LARGE TRIAXIAL TESTS ON FABRIC REINFORCED AND CEMENT MODIFIED MARGINAL SOIL

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1 IGC 2009, Guntur, INDIA LARGE TRIAXIAL TESTS ON FABRIC REINFORCED AND CEMENT MODIFIED MARGINAL SOIL G.V. Praveen Research Scholar, Faculty, S.R. Engg. College, Warangal, India. V. Ramana Murty Assistant Professor, Dept. of Civil Engg., National Institute of Technology, Warangal , India. ABSTRACT: In view of the inevitable use of marginal soils in mechanically stabilized earth construction due to the paucity of suitable backfill soils at many work sites, there arises the need to understand the behaviour of such materials to make suitable amendments in design. This paper presents the shear strength behaviour of fabric reinforced marginal soil without and with cement modification and compares its performance with that of reinforced sand. The provision of geotextile reinforcement also facilitates internal drainage to offset the low permeability of marginal soil. The cement modified reinforced marginal soil has shown significant improvement in shear strength both under undrained and drained s by overcoming the ill-effects of plasticity. This study revealed that the cement modified marginal soil has become non-plastic with its performance close to that of sand. 1. INTRODUCTION Since its inception in France by Henri Vidal in 1969, reinforced earth has made ever-increasing traits in the field of civil engineering. As a composite material, it offers resistance to both static and dynamic loads by interfacial frictional forces between the soil and reinforcement (Hausmann, 1990; Bergado, 1995; Saran, 2006). The effectiveness of soil reinforcement interaction depends on the characteristics of backfill soil in terms of its drainage and frictional properties (Ingold, 1981; Hausmann, 1990; Goel, 2006). Generally coarse grained soils with percent fines less than 15 are suitable for backfill. However, due to non availability of such favorable soils in many instances it is compelled to use poor backfill soils known as marginal soils all over the world (Glendinning et al. 2005; Won & Kim, 2007). Several failures of reinforced earth structures were reported by various investigators due to the use of such marginal backfills (Tatsuoka, 1997; Koerner, 2000; Goel, 2006). In order to overcome the ill effects of marginal soils in terms of their excess fines and plasticity, an attempt is made to modify them using cement as admixture. This improved marginal soil is characterized for its suitability as backfill in terms of shear strength behaviour of reinforced marginal soil without and with cement modification both under undrained and drained triaxial test s. s of varying tensile strength were used as reinforcement in order to understand the different failure mechanisms. 2. MATERIALS AND METHODOLOGY The present investigation is undertaken to understand the shear strength behaviour of reinforced marginal soil without and with cement modification for which the following materials and methodology were adopted. 2.1 Materials Sand Locally available river sand is used with the following properties: Specific Gravity = 2.65; Gravel = 5%; Sand = 95%; Silt = 0%; Clay = 0%; IS classification = SP; OMC = 12%; MDD = 1.58 Mg/m 3 ; c ' = 0 kpa; φ ' = 41 and Coefficient of permeability, k = cm/sec Upland Soil Locally available upland soil is used as marginal soil and its properties are: Specific Gravity = 2.68; Gravel = 0%; Sand = 58%; Silt = 24%; Clay = 18%; Liquid limit = 39%; Plastic limit = 17%; IS classification = SC; OMC = 16.5%; MDD = 1.78 Mg/m 3 ; c u = 52 kpa; φ u = 14 ; c ' = 20 kpa; φ ' = 29 and Coefficient of permeability, k = cm/sec Cement Ordinary Portland cement of 53 grade is used to modify the upland soil. 462

2 2.1.4 Soil cement The marginal soil cement mixes with different cement contents were tested for their Atterberg limits and UCS values as given in Table 1. From this Table, it can be seen that the soil has become non plastic (NP) at 2% cement content and for the subsequent shear strength studies 3, 5 and 10% cement contents were used. Table 1: Properties of Marginal Soil-Cement Mixes Property Cement content (%) Atterberg Limits (Immediately after adding cement) Liquid Limit, w l (%) NP Plastic Limit, w p (%) Atterberg Limits (At 3 days curing period) Liquid Limit, w l (%) NP NP NP NP Plastic Limit, w p (%) Unconfined compressive strength (UCS) (at 3 days curing period for cement modified samples) kpa s Two non-woven geotextiles (Q425D and Fibertex G 100) and a woven geotextile (PD381) were used as reinforcing materials and their properties are given in Table 2. Property Weight Thickness at 2 kpa Wide width tensile strength In plane permeability Apparent opening size, O 95% Table 2: Properties of s Fibertex G 100 (Non-woven) 100 g/m mm 4.0 kn/m 0.13 m/sec 110 micron Q 425 D (Nonwoven) 275 g/m mm 8.3 kn/m 0.05 m/sec 60 micron PD 381 (Woven) 235 g/m mm 18 kn/m m/sec 212 micron 3. TEST PROCEDURE The reinforced marginal soil samples without and with cement modification were tested in large triaxial cells (Fig. 1). This testing was aimed at understanding the shear strength behaviour of fabric reinforced marginal soil without and with cement modification and to compare its performance with that of reinforced sand. Fig. 1: Bulging Failure of Fabric Reinforced Marginal Soil Sample 3.1 Sample Preparation The test samples of 100 mm diameter and 200 mm height were prepared using a split cylindrical mould by static compaction by varying the number of geotextile reinforcing layers. For plain soil samples, the required dry weight of soil for each sample corresponding to maximum dry density was taken and the calculated saturated moisture content was added. This wet soil was divided into equal parts as per the number of reinforcing layers and pressed to the desired thickness in compression testing machine. In case of cement modified samples, the required dry weight of marginal soil for each sample was taken and the proposed cement content was added and then mixed with the optimum moisture content of virgin soil. This soil cement mix was compacted in the same manner as for virgin soil samples depending upon the number of reinforcing layers by static compaction. The prepared samples were kept in perforated polythene bags and placed in desiccators for 24 hours before they were moisture cured by submerging them in water tubs for 3 and 7 days of curing period. The reinforced sand samples were prepared using split mould as per the reinforcing layer configuration by static compaction Large Triaxial Tests These tests were carried out on fabric reinforced marginal soil samples without and with cement modification using large triaxial cell under undrained and drained s in order to understand the influence of low permeability of such a soil on the shear strength. For reinforced sand samples, the tests were conducted only under drained in view of their free draining property. The deviator load was applied at a constant strain rate of 1.20 mm/min for undrained and 0.01 mm/min for drained until failure or up to 20% strain. 463

3 4. RESULTS AND DISCUSSION 4.1 Stress-Strain Behaviour Typical stress-strain patterns for 3-layer fabric reinforced marginal soil without cement modification for σ 3 = 150 kpa are shown in Figures 2 & 3 under undrained and drained s respectively. Fig. 2: Stress-strain Patterns for 3-layer Fabric Reinforced Plain Marginal Soil for σ 3 = 150 kpa under drained, whereas in undrained it could not be distinctly measured due to premature failure of soil-cement as observed from failed samples. Further, it was observed that stress-strain patterns of cement modified and fabric reinforced marginal soil samples were similar to those obtained for reinforced plain sand samples Strength Parameters The shear strength of fabric reinforced plain marginal soil samples under undrained and drained s are given in Table 3. It can be observed from this table that the angle of internal friction is little influenced by fabric reinforcement under both the drainage s for any fabric used. However, the provision of fabric reinforcement resulted in considerable increase in cohesion component under drained with only a nominal variation under undrained. The nominal influence of fabric reinforcement on cohesion component under undrained could be attributed to the little interaction between the soil and fabric reinforcement. The increase in cohesion component under drained can be supported by the nominal stretching of fabric reinforcing layers without any signs of slippage, as could be observed from tested samples. Table 3: Strength Parameters of Marginal Soil (M.S.) for Different Reinforcement Configurations Fig. 3: Stress-strain Curves of Reinforced Marginal Soil under Condition for Different Fabric Reinforcement for σ 3 = 150 kpa As can be observed from Figure 2 that there is only a nominal increase in strength by the provision of reinforcement under undrained. However, in case of drained, the strength gain by marginal soil upon fabric reinforcement is more than twice that for undrained indicating the significance of drainage to ensure proper soil-fabric interaction (Fig. 3). Hence, the low permeable marginal soil with higher plasticity cannot be used in reinforced soil unless elaborate drainage arrangements are made to ensure proper soil reinforcement interaction. The fabric layers were observed to be subjected to sliding without any signs of rupture in plain soil under both the drainage s. Similar stress-strain patterns were observed for cement modified marginal soil samples also except for higher peak loads with gradual failure indicating more of its ductile nature. The woven (PD381) and non woven (Q425D) geotextile layers were partly stretched and partly slided whereas the Fibertex G-100 fabric layers were subjected to rupture failure. Further, the higher strength gain by soil-cement due to fabric reinforcement at greater curing periods could be reflected Woven (PD381) Non Woven (Q425D) Fibertex G 100 Test M. S. + 0 layer 1 layer 2 layers 3 layers c u φ u c' φ' c u φ u c' φ' c u φ u c' φ' The undrained and drained shear strength of fabric reinforced and cement modified marginal soil samples are presented in Table 4. As can be seen from this table that the increment in shear strength of soil cement is only nominal with increasing number of layers and cement content under undrained. The samples under undrained were tested for one curing period i.e. at 7 days. 464

4 Table 4: Strength Parameters of Cement Modified Marginal Soil (M.S.) Samples with Different Reinforcement Configurations under Undrained and Drained Conditions PD381 (woven) Q425D (nonwoven) Fibertex G 100 (nonwoven) Test c u(7days) + 0 lyr + 1 lyr + 2 lyrs + 0 lyr + 1 lyr + 2 lyrs 10%cem φ u(7days) c' 3 days φ' 3 days c' 7 days φ' 7 days c u(7days) φ u(7days) c' 3 days φ' 3 days c' 7 days φ' 7 days c u(7days) φ u(7days) c' 3 days φ' 3 days c' 7 days φ' 7 days The drained shear strength indicate that the angle of internal friction is significantly increased with cement modification of marginal soil; and the provision of fabric reinforcement further increased the cohesion component and angle of internal friction. This could be supported by the mixed failure pattern of partial stretching and partial slippage in case of woven (PD 381) and non woven (Q 425 D) fabric reinforcement. With Fibertex G 100, the angle of internal friction is increased only by 2 3 and cohesion component is increased significantly. The distinct rupture failure (Fig. 4) of Fibertex G 100 fabric with considerable extensibility could be attributed to this observed trend. It is interesting to note that the drained shear strength of fabric reinforced and cement modified marginal soil are close to those obtained for fabric reinforced sand (Table 5). Table 5: Strength Parameters of Reinforced Sand ( Condition) sand + 0 layer Sand + 1 layer sand + 2 layers Sand + 3 layers PD 381 c' φ' Q425D c' φ' Fibertex c' G 100 φ' Fig. 4: Rupture of Fibertex G-100 reinforcing layer. 5. SUMMARY AND CONCLUSIONS The fabric reinforced marginal soil upon cement modification could ensure proper soil-reinforcement interaction resulting in higher shear strength due to its nullified plasticity with added cementation. The shear strength of cement modified reinforced marginal soil are almost similar to those obtained for sand. This could be supported by the non-plastic nature of cement modified marginal soil coupled with enhanced internal drainage provided by fabric reinforcement. The failed samples have shown a mixed failure of stretching coupled with slippage in 465

5 case of woven (PD 381) and non-woven (Q425D) geotextiles up to 5% cement content in marginal soil. However, in case of Fibertex G-100 non-woven geotextile, predominantly rupture failure is observed in all the drained test s for cement modified marginal soil samples. The reinforced samples of both the marginal soil and sand for different test s have shown progressive failure as against the post peak yielding of plain soil samples. With increasing stiffness of cement modified marginal soil at higher cement contents, the cohesion component is considerably increased with nominal variation in the angle of internal friction. REFERENCES Bergado, D.T., Werner, G., Ten, M.H., and Zen, X.H. (1995). Interaction Between s and Silty Sand by Large Direct and Triaxial Tests, Proceedings of Geosynthetics 95, Nashville, U.S.A., Vol. 3, Glendinning, S., Jones, C.J.F.P. and Pugh, R.C. (2005). Reinforced Soil Using Cohesive Fill and Electro Kinetic Geosynthetics, International Journal of Geomechanics, ASCE, Vol. 5, No. 2, Goel, R. (2006). Mechanically Stabilized Earth Walls and Reinforced Soil Slopes: Indian scenario A Comprehensive Review, Journal of the Indian Roads Congress, Vol. 67, No. 1, Hausmann, M.R. (1990). Engineering Principles of Ground Modification, McGraw-Hill Publishing Company, New York. Ingold, T.S. (1981). A Laboratory Simulation of Reinforced Clay Walls, Geotechnique, Vol. 31, No. 3, Koerner, R.M. (2000). Emerging and Future Developments of Selected Geosynthetic Applications, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 126, No. 4, Swami Saran. (2006). Reinforced Soil and its Engineering Applications, I.K. International Pvt. Ltd, New Delhi, India. Tatsuoka, F., Tateyama, M., Uchimura, T. and Koseki, J. (1997). Geosynthetic-Reinforced Soil Retaining Wall as Important Permanent Structures, Geosynthetics International, Vol. 4, No. 2, Vidal, H. (1969). The Principle of Reinforced Earth, Highway Research Record, Vol. 282, Wang, Y.H. and Wang, M.C. (1993). Internal Stability of Reinforced Soil Retaining Structures with Cohesive Backfills, Transportation Research Record No. 1414, Transportation Research Board, Washington, D.C., Won, M.S. and Kim, Y.S. (2007). Internal Deformation Behaviour of Geosynthetic-reinforced Soil Walls, s and Geomembranes, Elsevier, No. 25,