MONOTONIC BEHAVIOR OF GEOTEXTILE REINFORCED SOILS UNDER DISCRETE ROTATION OF PRINCIPAL STRESSES *

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1 IJST, Transactions of Civil Engineering, Vol. 8, No. C2, pp 25-5 Printed in The Islamic Repblic of Iran, 214 Shiraz University MONOTONIC BEHAVIOR OF GEOTEXTILE REINFORCED SOILS UNDER DISCRETE ROTATION OF PRINCIPAL STRESSES * M.R. HABIBI 1, A. SHAFIEE 2** AND M.K. JAFARI 1 Islamic Azad University, Science and Research Branch, I. R. of Iran Tehran 2 IIEES, Los Angeles, United States alishafiee6572@gmail.com IIEES, I. R. of Iran Abstract Monotonic triaxial compression, triaxial extension and torsional shear tests were carried ot on geotextile reinforced sand and reinforced clay, mainly to investigate the effects of rotation of principal stresses on the mechanical behavior of the reinforced soil materials. The tests were carried ot on nreinforced and reinforced specimens with 2, and 4 geotextile layers nder three different confining pressres. Investigation of the monotonic behavior of the reinforced materials nder different stress paths, i.e. triaxial compression, triaxial extension, and torsional shear shows that direction of principal stresses can have profond effects on the stress-strain crve, shear strength, and slope and intercept of failre envelope. Test reslts reveal that geotextiles improve the mechanical properties of the sand and clay, since both strain at failre and ndrained shear strength increase with the nmber of geotextile layers in sand and clay. In addition, test reslts indicate that geotextile inclsion enhances the mechanical properties of geotextile reinforced sand and clay, however, geotextiles seems to be more effective when sed to reinforce sands. Keywords Principal stress rotation, geotextiles, sand, clay 1. INTRODUCTION Reinforcement with geosynthetics has gained in poplarity de to its versatile applications in varios problems sch as retaining walls, pavements, fondations, and embankments [1]. With regard to the cost of the in sit tests on geosynthetic reinforced soils, there is more tendency to se physical modeling in different scales, however, in some cases, element testing like monotonic/cyclic triaxial [2-4] have been carried ot to qalitatively evalate the mechanical behavior of geosynthetic reinforced soils. Geotextiles pgrade the mechanical properties of soils by three mechanisms: 1-enhancing the bonds in the soil de to the interlocking of the soil particles with the reinforcement apertres [5], 2- contribting to the shear resistance depending on the direction of reinforcements with respect to the failre plane [5], and -increasing lateral stress throgh limiting lateral deformations [6]. A review of the geotechnical literatre of the geotextile-reinforced soils in the scale of element testing reveals that a majority of the stdies have been carried ot either in the triaxial device (compression loading) or in the direct shear device. The direct shear tests mainly focs on the soil-geotextile interface behavior [7-9]. Chandrasekaran et al. [1] tested polyester reinforced sands nder drained triaxial conditions, and fond that deviatoric stress and dilation increase with the nmber of polyester layers. Krishnaswamy and Isaac [11] fond that liqefaction resistance of satrated sands significantly increases with the nmber of Received by the editors Jly 28, 21; Accepted December 28, 21. Corresponding athor

2 26 M. R. Habibi et al. geotextile layers. Haeri et al. [2] performed triaxial compression tests on nreinforced and geotextile reinforced satrated sandy specimens. The reslts demonstrated that geotextile inclsion increases the peak strength, axial strain at failre, and dctility, however, it redces dilation. Failre envelopes for the reinforced sand were observed as bilinear or crved. Latha and Mrthy [12] showed that geotextile reiforced dry sands exhibit cohesion in triaxial compression. The cohesion increases with the nmber of geotextile layers. Moghaddas Tafreshi and Asakereh [1] demonstrated that strength of the geotextile reinforced silty sands increases nonlinearly with confining pressre in sch a manner that at high confining pressres the reinforcement is not effective. As a reslt of the growing interest in tilizing on-site cohesive soils in reinforced soil strctres associated with significant cost redction, the research on the sbject of the mechanical behavior of geotextile reinforced clays has been intensified (e.g., [7,14]). Ingold [15] performed a limited nmber of tests on partly satrated clay with impermeable reinforcement. The reslts revealed that the strength is redced in clay with high degree of satration. Ingold and Miller [16] fond that the drained shear strength and secant deformation modls of the reinforced clay increase with the decrease in spacing between the layers of reinforcement. Ling and Tatsoka [17] demonstrated that enhanced confinement is offered to the geotextile reinforced clay when it is consolidated anisotropically. Unnikrishnan et al. [], by the help of monotonic compression and cyclic triaxial tests, showed the enhancement of strength and deformation properties of geotextile reinforced clays throgh sing a thin layer of high-strength sand provided on both sides of the reinforcement. Mofiz ad Rahman [1] showed that reinforced clayey soils exhibit higher failre strains and volme contraction than nreinforced soils nder drained triaxial condition. Noorzad and Mirmoradi [4] carried ot nconsolidated ndrained tests on getotextile reinforced clays. They showed that moistre content, relative compaction and nmber of geotextile layers affect the strength properties of reinforced soils. They fond that peak strength increases with relative compaction and nmber of geotextile layers, and decreases with moistre content. On the other hand, in the field, soil elements ndergo different loading paths depending pon the loading conditions. It is well known that mechanical properties of sands and clays are highly dependent on the principal stress rotation. There exists a nmber of stdies that show how drained and ndrained behavior of sands is affected by rotation of principal stresses. Arthr et al. [18] tested dense and loose sands in the directional shear device, and showed that drained shear strength decreases with ( defined as the angle between major principal stress and vertical axis), while effective friction angle is not affected remarkably by the rotation of principal stress axes. Symes et al. [19] showed how pore pressre bild-p in sands is affected by rotation of principal stresses at constant shear stress. It was fond that excess pore pressre increases and deviatoric stress decreases when is increased. They revealed that accmlation of pore pressre dring cyclic principal stress rotation may even lead to failre. Yoshimine et al. [2] made it clear that the sand exhibits highest resistance with lowest contractancy in triaxial compression ( ), while triaxial extension ( 9 ) gives the opposite extreme in the assessment of flow failre. The mechanical behavior of clayey soils is also affected by rotation of principal axes. Hicher and Lade [21] fond that stress-strain behavior and pore pressre are affected by principal stress rotation, and strength and pore pressre developments occr faster in monotonic tests withot stress rotation. Hight et al. [22] tested Bothkennar clay nder triaxial compression, triaxial extension and simple shear conditions and fond that the highest ndrained shear strength is obtained in triaxial compression, while the lowest one is attained in triaxial extension. Albert et al. [2] condcted a series of laboratory tests to evalate the effect of the continos rotation of principal stresses on Bothkennar clay and fond that the effective IJST, Transactions of Civil Engineering, Volme 8, Nmber C2 Agst 214

3 Monotonic behavior of geotextile reinforced soils 27 stress paths are not as brittle as those observed in triaxial compression tests. In addition, failre of all specimens occrred when reached, on average,. Lade and Kirkgard [24] performed a series of consolidated-ndrained torsion shear tests on the K -consolidated specimens of San Francisco Bay Md. It was shown that the ndrained shear strength decreases systematically with inclination of the major principal stress, from triaxial compression with to triaxial extension with 9. Lin and Penmad [25] presented the effective friction angles, ndrained shear strengths, stress-strain relationships, pore water pressres and stress-paths as a fnction of the angle of principal stress rotation. The present stdy aims to determine the effect of rotation of principal stresses on the monotonic behavior of geotextile reinforced sand and clay. The monotonic tests have been condcted on the isotropically confined specimens nder triaxial compression (TC), triaxial extension (TE), and torsional shear (TS) conditions. Figre 1 shows the total stress path in each of the tests. Herein, p and q represent 2 the mean and deviatoric total stresses respectively, and are obtained by: 1 p, and q 1 where 1 and are major and minor principal stresses respectively. The sand and clay were reinforced with mlti layers of geotextiles, and tested nder different confining pressres. TC TS TE q 1 1 Fig. 1. Total stress path in different tests performed in this stdy Althogh all the element tests performed in this stdy can be regarded as small scales physical model tests, since the thickness (and conseqently the tensile strength) of the geotextiles has not been decreased in the model tests, the reslts shold be investigated qalitatively. As Viswanadham and Mahajan [26] mentioned, two reqirements shold be met in order to model reinforcement layers correctly: (1) scaling of tensile strength-strain behavior and (2) modeling of the bond between soil and geotextile. p 2. TEST MATERIALS A silica sand and commercial clay were selected to be reinforced with geotextile to investigate the effects of principal stress rotation on the mechanical behavior of granlar and cohesive soils. The sand was retrieved from a mine 6 km north of Tehran, Iran. It is a niformly graded silica sand with a mean grain size, D 5 of.25 mm, coefficient of niformity, C of 1.75 and a specific gravity of Its grains are sb-anglar to sb-rond in shape. It has a maximm and minimm void ratio of.82, and.4 respectively. The commercial clay had a specific gravity of 2.7, liqid limit of 42%, plasticity index of 18%, a maximm dry nit weight of 1.92 gr/cm, and an optimm water content of 2% [27]. Two, three or for layers of a woven geotextile prodced from polypropylene were interfaced between the soil layers. The properties provided by the manfactrer are given in Table 1. Agst 214 IJST, Transactions of Civil Engineering, Volme 8, Nmber C2

4 28 M. R. Habibi et al. Table 1. Properties of the geotextile sed in this stdy C.B.R Pnctre (N) Physical Properties Fabrication process Weight (gr/m 2 ) Normal thickness (mm) Woven Mechanical Properties Max. Tensile strength Max. Elongation (%) Pnctre (kn/m) resistance (N) Longitdinal Transverse Longitdinal Transverse >5 >5 42. EXPERIMENTAL PROGRAM Triaxial compression, triaxial extension and hollow cylinder torsional shear tests were performed to investigate the effects of principal stress rotation on the monotonic behavior of geotextile reinforced sand and clay. If is the angle of major principal stress ( 1 ) with vertical axis, then it wold be, 9, and 45 nder triaxial compression, triaxial extension, and torsional shear conditions respectively, for an isotropically confined specimen. Figre 2 shows how the direction of major principal stress in torsional shear test makes an angle eqal to 45 with the vertical axis. All the tests were performed nder nconsolidated ndrained conditions at three different confining pressres of 1,, and 5 kpa. Varios arrangements of geotextiles were sed in this stdy (Fig. ). The sand/clay were reinforced with either 2,, or 4 layers of geotextile. Clean sand and pre clay specimens were also tested to provide a basis for the comparison of the test reslts. A total of 72 triaxial compression, triaxial extension, and hollow cylinder torsional shear tests were performed. Fig. 2. Principal stress direction for torsional shear test on an isotropically confined specimen ( p =confining pressre, and =shear stress) Fig.. Geotextile arrangements sed in this stdy The samples were compacted in several layers ntil achieving the target density. The sandy specimens were compacted with a relative density of 7% and a water content of %. The clayey samples were also compacted at a dry nit weight eqal to 95 % of their maximm dry nit weight (γ d,max ) and with an IJST, Transactions of Civil Engineering, Volme 8, Nmber C2 Agst 214

5 Monotonic behavior of geotextile reinforced soils 29 optimm water content obtained from compaction test (ASTM D698, 212). After compacting and leveling each layer of soil, the reinforcement was placed horizontally in the specimen. The diameter of the reinforcement was slightly less than that of the specimen. The nmber of layers for preparation of the specimen was selected between 2 and 4 depending on the geotextile arrangement. The triaxial specimens had a diameter, and height of 5 cm, and 1 cm respectively. The specimens tested in the hollow torsional device had an otside diameter of 1 cm, inside diameter of 5 cm, and a height of 1 cm. All the tests, either triaxial or torsional shear were condcted nder strain-controlled conditions. The strain rate was.% /min in triaxial tests, and the tests were contined p to a strain level of 15%. Corrections sch as membrane penetration, membrane force, cell expansion, and cross-sectional area were considered and applied in the calclations. In hollow cylinder torsional shear tests, the rotation rate was.5 rad/min, and the tests were contined ntil achieving shear strains as high as 25%. 4. TEST RESULTS AND DISCUSSIONS a) Stress-strain behavior of geotextile reinforced soils Figres 4 to 6 present stress-strain crves of the reinforced sand and clay nder triaxial compression, torsional shear, and triaxial extension at different confining pressres ( p ) respectively. Vertical axis in all the triaxial tests (Figs. 4 and 6) is deviatoric stress, which is eqal to ( 1 ). The vertical axis in torsional shear tests (Fig. 6) is shear stress on the horizontal plane as shown in Fig. 2. As seen, in triaxial compression (Fig. 4), where, geotextiles (GT) have no remarkable effects on the stress-strain crves ntil axial strains as high as 2%. Then, deviatoric stress increases with the nmber of geotextile layers. On the other hand, when increases, i.e. in torsional shear and triaxial extension, geotextiles show their reinforcing effects on the stress-strain crves, almost from beginning of the loading (Figs. 5 and 6). The geotextile reinforced sand and clay exhibit brittle behavior nder all types of loading patterns, and strain at failre (where peak deviatoric stress occrs) increases with the geotextile layers. Deviatoric Stress (kpa) Deviatoric Stress (kpa) (a) Axial Strain (%) (b) Sand,Po=1kPa Sand,Po=kPa Sand,Po=5kPa 2 layer GTX,Po=1kPa 2 layer GTX,Po=kPa 2 layer GTX,Po=5kPa layer GTX,Po=1kPa layer GTX,Po=kPa layer GTX,Po=5kPa 4 layer GTX,Po=1kPa 4 layer GTX,Po=kPa 4 layer GTX,Po=5kPa Clay,Po=1kPa Clay,Po=kPa Clay,Po=5kPa 2 layer GTX,Po=1kPa 2 layer GTX,Po=kPa 2 layer GTX,Po=5kPa layer GTX,Po=1kPa layer GTX,Po=kPa layer GTX,Po=5kPa 4 layer GTX,Po=1kPa 4 layer GTX,Po=kPa 4 layer GTX,Po=5kPa Axial Strain (%) Fig. 4. Stress-strain crves nder triaxial compression test (a) reinforced sand, (b) reinforced clay Agst 214 IJST, Transactions of Civil Engineering, Volme 8, Nmber C2

6 M. R. Habibi et al. Shear Stress (kpa) Shear Stress (kpa) (a) Shear Strain (%) (b) Sand,Po=1kPa Sand,Po=kPa Sand,Po=5kPa 2 layer GTX,Po=1kPa 2 layer GTX,Po=kPa 2 layer GTX,Po=5kPa layer GTX,Po=1kPa layer GTX,Po=kPa layer GTX,Po=5kPa 4 layer GTX,Po=1kPa 4 layer GTX,Po=kPa 4 layer GTX,Po=5kPa Clay,Po=1kPa Clay,Po=kPa Clay,Po=5kPa 2 layer GTX,Po=1kPa 2 layer GTX,Po=kPa 2 layer GTX,Po=5kPa layer GTX,Po=1kPa layer GTX,Po=kPa layer GTX,Po=5kPa 4 layer GTX,Po=1kPa 4 layer GTX,Po=kPa 4 layer GTX,Po=5kPa Shear Strain (%) Fig. 5. Stress-strain crves nder torsional test (a) reinforced sand, (b) reinforced clay Deviatoric Stress (kpa) Deviatoric Stress (kpa) (a) Axial Strain (%) (b) Sand,Po=1kPa Sand,Po=kPa Sand,Po=5kPa 2 layer GTX,Po=1kPa 2 layer GTX,Po=kPa 2 layer GTX,Po=5kPa layer GTX,Po=1kPa layer GTX,Po=kPa layer GTX,Po=5kPa 4 layer GTX,Po=1kPa 4 layer GTX,Po=kPa 4 layer GTX,Po=5kPa Clay,Po=1kPa Clay,Po=kPa Clay,Po=5kPa 2 layer GTX,Po=1kPa 2 layer GTX,Po=kPa 2 layer GTX,Po=5kPa layer GTX,Po=1kPa layer GTX,Po=kPa layer GTX,Po=5kPa 4 layer GTX,Po=1kPa 4 layer GTX,Po=kPa 4 layer GTX,Po=5kPa Axial Strain (%) Fig. 6. Stress-strain crves nder triaxial extension test (a) reinforced sand, (b) reinforced clay b) Undrained shear strength of geotextile reinforced soils Undrained shear strength ( S ) for specimens in triaxial compression and extension tests is obtained as the maximm vale of the deviatoric stress (q) from Figs. 4 and 6. On the other hand, as shown in Fig. 2, for torsional shear test on a specimen nder isotropic confining pressre, the vale of deviatoric stress is: q 1 2 where is the shear stress on the horizontal plane, i.e. the vertical axis in Fig. 5. Conseqently, S will be two times the maximm vale of the shear stress obtained from Fig. 5. As IJST, Transactions of Civil Engineering, Volme 8, Nmber C2 Agst 214

7 Monotonic behavior of geotextile reinforced soils 1 shown in Fig. 7, S decreases with so that S in reinforced sand and clay is highest in triaxial compression, and lowest in triaxial extension. In general, S of the reinforced sand is higher than that of reinforced clay. Figre 8 presents the effects of geotextiles on the ndrained shear strength of sand and clay respectively. Herein, ndrained shear strength of reinforced material ( S,Reinf. ) is normalized to the ndrained shear strength of nreinforced material ( S,Unreinf. ). The ratio is compted at each confining pressre, and then averaged over all the confining pressres. As seen, shear strength increases with the nmber of geotextile layers irrespective of the test type or parent material. However, geotextile is more beneficial when sed in sands, and the vale of S, Re inf. S, Unre inf. in the reinforced sand is higher than that of reinforced clay. This shows that the bond strength between the sand and geotextile is higher than that of the clay and geotextile. S (MPa) S (MPa) (a) No. of Geotextile Layers (b) TC,Po=1kPa TS,Po=1 kpa TE,Po=1kPa TC,Po=kPa TS,Po=kPa TE,Po=kPa TC,Po=5kPa TS,Po=5kPa TE,Po=5kPa TC,Po=1kPa TS,Po=1 kpa TE,Po=1kPa TC,Po=kPa TS,Po=kPa TE,Po=kPa TC,Po=5kPa TS,Po=5kPa TE,Po=5kPa No. of Geotextile Layers Fig. 7. Undrained shear strength in (a) reinforced sand, and (b) reinforced clay (S,Reinf. /S,Unreinf. ) avg 2.4 TE, Sand 2.2 TC, Sand TS, Sand 2 TE, Clay TC, Clay 1.8 TS, Clay No. of Geotextile Layers Fig. 8. Effect of geotextiles on the ndrained shear strength of sand and clay for three different types of test Figre 9 presents the effect of confining pressre on the ndrained shear strength of the geotextile reinforced sand and clay respectively. Herein, ndrained shear strength at each confining pressre ( S, p ) has been normalized to the ndrained shear strength at lowest confining pressre, i.e. 1 kpa ( S, p 1kPa ). The ratio has been compted for each specimen, and then averaged over all the specimens at that particlar confining pressre. As seen, shear strength increases linearly with confining pressre irrespective of or parent material. In geotextile reinforced sand, S, p S, p1kpa does not depend on, however, in reinforced clays, S, p S, p 1kPa increases with. In addition, increasing confining Agst 214 IJST, Transactions of Civil Engineering, Volme 8, Nmber C2

8 2 M. R. Habibi et al. pressre leads to more shear strength increase in reinforced sands than clays. For instance, S, p S, p 1kPa reaches as high as.2 in the reinforced sand when confining pressre is 5 kpa, while the ratio reaches to 2. in the reinforced clay nder the same conditions..5 TE, Sand TS, Sand 2.5 TC, Sand TE, Clay 2 TS, Clay TC, Clay Confining Pressre (kpa) Fig. 9. Effect of confining pressre on the ndrained shear strength of reinforced sand and clay (S,p /S,p=1 kpa ) avg c) Failre envelope of geotextile reinforced soils Figres 1a and b show failre envelopes in terms of total stresses in q p plane for the geotextile reinforced sand and clay respectively. Failre in all the reinforced soils happened within the soil, since no evidence of geotextiles failre (either ripping or plling ot) was captred dring and after the tests. As seen, failre envelopes are almost non-linear, and the lowest failre envelopes belong to the triaxial extension tests. On the other hand, failre envelopes from torsional shear tests fall above that of triaxial compression tests in geotextile reinforced sands (Fig.1a), however, the trend is reversed in the geotextile reinforced clays, and except for pre clay in which failre envelope from torsional shear and triaxial compression tests are identical, the failre envelopes from triaxial compression tests lie above that of triaxial compression tests (Fig. 1b). q (MPa) q (MPa) (a) p (MPa) (b) TC Sand TS Sand TE Sand TC 2 layer GTX TS 2 layer GTX TE 2 layer GTX TC layer GTX TS layer GTX TE layer GTX TC 4 layer GTX TS 4 layer GTX TE 4 layer GTX TC Clay TS Clay TE Clay TC 2 layer GTX TS 2 layer GTX TE 2 layer GTX TC layer GTX TS layer GTX TE layer GTX TC 4 layer GTX TS 4 layer GTX TE 4 layer GTX p (MPa) Fig. 1. Total stress failre envelopes in (a) reinforced sands, and (b) reinforced clay In addition, it can be inferred from Fig. 1a that the slope of failre line increases with the nmber of geotextile layers in the geotextile reinforced sands. The slope is highest in torsional shear ( 45 ) and IJST, Transactions of Civil Engineering, Volme 8, Nmber C2 Agst 214

9 Monotonic behavior of geotextile reinforced soils lowest in triaxial extension ( 9 ). On the other hand, the slope of failre line in geotextile reinforced clays (Fig. 1b) does not show strong dependency on the nmber of geotextile layers and rotation of principal stresses. Althogh geotextile reinforced sands (Fig. 1a) do not show remarkable cohesion intercept (i.e. where failre line meets vertical axis), cohesion intercept increases with the nmber of geotextile layers and decreases with in geotextile reinforced clays. Since, the trend of variation of cohesion intercept with in the reinforced clays, and the slope of failre line with in the reinforced sand is the same as the nreinforced material (either clay or sand), one can conclde that in the types of tests (i.e., triaxial compression, triaxial extension and torsional shear) failre has occrred within the soil. A smmary of the effects of rotation of the direction of principal stresses on the mechanical behavior of geotextile reinforced sand and clay is presented in Table 2. Table 2. Effects of rotation of principal stresses on the mechanical behavior of reinforced sand and clay Parameter/Characteristic Geotextile reinforced sand Geotextile reinforced clay stress-strain crve when increases geotextiles show their when increases geotextiles show their reinforcing effects on the stress-strain reinforcing effects on the stress-strain crves from beginning of the test crves from beginning of the test strain at failre does not depend on does not depend on S decreases with decreases with S Highest at 9, lowest at 45 Highest at 9, p S, p kpa does not depend on increases with Highest at 45, lowest at 9 moves down with Highest at 45, lowest at does not depend on, Re inf. S, Unre inf. S 1 failre envelope slope of failre line cohesion intercept N.A. decreases with, lowest at CONCLUSION A laboratory stdy was carried ot to investigate the effects of rotation of principal stresses on the mechanical behavior of geotextile reinforced sand and clay. The isotropically confined reinforced soils were tested nder triaxial compression ( ), torsional shear ( 45 ) and triaxial extension ( 9 ) conditions, where is the angle between the direction of major principal stress and vertical axis. The following conclsions may be drawn based on this experimental stdy: 1. Geotextiles improve the mechanical properties of the sand and clay. The geotextile reinforced sand and clay exhibit brittle behavior nder all types of loadings. Strain at failre and ndrained shear strength increases with the nmber of geotextile layers in both reinforced sand and clay. In reinforced sand, slope of failre line increases with the nmber of geotextile layers, while in reinforced clay, cohesion intercept increases with the nmber of geotextile layers. In light of the increase in the ndrained shear strength, strain at failre, slope of failre line (for the sand) and cohesion intercept (for the clay) with the nmber of geotextile layers (which is in accordance with the previos stdies), and since failre has occrred within the soil, it can be conclded that the increase in aspect ratio of the soil confined between two geotextile layers, and lateral constraint provided by the geotextiles are the main cases of the enhancement of strength and stiffness properties of the geotextile reinforced soils. 2. In both geotextile reinforced sand and clay, ndrained shear strength ( S ) decreases with. In other words, the trend of variation of S with in the reinforced soil is the same as the parent material (either sand or clay), which means that in all types of tests (i.e., triaxial compression, triaxial extension and torsional shear) failre has occrred within the soil. In both reinforced sand Agst 214 IJST, Transactions of Civil Engineering, Volme 8, Nmber C2

10 4 M. R. Habibi et al. and clay S, Re inf. S, Unre inf. is highest at 9 and lowest at 45. In reinforced sand S, p S, p 1kPa does not depend on, while in reinforced clay it increases with.. Total-stress failre envelope depends on. Failre envelopes from triaxial extension tests are at their lowest position for both reinforced sand and clay. In reinforced sand, they are at their highest position at 45, while in reinforced clay the highest position is achieved when. 4. Althogh the slope of failre line does not depend on for reinforced clay, it is highest at 45 and lowest at for reinforced sand. In addition, cohesion intercept decreases with in reinforced clay. 5. It appears that geotextiles are more beneficial when sed to reinforce granlar material, since parameters like S, Re inf. S, Unre inf., and S, p S, p 1kPa are higher in geotextile reinforced sand compared with the corresponding vales for geotextile reinforced clay. This means that sand particles exhibit better interlocking with geotextiles than the clay particles. Acknowledgement: The torsional shear tests were performed at the International Institte of Earthqake Engineering and Seismology which is acknowledged. REFERENCES 1. Mofiz, S. A. & Rahman, M. M. (21). Evalation of failre load-deformation characteristics of geo-reinforced soil sing simplified approach. 11th IAEG Congress, Ackland, New Zealand, pp Haeri, S. M., Noorzad, R. & Oskoorochi, A. M. (2). Effect of geotextile reinforcement on the mechanical behavior of sand. Geotextiles and Geomembranes, Vol. 18, No. 6, pp Unnikrishnan, N., Rajagopal, K. & Krishnaswamy, N. R. (22). Behavior of reinforced clay nder monotonic and cyclic loading. Geotextiles and Geomembranes, Vol. 2, No. 2, pp Noorzad, R. & Mirmoradi, S. H. (21). Laboratory evalation of the behavior of a geotextile reinforced clay. Geotextiles and Geomembranes, Vol. 28, No. 4, pp Mofiz, S. A., Rahman, M. M. & Alim, M. A. (24). Stress-strain behavior and model of reinforced residal soil. 15th Soth Asian Geotechnical Society Conference, Bangkok, Thailand, pp Broms, B. B. (1977). Triaxial tests with fabric reinforced soil. Proc. Int. Conf. on the Use of Fabrics in Geotechnics, Vol., Ecole Nationale des Ponts et Chassees, Laboratoire Central des Ponts et Chassees, paris, pp Athanasopolos, G. A. (1996). Reslts of direct shear tests on geotextile reinforced cohesive soil. Geotextiles and Geomembranes, Vol. 14, No. 11, pp Lee, K. M. & Manjnath, V. R. (2). Soil-geotextile interface friction by direct shear tests. Canadian Geotechnical Jornal, Vol. 7, No. 1, pp Ghazavi, M. & Ghaffari, J. (21). Experimental investigation of time-dependent effect on shear strength parameters of sand-geotextile intersect. Iranian Jornal of Science and Technology, Transactions of Civil Engineering, Vol. 7, No. C1, pp Chandrasekaran, B., Broms, B. B., Wong, K. S. (1989). Strength of fabric reinforced sand nder axisymmetric loading. Geotextiles ad Geomembranes, Vol. 8, pp Krishnaswamy, N. R. & Issac, N. T. (1994). Liqefaction potential of reinforced sand. Geotextiles ad Geomembranes, Vol. 1, pp Latha, G. M. & Mrthy, V. S. (26). Effects of reinforcement form on the behavior of geosynthetic reinforced sand. Geotextiles and Geomembranes, Vol. 25, No. 1, pp IJST, Transactions of Civil Engineering, Volme 8, Nmber C2 Agst 214

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