interaction of nonwoven needle-punched

Geotextiles and Geomembranes 19 (2001) 299–328

Interaction of nonwoven needle-punchedgeotextiles under axisymmetric loading

conditions

D.T. Bergadoa,*, S. Youwaib, C.N. Haic, P. Voottipruexd

aGeotechnical Engineering Program, School of Civil Engineering, Asian Institute of Technology, P.O. Box 4,

Klong Luang, Pathumthani 12120, ThailandbSchool of Civil Engineering, Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathumthani 12120,

ThailandcThe Polytechnic University of HoChiMinh City, HoChiMinh City, Viet Nam

dKing Mongkut’s Institute of Technology, North Bangkok, Piboonsongkram Rd., Bangsue District,

Bangkok, Thailand

Received 28 May 2000; received in revised form 16 February 2001; accepted 03 March 2001

Abstract

Geotextiles have been successfully used for reinforcement of unpaved roads on softsubgrade to improve the performance of a reinforced fill layer placed on soft ground. Thetension–strain behavior of a nonwoven needle-punched geotextile under axisymmetric loading

condition as well as the mechanism and effects of the different grades of geotextile on theincrease in bearing capacity of reinforced unpaved roads over weak subgrade under trafficload were considered. The strain energy capacity concept is proposed to describe the tension–

strain of geotextile under an axisymmetric loading condition. Modified CBR tests on soft andweathered clay overlain by compacted sand as well as on soft and weathered clay overlain bycompacted sand reinforced with fix- or free-end nonwoven needle punched geotextile werecarried out. Finite element method (FEM) using the PLAXIS software was utilized to back-

analyze the results of the modified CBR tests. No significant difference between in-air and in-soil stiffness has been found for geotextile reinforcement of unpaved road. The calculatedresults indicate an additional load capacity due to the presence of the geotextile using an

axisymmetric stiffness which demonstrated a significant contribution of membrane action bythe different types of geotextile on the increase in bearing capacity of soil–geotextile system.The effects of the different types of geotextile obtained in this study can be used to preliminary

select the appropriate grades of nonwoven needle-punched geotextile corresponding to the

*Corresponding author. Tel.: +66-2-524-5512; fax: +66-2-524-6050.

E-mail address: [email protected] (D.T. Bergado).

0266-1144/01/$ - see front matter # 2001 Published by Elsevier Science Ltd.

PII: S 0 2 6 6 - 1 1 4 4 ( 0 1 ) 0 0 0 1 0 - 3

allowable rut depth in the design of the reinforced unpaved roads under traffic load. # 2001

Published by Elsevier Science Ltd.

Keywords: Geotextiles; Modified CBR; Unpaved roads

1. Introduction

The reinforcement which is strong in tension effectively combines with the soilwhich is strong in compression, forming a strong and semi-rigid composite material.With the availability of competent geotextile, the uses of geotextile in manyengineering applications have become more apparent and have proven to be aneffective means of soil improvement. In early applications in roads and airfieldconstruction, emphasis was laid on the separation function of the geotextile. The

Nomenclature

Tf tensile strength at break in uniaxial tensile test (kN/m)Pf puncture force at break in CBR puncture test (kN)r radius of plunger in CBR test (m)T tensile force per unit width of fabric (kN/m)F puncture force in (kN)a angle between geotextile plane and initial horizontal position (8)e tensile strain (%)l diagonal length of the geotextile (mm)a horizontal distance between the outer edge of the plunger and the inner

edge of the mold (mm)uy vertical displacement of puncture rod (mm)d diameter of puncture rod (mm)Fr applied load in the case of with reinforcement (kN)Fu applied load in the case of without reinforcement (kN)C dilatancy angle (8)fmax maximum internal friction angle (8)fcrit internal friction angle at critical state condition (8)Eu undrained modulus of soil (kPa)Suv undrained shear strength (kPa)G shear modulus of soil that is in contact with the reinforcement (kPa)Gi shear modulus of interface element (kPa)Ri interface coefficient (kPa)c shear strength of soil that is in contact with reinforcement (kPa)ci shear strength of interface element (kPa)d angle of friction of interface element (kPa)

D.T. Bergado et al. / Geotextiles and Geomembranes 19 (2001) 299–328300

geotextile sheeting partially replaced the conventional sand filter-separation layer(Hausmann, 1987). A full-scale field test was constructed at Washington StateHighway by Tsai et al. (1993) to compare the ability of different geotextiles to stabilizea soft subgrade during construction and found that nonwoven, needle-punchedgeotextiles had the best overall performance due to their ability for in-plane drainage.The uniaxial loading tensile test has been used to determine the in-isolated tensile

strain properties of geotextiles. Maneecharoen (1997) studied the factors affectingthe in-isolation geotextile laboratory testing and discovered that the tensile strengthof nonwoven, needle-punched geotextiles slightly increased with increasing strainrate. However, McGown et al. (1981) summarized that nonwoven polymergeotextile, which are mainly made by Polypropylene, in-isolation test is notsignificantly affected by changing the strain rate.Resl and Werner (1986) carried out the laboratory tests under an axisymmetric

loading condition using nonwoven, needle-punched geotextiles. The results showedthat the geotextile layer placed between subbase and subgrade can significantlyincrease the bearing capacity of soft subgrades. Model laboratory tests and full scaletests were conducted by Douglas (1993) and Tsai et al. (1993) lead to the conclusionof increased bearing capacity and reduced rut depth achieved under traffic load dueto the membrane effect of geotextile stabilized soft soil. Fannin and Sigurdsson(1996) carried out a full-scale field trial to observe the performance of differentgeosynthetics in unpaved road construction over soft ground. The resultingcorrelation between tensile stiffness of the geotextile and improved trafficabilitywas attributed to a significant tension generated by membrane effect of the geotextilereinforcement.In this study, the in-air CBR puncture test on nonwoven, needle-punched

geotextile was utilized to find a suitable method to describe the tension–strainbehavior of geotextile under axisymmetric loading condition. In addition, FEM wasused to back-analyze the results of modified CBR tests on soft and weathered clayoverlain by sand with and without geotextile reinforcement. The comparisonbetween in-air and in-soil stiffness, the improvement of bearing capacity of soil–geotextile system, and the mechanism of geotextile-reinforced unpaved road overclay subgrade were investigated.

2. In-air stress–strain behavior

Cazzuffi and Venesia (1986) investigated the comparison between the tensilestrength and puncture force at break of nonwoven geotextiles obtained from widewidth tensile test (ASTM D4595-86, 1994) and CBR puncture test, respectively.Consequently, an empirical relation was obtained between the tensile load underuniaxial loading with the puncture force at break of geotextile obtained from CBRpuncture test as follows:

Tf ¼Pf

2pr; ð1Þ

D.T. Bergado et al. / Geotextiles and Geomembranes 19 (2001) 299–328 301

where Tf=tensile strength at break in uniaxial tensile test, Pf=puncture force atbreak in CBR puncture test, and r=radius of pluger in CBR test. In practice, thedimension ratio (L/B) of the wheel contact area for traffic loading is less than twoand is quite similar to an axisymmetric case which can be simulated by CBRpuncture test.In-air or in-isolation stress–strain behavior of geotextiles was investigated. Two

types of in-air tests, namely: wide width tensile test and CBR puncture test werecarried out using low strength nonwoven needle-punched Polyfelt geotextiles withthree different types, namely: TS20, TS40 and TS65. The physical properties for eachtype of geotextile are tabulated in Table 1.

3. The method to analyze an axisymmetric stress–strain behavior

In CBR puncture test, the test data recorded only the vertical displacement andthe applied force of the puncture rod. The test results described in the form ofpuncture force versus vertical displacement, did not describe the tension load–strainin geotextile. In this paper, a method to analyze and calculate the tensile strength andthe tensile strain of geotextile under an axisymmetric condition is proposed based onMcGown et al. (1998) and McGown and Khan (1999). The proposed method isrelated to the relationship between the puncture force and vertical displacement ofpuncture rod with the tensile load in geotextile at specified displacement (Fig. 1) asfollows:

T ¼F

2pr sin a; ð2Þ

where T=tensile force per unit width of fabric in kN/m, F=puncture force in kN,r=radius of plunger (r ¼ 25mm) in m, and a=angle between geotextile plane andthe initial horizontal position as illustrated in Fig. 1.The angle, a, is obtained from the following relationship:

sin a ¼y

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiy2 þ 502

p ; ð3Þ

Table 1

Physical property of nonwoven needle-punched geotextiles

Property Unit TS20 TS40 TS65

Physical characteristic polymer Continuous filament nonwoven needle punched 100% poly-

propylene

Nominal mass g/m2 125 180 285

Thickness mm 1.2 1.7 2.5

Apparent opening size (O95) mm 0.26 0.24 0.18


where y is vertical displacement of puncture rod as shown in Fig. 1. According to theGerman standard (DIN), the tensile strain (e) is calculated as follows:

e ¼ðl � aÞ

a100 ð4Þ

where l=the diagonal length of the geotextile and a=the horizontal distancebetween the outer edge of the plunger and the inner edge of the mould (a ¼ 50mm).Based on the above-proposed analysis, the CBR test result can be plotted in the

form of tensile load versus strain of geotextile, which is represented for the load–strain behavior of geotextile under an axisymmetric loading condition. The stiffnessof the geotextile under an axisymmetric condition can be determined from this curve,and applied to the design of geotextile reinforcement in unpaved road under trafficload. In practice, the loading application of wheel load caused by traffic can besimilar to the axisymmetric case.

Fig. 1. Proposed geometry assumed to interpret an axisymmetric tension–strain of geotextile in CBR

puncture test.


4. Wide width tensile test

A series of wide width tensile tests were carried out to investigate the uniaxialstress–strain behavior of nonwoven, needle-punched geotextiles. A 200mm wide by200mm long specimens were used. Three grades of nonwoven, needle-punchedPolyfelt geotextile TS20, TS40 and TS60 were tested in two directions such asmachine and cross machine direction at a constant rate of strain of 10%/min. Theresults of the wide width tensile tests of the TS20, TS40 and TS60 geotextile inmachine direction based on five samples are plotted in Figs. 2–4, respectively. Thescatter of the result are from the non-uniform distribution of mass per unit area inlightweight nonwoven geotextile layer. However, the difference in the testing result isnot significant. Thus, the average value was employed as shown in Figs. 2–4. Theresulting data indicate a difference of tensile strain at break of geotextile samplebetween machine and cross machine direction. The pre-tension of the geotextile inthe machine direction during manufacture process resulted in lower elongation atbreak. The value of tensile strain at break of cross machine direction was found to beabout two times higher than in machine direction. The ultimate tensile strengths,however, were nearly the same for both directions.

5. CBR puncture tests

The puncture resistance of the geotextiles was determined by the following theBritish standard method (BS6906: Part 4, 1989) of test. A test specimen was securely

Fig. 2. Wide width tensile test of TS20 by ASTM-D-4595.


clamped between the clamping ring of the test rig ensuring no slippage and damageto the sample. The test rig was then placed in the tensile testing machine wherein the50mm diameter plunger was punctured into the geotextile. The CBR puncture tests

Fig. 3. Wide width tensile test of TS40 by ASTM-D4595.

Fig. 4. Wide width tensile test of TS65 by ASTM-D4595.


were carried out at three different puncture speeds of 20, 40 and 80mm/min,respectively, for each grade of geotextile. The test results described in the form ofvertical displacement of puncture rod versus puncture force is typically shown inFig. 5 as average values of five samples. To investigate the stress–strain behavior ofnonwoven geotextiles under CBR puncture test, the tensile load in geotextile wasinterpreted by Cazzuffi’s empirical formula (Eq. (1)) and the proposed formula(Eq. (2)). The tensile strain was calculated according to the German standard (DIN)using Eq. (4). The CBR testing results were described in the form of tensile load perunit width versus tensile strain of geotextile sample as typically plotted in Fig. 6.These results indicate that the puncture force and the vertical displacement at breakwere slightly effected by changing puncture speed. The maximum puncture forceslightly increased simultaneously to reduce the vertical displacement of puncture rodat break when the strain rate increased ranging from 20 to 80mm/min, as typicallyshown in Fig. 5.In the CBR puncture test, the plunger was pushed perpendicular into the

geotextile sample at a constant rate of vertical displacement. However, the tensilestrain in the geotextile sample may not be at a constant rate of strain. Assuming anaverage rate of strain in the geotextile specimen as defined by the tensile strain atbreak divided by the time to reach failure; the tensile strength at breakage and thecalculated results of the average tensile strain with varying strain rate are shown inTables 2 and 3, respectively. These results indicate that a rate of vertical puncture of20, 40 and 80mm/min correspond to an average rates of strain in geotextile sampleof 14%/min, 28%/min and 55%/min, respectively. The test results also show a slight

Fig. 5. CBR puncture test of TS20 at different puncture speeds.


Fig. 6. Tension–strain of TS20 interpreted by Cazzuffi’s formula.

Table 2

Tensile strength at break of geotextile at different puncture speeds of CBR puncture test

Puncture speed

(mm/min)

Tensile strength at break (kN/m)

Proposed formula (Eq. (3)) Cazzuffi’s formula (Eq. (1))

TS20 TS40 TS65 TS20 TS40 TS65

20 13.4 20.1 31.4 8.6 13.8 20.9

40 14.4 20.8 32.4 9.1 14.1 21.5

80 15.2 22.3 33.6 9.5 14.3 21.8

Table 3

Average tensile strain rates of geotextile at different puncture speeds of CBR puncture test

Puncture

(mm/min)

Tensile strain at break (%) Average tensile strain rate (%/min)

TS20 TS40 TS65 TS20 TS40 TS65

20 30.8 37.5 34.1 13.8 15.0 14.4

40 29.4 35.8 33.5 37.1 29.5 28.7

80 28.6 30.2 31.2 53.6 54.9 55.6


effect of strain rate on tensile strain of geotextile sample. For example, when theaverage rate of strain increased twice, from 14%/min to 28%/min or from 28%/minto 55%/min (100% increase), the tensile strength of geotextile at break increasedfrom 2% to 7%. The deviation of tensile strength at break is about of 1 kN/m whenthe average strain rate increased by 100%. It is concluded that the effect of variablestrain rate is not significant for the nonwoven polymer geotextile tested with the highrate of strain in the range of 14%/min–28%/min. This conclusion is consistent withthe results of McGown et al. (1981) and Maneecharoen (1997).

6. Comparison of strain energy capacity

The concept of strain energy capacity defined as the area under the load–straincurve at a given strain (see Fig. 8) can be applied to verify the proposed formula toanalyze the tension–strain behavior of geotextile in CBR test. The strain energystored in geotextile sample at break obtained from CBR puncture test is interpretedby the proposed method and compared with the results obtained from wide widthtensile test. These areas should be compared at the same tensile rate. The CBRpuncture tests were carried out at three different puncture speeds of 20, 40 and80mm/min. The purpose is to select a suitable load–strain curve to compare thestrain energy capacity of the geotextile with the results obtained from wide widthtensile test carried out at a constant rate of strain of 10%/min. As discussedpreviously for nonwoven geotextiles, the results were not significantly affected bychanging the strain rate. Hence the deviation between two load–strain curves havingaverage strain rates of 10%/min and 14%/min for CBR puncture and wide widthtensile test, respectively, can be neglected. Therefore, the area under tension–strainwith the average strain rate of 14%/min in CBR puncture test can be used tocompare with the strain energy capacity obtained from wide width tensile tests whichwas carried out at the strain rate of 10%/min. The uniaxial tensile-strain andaxisymmetric tensile-strain curves of the geotextile at a strain rate of 10%/minobtained from wide width tensile test and CBR puncture test, respectively, aretypically plotted in Fig. 7. The axisymmetric tensile-strain of geotextile obtainedfrom CBR puncture tests are interpreted by the proposed method and Cazzuffi’sempirical equation, respectively. The deviation of the strain between the axisym-metric and the uniaxial conditions due to the increment of the strain in the geotextiledepends on the loading condition and the stress state. However, at the failure state,the strain energy storage in the geotextile by the same applied strain rate fordeveloping the failure condition should be the same. This is one of the concepts ofthe ‘‘Isochronous Strain Energy’’ (McGown et al., 1998; McGown and Khan, 1999).The comparison of strain energy capacity of TS20, TS40 and TS65 obtained from

wide width tensile test (uniaxial tensile test) and CBR puncture test (axisymmetrictensile test) interpreted by the proposed method and using Cazzuffi’s strain equationare shown in Fig. 8 and tabulated in Table 4. The load–strain curves of geotextile inCBR puncture test interpreted by the proposed method can yield the value of strainenergy capacity at break close to the value obtained from wide width tensile tests at


the same strain rate. Thus, the axisymmetric load–strain behavior of geotextile underCBR puncture test can be reasonably interpreted by the proposed method. In Fig. 8,there is some difference between the strain energy capacities at failure condition

Fig. 7. Comparison of tension–strain of TS20 at strain rate of 10%/min.

Fig. 8. Comparison of strain energy capacity of TS20 at strain rate of 10%/min.


obtained from axisymmetric and uniaxial tensile loading which can be acceptabledue to the difficulty in the simulation and the effect of slightly changing the directionof tensile force in the geotextile under CBR puncture test during the testing process.Actually, during the puncture process the geotextile plane becomes slightly curvedbetween the edge of puncture rod and the circular clamps. It is not exactly as adiagonal line between the edge of puncture rod and the circular clamps as assumed.

7. Nonwoven needle-punched geotextiles

7.1. Axisymmetric tensile load–strain behavior

The test results produced a proposed suitable method to interpret the tensile load–strain behavior of nonwoven geotextile samples under CBR puncture tests in theform of tensile load–strain curve of geotextile under axisymmetric loading. Thestiffness of geotextile under axisymmetric loading condition can be determined fromthis curve and then applied for design of geotextile reinforcement under suchcondition. The results of CBR puncture test in the form of axisymmetric tensile loadversus strain interpreted by the proposed method are shown in Fig. 9 for the differentgrades of nonwoven needle-punched geotextiles consisting of TS20, TS40 and TS65.The secant modulus (or stiffness) for individual grade of geotextile underaxisymmetric condition can be determined from Fig. 9 and applied for design ofthe geotextile reinforcement under such condition.

7.2. Axisymmetric stress–strain behavior

The result of CBR puncture test in the form of load–strain as shown in Fig. 10indicated that the slope of nonlinear load–strain curve of TS20, TS40, TS65 are quitedifferent from each other. From these curves, the stiffness of the individual grade ofgeotextile at a given strain can be determined. However, when the tensile load (inkN/m) is divided by the thickness (in m) geotextile sample, the results can bedescribed in the form of tensile stress–strain behavior as in Fig. 10. The stress–straincurves of the different grades of nonwoven needle-punched geotextile occur in anarrow band. In order to facilitate for design, a suitable average stress–strain curve

Table 4

Comparison of strain energy capacity obtained from CBR puncture test and wide width tensile test

Method of analyze load–strain curve Strain energy capacity

TS20 TS40 TS65

Proposed method (Axisymmetric tensile) 334.2 518.2 686.3

Cazzufi (1986) formula 171.5 285.9 370.9

Wide width tensile test (Uniaxial tensile) 355.5 417.0 926.8


representing several grades of low strength, nonwoven geotextile can be described asshown in Fig. 10. For finite element analyzes of geotextile reinforcement, thisnonlinear stress–strain behavior can be simulated by a bilinear curve having

Fig. 9. Axisymmetric tension–strain of TS20, TS40 and TS65 interpreted by the proposed method.

Fig. 10. Axisymmetric stress–strain behavior of nonwoven, needle-punched geotextile.


intersection at the strain of 2.5% as shown by the continuous and dashed lines inFig. 10. The stiffness of the geotextile under axisymmetric loading conditions canalso be determined from the slope of this bilinear curve by multiplying with thecorresponding thickness of geotextile sample. The stiffness of nonwoven needle-punched geotextiles under axisymmetric conditions obtained from the bilinear curveis shown in Table 5.

8. In-soil test results

8.1. Modified CBR test

The modified CBR test aims to simulate the wheel load on an unpaved road over aweaker subgrade. A modified CBR test mold based on the idea from modified CBRdesign method of US Army Corps of Engineers was introduced in this study. Amodified cylindrical mold with a diameter of 300mm and a height of 230mm wasmade to investigate the behavior of sand–clay system reinforced and unreinforcedwith geotextiles. The size of the mold of the modified CBR test is larger than theconventional CBR testing method to minimize the boundary effect. From thenumerical analysis result, the plastic zone expand to a distance of approximately 1.0D from the center of the footing and to a depth of 1.5 D under the foundation, whereD is the diameter of the circular footing (Taiebat and Carter, 2000). Therefore, theboundary condition of this testing system has little effects to the result of themodified CBR test.The mold has an upper and lower sections with heights of 50 and 180mm,

respectively, as shown in Fig. 11. The 30mm wide flanges were welded to the upperand lower sections to clamp and bolt the geotextile in place for the fix-end geotextilecase of the test. The lower part of the mold contained undisturbed soft or weatheredclay representing a weak subgrade soil. The upper part contained compacted siltysand representing the subbase soil layer. The sand layer was compacted at watercontent of 13% and dry density of 17 kN/m3 corresponding to 90% standard Proctorcompaction in wet side of optimum in two equal lifts. A Marshall compactorhammer with a weight of 45.3N falling freely from 457mm height was used to apply

Table 5

In-isolation stiffness of nonwoven needle-punched geotextile Polyfelt TS under axisymmetric load

condition

Material Thickness

(mm)

Slope of bilinear curve

(kN/m2)

In-isolation axisymmetric

stiffness (kN/m)

e52:5% e42:5% e52:5% e42:5%

TS20 (125 g/m2) 1.2 180 000 24 000 216 29

TS20 (180 g/m2) 1.7 180 000 24 000 306 41

TS20 (285 g/m2) 2.5 180 000 24 000 450 60


51 blows per lift. This number of blows per lift was calculated based on equal energywith standard Proctor test.Prior to CBR testing, a surcharge equivalent to 5.3 kPa was applied on top the

compacted sand layer by placing 11 circular steel plates having diameter of 298mmand thickness of 6.2mm (Fig. 11). The center of each plate has a circular hole with a52mm diameter to facilitate the penetration of the 50mm CBR plunger. The appliedforce and vertical displacement of the plunger was recorded by the automatic dataacquisition system.

8.2. Sand over soft and weathered clay layers

To investigate the bearing capacity of the unreinforced sand–clay system underaxisymmetric loading, two tests of modified CBR were carried out on 50mmthickness of compacted silty sand over 180mm thickness of weathered clay(Su ¼ 25 kPa). The other two tests were done using soft clay (Su ¼ 12 kPa) as thesubgrade soil. The undrained shear strength values of weathered clay and soft claywere obtained from corrected field vane shear test before the undisturbed sampling.The applied force and vertical displacement (uy) were recorded. The tests results areshown in Fig. 12. The shapes of the load-displacement curves fall into two distinctportions. The applied load becomes reasonably constant or slightly decrease atvertical displacement (uy) of the puncture rod greater than 5mm. It is equivalent to

Fig. 11. Test set-up and sketch of modified apparatus.


displacement ratios (uy=d) greater than about 10%. The term displacement ratio(uy=d) is defined as a fraction of the diameter of puncture rod.

8.3. Geotextile in sand and soft and weathered clay interfaces

A series of modified CBR tests were carried out with a reinforced geotextile layerplaced at the interface between the compacted silty sand and undisturbed weatheredclay layer in two cases of fix- and free-end geotextile reinforcement. Three grades ofnonwoven needle-punched geotextile such as: TS65 (285 g/m2), TS40 (180 g/m2) andTS20 (125 g/m2) were used. The reinforced geotextile TS65 was used with two casesof weak subgrade soils i.e. weathered clay (Su ¼ 25 kPa) and soft clay (Su ¼ 12 kPa),respectively. The typical results are shown in Fig. 13 for TS65. With the presence ofgeotextile, the shapes of all load-displacement curves are similar. None of the curvesexhibited a peak load. In the initial stages, the applied load increased nonlinearlywith the vertical displacement but as the displacement ratio increased, the appliedload increased at a virtually constant rate. The slope of the curves is greatest atvertical displacements smaller than 5mm or displacement ratio below 10%.The load-displacement curves obtained from fix- and free-end reinforcement cases

are quite similar in terms of footing settlement. The difference appears at the verticaldisplacement greater than 15mm (equivalent to displacement ratios beyond of 30%)and become clear as a large displacements occurs. In the free-end case, the sametrend to increase bearing capacity of sand–clay system as in fix-end case when the

Fig. 12. Modified CBR test results of sand over clay (no geotextile reinforcement).


vertical displacement increased. However, the load-displacement curves recordedfrom free-end modified CBR tests seem to be less stable than the fix-end cases. Thisbehavior can be attributed to the slippage of the geotextile inclusion in the soil.Therefore, the main comparisons and investigations are based on the fix-end testresults.

8.4. Comparison between unreinforced and reinforced sand over weathered clay system

The load-displacement curves obtained from unreinforced and reinforced sandover weathered Bangkok clay (Su ¼ 25 kPa) with different grades of geotextile TS20,TS40, TS65 are plotted in Fig. 14. These results have confirmed the findings of anumber of previous investigations by showing the improvement of the bearingcapacity of the sand–clay system when a layer of nonwowen, needle-punchedgeotextile was placed between the two soil interfaces. The relative bearing capacityfactors (Fr=Fu) versus the relative displacement ratio (uy=d) are shown in Fig. 15.The relative bearing capacity factor (Fr=Fu) was calculated as fraction of the appliedload in the case of with reinforcement (Fr) and without reinforcement (Fu) at a givendisplacement ratio. The displacement ratio (uy=d) is defined as a fraction of thevertical displacement (uy) and the diameter (d) of the puncture rod. Theimprovement in applied load is more pronounced at the vertical displacement ratiosbeyond about of 20%. The effects of different grades of geotextile in theimprovement bearing capacity can be recognized at the displacement ratios greater

Fig. 13. Modified CBR test results of soft clay (Su ¼ 12 kPa) overlain by sand reinforced with TS65.


than about of 30%. The relative bearing capacity factor of TS65, TS40 and TS20 isthe same at about 1.35 at the displacement ratio of 25%. Beyond of the displacementratio of 30%, the difference in the relative bearing capacity between the grades ofgeotextile were recorded and confirmed.Comparing the improvement predicted due to the reinforcement by Giroud and

Noiray (1981) method, the bearing capacity is increased by the membrane effect fromelastic (pcu) to the ultimate bearing capacity (2þ p). Similarly, in Steward et al.(1977) method, the bearing capacity factor (Nc) increased from 2.8 to 5 due toreinforcement for rut depth less than 50mm. The relative bearing capacity factor(Fr=Fu) were calculated as a fraction of the applied load in the case of reinforced (Fr)and unreinforced (Fu) at a given displacement ratio, where the displacement ratio(uy=d) is defined as a fraction of the vertical displacement (uy) and the diameter (d) ofthe puncture rod. The relative bearing capacity factor (Fr=Fu) is of about 1.6 and 1.7,when using Giroud and Noiray (1981) method and Steward et al. (1977) method,respectively. Barenberg et al. (1980) also proposed the allowable subgrade stresslevels could be increased by a factor of about 1.8 compared to the unreinforced case.In these methods, the relative bearing capacity factor is assumed to be independentof the grades or types of geotextile and the rut depth.The relative bearing capacity factor obtained from the test results is illustrated in

Fig. 15. The displacement ratio varied depending on which grade of geotextile wasselected. Thus, the relative bearing capacity factor can be different depending on the

Fig. 14. Compared the modified-CBR test results of weathered clay (Su ¼ 25 kPa) overlain by sand

reinforced with different grades of geotextiles.


type of geotextile used. The effect of different grades of geotextile, which is differentin strength and deformation characteristic, on the improvement of bearing capacityis not included in the existing design methods. Therefore, the relative bearingcapacity factor can be used to preliminary select the suitable grades of geotextile fordesign in the geotextile reinforced unpaved road under traffic load corresponding tothe allowable rut depth.

9. Finite element modeling

9.1. Finite element results

The PLAXIS finite element program version 7.1 was utilized (Brinkgreve andVermeer, 1998). The finite element modelling of modified CBR test for unreinforcedcase was carried out first. Subsequently, the FE simulation was applied to thereinforced sand over weathered clay with three different types of geotextile, namely:TS20, TS40 and TS65.The soil was modelled by 15-noded triangular elements. The soil–geotextile

interface was considered by using 10-noded interface elements. The Mohr–Coulomb’s elastic perfectly plastic model was used for both soil and soil–geotextile

Fig. 15. Relation of bearing capacity of weathered clay (Su ¼ 25 kPa) overlain by sand reinforced with

different grades of geotextiles.


interface. The 5-noded bar elements using elastic stress–strain relation wereemployed for the geotextile reinforcement. The finite element analysis was carriedout by applying vertical prescribed displacements of 30mm and zero horizontalprescribed displacement on top of the loading beam elements to simulate thepuncturing process. A beam element with high rigidity was used to modelthe puncture rod. It was necessary to limit the applied displacement to 30mm inthe finite element calculation in order to avoid excessive element distortions in thearea close to the footing.The back-analysis of the modified-CBR test by using numerical analysis without

reinforcement were carried out to determine the suitable parameters and toinvestigate the influence of the boundary condition. The compacted silty sand layer ismodeled using the Mohr–Coulomb’s elastic-perfectly-plastic model with the Poison’sratio of 0.33, friction angle and cohesion of compacted silty sand layer were adoptedfrom the previous studies (Zou, 1994; Parwaiz, 1994; Long, 1997). The dilation angleis obtained from the relation between the dilation angle and the internal frictionangle as follows (Bolton, 1986):

C ¼ 0:8ðfmax � fcritÞ; ð5Þ

where C is dilatancy angle and fmax is maximum internal friction angle and fcrit isinternal friction angle at critical state condition 308 (Brinkgreve and Vermeer, 1998).The elastic modulus of the compacted silty sand was obtained from the back-

analysis of the displacements. The undrained shear strengths of weathered andsoft clay were obtained from corrected in-situ vane shear test results. Theundrained modulus of Bangkok clay can be correlated to field vane shear strengthas follows:

Eu ¼ aSuv; ð6Þ

where Eu is the undrained modulus of soil and a is the correlation factor between 70and 300 (Balasubramaniam and Brenner, 1981) and Suv is undrained shear strength.The value elastic modulus was obtained from back-analysis of the displacement forthe modified CBR mold without reinforcement.For the modeling of the geotextile, the stress–strain behavior at soil-interface is

simulated by elastic-perfectly-plastic model. The model parameters at soil interfacecan be generated from that soil using the interaction coefficient Ri, defined as theratio of shear strength of soil structure interface to corresponding shear strength ofsoil (Brinkgreve and Vermeer, 1998) as follows:

tan d ¼ Ri tan f; ð8Þ

ci ¼ Ric; ð9Þ

Gi ¼ R2iG; ð10Þ

where G is shear modulus of the soil that is in contact with the reinforcement, Gi isshear modulus of interface element, Ri is interface coefficient, c is shear strength ofthe soil that is in contact with reinforcement, ci shear strength of interface element, fis angular of friction of soil contact with reinforcement and d is angular of friction of


interface element The magnitude of interaction coefficient (Ri) for all grades ofgeotextile were 1.0–0.3 for sand-geotextile interface and clay-geotextile interface,respectively, as obtained from Long et al. (1997) and Long (1997). The selectedparameters for the geotextile in FEM back-analysis are shown in Table 6.The selected stiffness for the geotextile in FEM back-analyzes are shown in Table

7. The finite element meshes and applied boundary conditions to simulate thepuncturing process in modified CBR tests are illustrated in Fig. 16. The results in thefinite element analyzes of unreinforced and reinforced cases are typically shown, inFigs. 17–19, in the form of applied load versus vertical displacement relations. Theseresults indicated that the PLAXIS program could be used successfully in modelingthe unreinforced and geotextile reinforced sand over soft subgrade in unpaved roads.The finite element results are seen to provide a reasonable agreement with theexperimental data measured from modified CBR test. The confining effect to thegeotextile from transient vehicle wheel loads could be much less than the uniformpermanent loads and may not significantly affect the in-soil stiffness in theunderlying geotextile reinforcement. Moreover, the confining pressure on geotextileinterface caused by the thickness of subbase soil layer overlying the geo-textile reinforcement in unpaved road is small and may not affect the in-soilgeotextile stiffness. Thus, the in-air stiffness of geotextile under axisymmetric loadingobtained from the proposed method can be suitably used for design of geotextilereinforced unpaved road.

9.2. Mechanism of geotextile reinforced unpaved road over soft subgrade

The improvement in the bearing capacity of soil–geotextile system and themechanism of geotextile reinforced in unpaved road can also be investigated fromFEM analysis. The results of the finite element analyzes of the unreinforced and

Table 7

Selected stiffness of geotextile in FEM analysis

Material Stiffness of geotextile (kN/m)

TS20 TS40 TS65

Strain e52:5% 224 317 410

Strain e52:5% 32 45 60

Table 6

Selected parameters for soils in FEM analysis

Materials Model c0 (kPa) f0 (8) Eref (kPa) n0 c0 (8) gt (kN/m3)

Compacted silty sand M-C 10 30 15 960 0.33 0 19.2

Weathered clay M-C 25 0 7884 0.35 0 16.3

Soft clay M-C 12 0 3780 0.35 0 15.1


reinforced case compared with the testing results are illustrated in Figs. 20 and 21.When a vertical load is applied at the surface of the subbase layer, it causes highhorizontal as well as vertical stress, under the loaded area. The soil under vertical

Fig. 16. Finite element mesh of modified-CBR test. (a) Sand–clay, (b) sand–clay-fix-end geotextile,

(c) sand–clay-free-end geotextile.

Fig. 17. FEM prediction versus laboratory tests results of clay overlain by sand.


Fig. 18. FEM predictions versus laboratory test results of soft clay (Su ¼ 12 kPa) overlain by sand

reinforced with TS65.

Fig. 19. FEM predictions versus laboratory test results of weathered clay (Su ¼ 25 kPa) overlain by sand

reinforced with TS65.


Fig. 20. Typical deformed mesh of modified CBR test on weathered clay overlain by sand without

geotextile.

Fig. 21. Typical deformed mesh of modified CBR test on weathered clay overlain by sand with geotextile.


loaded area tends to move laterally. In the unreinforced case, the high horizontaldisplacement occurred in the subbase soil layer. The resulting horizontal thrust in thesoil is partly resisted by the horizontal stress in the fill outside the loaded area, butalso resulted from the outward shear stresses on the surface of the clay below. Thepresence of such outward shear stresses reduced the appropriate bearing capacityfactor for the clay.With the presence of geotextiles, the horizontal shear stresses are carried by the

geotextile reinforcement through the reinforcement tension. Consequently, onlythe vertical stresses are mainly transmitted to the underlying subbase layer. Thus, thehorizontal displacements are also reduced. The vertical loads are also partiallysupported by the deformed geotextile through the membrane effect. The differenttensile force in geotextile at the same value of vertical displacement of puncture rodobtained from FEM results are shown in Fig. 22. The effects of the different grades

Fig. 22. The tensile force in different grades of geotextile at same footing displacement from FEM

analysis.

Fig. 23. Typical deformed of geotextile in FEM analysis.


Fig. 24. Comparison of horizontal displacement between unreinforced and reinforced with TS65 from

FEM analysis. (a) Sand–clay, (b) sand–geotextile–clay, (c) comparison of horizontal displacements.


of geotextile are illustrated. The typical deformation of geotextile under vertical loadobtained from FEM analysis is shown in Fig. 23. To investigate the reduction ofhorizontal displacement in subbase layer, the typical FEM results of the horizontaldisplacement at the cross-section beneath the edge of puncture rod in theunreinforced and reinforced cases are plotted in Fig. 24. These results indicate thatthe horizontal displacement in the reinforced case is divided into two zones by thegeotextile layer. This effect is transferred to the deeper level under the geotextile layerand reduced the horizontal displacement in the subbase soil compared with theunreinforced case. The transmission effect to the underlying layer allowed the fullmobilization of the bearing capacity. Therefore, the bearing capacity of soil–geotextile system is increased.The finite element analyzes described previously illustrated well the tendency of

the reinforcement to reduce the magnitude of the shear stress transmitted to thesubgrade as well as the membrane mechanism that becomes increasingly importantat large deformations. The axial stiffness of the reinforcement becomes, however, animportant consideration if large tensions are to develop at small rut depths. Theresults of the investigation also explain why the reinforcement is able to provide animprovement in road performance and the importance of geotextile separationcannot be denied.

Fig. 25. Comparison of increase in the applied force due to membrane effect calculated by applying

Giroud and Noiray (1981) method using EAxisymmetric and EUniaxial.


9.3. Increase of bearing capacity due to membrane effect

The comparison between the calculated results of the membrane support usingaxisymmetric stiffness and uniaxial stiffness obtained using Giroud and Noiray(1981) method are plotted in Fig. 25 for geotextiles TS20 and TS40. These resultsare compared with the modified CBR test results from this study. It was observedthat the membrane force does make a significant contribution to the increase inthe bearing capacity of the soil–geotextile system. The results indicated that the valueof the geotextile stiffness under axisymmetric condition provide a more reasonableresult than when using uniaxial stiffness. However, the predicted value using Giroudand Noiray’s solution overpredicted the vertical displacement because it is basedon plane strain system. Therefore, the significant improvements in the load carryingcapacity of unpaved roads due to membrane action have been confirmed andthe effect of the different grades of the geotextile in increasing the bearing capacityof the soil–geotextile system has been demonstrated.

10. Conclusions

The in-air stiffness under the axisymmetric loading condition of nonwovenneedle-punched geotextile TS20, TS40 and TS65 can be determined fromthe corresponding tensile load–strain curve obtained from CBR puncturetests which are interpreted by the proposed method based on the strain energycapacity concept. The stress–strain behavior under the axisymmetric loadingcondition of all grades of nonwoven needle-punched geotextile can be representedby an average stress–strain curve and also simulated by a bilinear curve. The stiffnessunder such condition of individual grade of geotextile can also be determinedfrom this bilinear curve by multiplying with the corresponding thickness ofeach grade. The contact area of wheel load in unpaved roads can be assumedto be uniform distributed as an equivalent ‘‘circular distributed load’’. Consequently,the loading from individual wheels is approximated to an axisymmetric rather thana plane strain loading. This paper has confirmed the findings of a number of previousinvestigations by showing that the bearing capacity of a sand layer over a softclay subgrade is increased by the incorporation of a layer of geotextile at thesoil–clay interface. The calculated results in the form of additional load due tothe presence of the geotextile using an axisymmetric stiffness have demonstrateda significant contribution of membrane action by the different grades of geotextileon the increase in bearing capacity of soil–geotextile system. No significant differencehas been observed between in-air and in-soil stiffness of geotextile reinforcedunpaved roads. Thus, the in-air stiffness of geotextile under axisymmetric loadingcondition obtained from the proposed method can be applied for design of geotextilereinforced unpaved road under traffic load. The relative bearing capacityfactor obtained in this study which has considered the effect of different typesof geotextile can be used to preliminary select the grades of nonwovenneedle-punched geotextile corresponding to the allowable rut depth in the


design of unpaved road. The PLAXIS finite element program was appliedsuccessfully in modeling the unreinforced and reinforced sand over weak subgrade.The results of finite element analyzes has confirmed the improvement in the loadcarrying capacity of unpaved roads due to membrane action and the effect of theaxial stiffness of geotextile even at small rut depth. The importance of geotextileseparation cannot be denied.

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interaction of nonwoven needle-punched

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