amer-ica, Madison, Wisc. pp. 493–544.

GEO-SLOPE International Ltd. 2001a. SEEP/W Ver.5 user’sguide. GEO-SLOPE International Ltd., Calgary, Alta.

GEO-SLOPE International Ltd. 2001b. SLOPE/W Ver.5 user’sguide. GEO-SLOPE International Ltd., Calgary, Alta.

Iryo, T., and Rowe, R.K. 2003. On the hydraulic behaviour of un-saturated nonwoven geotextiles. Geotextile and Geomembranes,21: 381–404.

Iryo, T., and Rowe, R.K. 2004. Numerical study on infiltration intosoil-geotextile column. Geosynthetics International, 11(5): 377–389.

Ju, S.-H., and Kung, K.-J.S. 1997. Mass types, element orders andsolution schemes for the Richards equation. Computers &Geosciences, 23: 175–187.

Kamon, M., Akai, T., Matsumoto, A., Suwa, S., Fukuda, M.,Simonodan, T., et al. 2001. High embankment of clay reinforcedby GHD and its utilities. In Proceeding of International Sympo-

sium on Landmarks in Earth Reinforcement, Fukuoka, Japan,14–16 November 2001. Edited by H. Ochiai, J. Otani, N.Yasufuku, and K. Omine. A.A. Balkema, Rotterdam. The Neth-erlands. pp. 225–229.

Michalowski, R.L. 1998. Limit analysis instability calculations ofreinforced soil structures. Geotextiles and Geomembranes, 16:311–331.

Miki, H. 1997. Reinforcing mechanism and analysis method ofgeotextile reinforced embankment. Report of public works re-search institute, Ministry of Construction, Tsukuba, Japan.Vol. 197. [In Japanese.]

Mitchell, J.K., and Zornberg, J.G. 1995. Reinforced soil structureswith poorly draining backfills. Part II: Case histories and appli-cations. Geosynthetics International, 2: 265–307.

Miyazaki, T. 1988. Water flow in unsaturated soil in layeredslopes. Journal of Hydrology, 102: 201–214.

Nishigaki, M., Umeda, Y., and Kono, I. 1993. A study of functionof geofilters spread in the compacted soil embankment. In Pro-ceedings of the Symposium on Investigation, Design and Con-struction for Unsaturated Ground, The Japanese GeotechnicalSociety. pp.135–138. [In Japanese.]

PWRI, Toyobo Co. Ltd., Toray Industries Inc., and Unitika Ltd.1988. Development of reinforcing method for embankment us-ing spunbond nonwoven geotextile. Performance of geotextilewithin soil and its effectiveness, Cooperative research report ofPWRI. Public Works Research Institute, Ministry of Construc-tion, Tsukuba, Japan. Vol. 6, No. 2, pp. 77–97. [In Japanese.]

Richards, L.A. 1931. Capillary conduction of liquids through po-rous mediums. Physics, 1: 318–333.

Tan, S.A., Chew, S.H., Ng, C.C., Loh, S.L., Karunaratne, G.P.,Delmas, Ph., and Loke, K.H. 2001. Large-scale drainage behav-iour of composite geotextile and geogrid in residual soil.Geotextiles and Geomembranes, 19: 163–176.

Vanapalli, S.K., Fredlund, D.G., Pufahl, D.E., and Clifton, A.W.1996. Model for the prediction of shear strength with respect tosoil suction. Canadian Geotechnical Journal, 33: 379–392.

van Genuchten, M.Th. 1980. A closed-form equation for predictingthe hydraulic conductivity of unsaturated soils. Soil Science So-ciety of America Journal, 44: 892–898.

Walters, D.L., Allen, T.M., and Bathurst, R.J. 2002. Conversion ofgeosynthetics strain to load using reinforcement stiffness.Geosynthetics International, 9: 483–523.

Wesley, L.D., and Leelaratnam, V. 2001. Shear strength parametersfrom back-analysis of single slips. Géotechnique, 51: 373–374.

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