Experimental Study of Stone Columns Installation in Kaolin Clay

EXPERIMENTAL STUDY OF STONE COLUMNS INSTALLATION IN KAOLIN CLAY Mounir Bouassida1,a , Wissem Frikha1,b and Jean Canou2 1 Civil Engineering Department, University Tunis El Manar/National Engineering School of Tunis, BP 37 Le Belvédère 1002 Tunis, Tunisia. E-mail: a [email protected], b frikha [email protected] CERMES, Institut Navier. ENPC, 6 et 8 Avenue Blaise Pascal - Cité Descartes Champs-sur-Marne, 77455 Marne la Vallée cedex 2, France. E-mail: [email protected] The behavior of remolded kaolin clay reinforced by stone column is investigated in laboratory. The installation of stone columns was simulated by performing a lateral expansion at different rates within hollow cylindrical remolded kaolin specimens initially subjected to K0 consolidation path. After the simulated column installation, specimens were subjected to classical consolidated undrained triaxial tests with recorded excess pore pressure. The experimental program analyzed the effects of consolidation stress and the simulated installation of stone column on the improvement of undrained Young modulus and shear strength of kaolin clay. Obtained results showed, as result of column installation, a significant improvement of Young modulus when the cavity expansion ratio and the consolidation stress increase. Additionally, the undrained shear strength of improved kaolin clay mainly increases at lower consolidation stress. Whilst, the ratio between undrained Young modulus and cohesion increases when the consolidation stress decreases. Keywords: Stone column, Remolded kaolin, Cavity expansion, Improved characteristics. 1. INTRODUCTION Several theoretical and experimental studies have shown that the improvement of soft soils by vibro-stone columns leads to the increase of bearing capacity and reduction of settlement of foundations. These performances mainly depend on the strength of column material, type of in situ soil, improvement area ratio, column length and grain size of column material: Bouassida (1996), Wood et al. (2000), Sivakumar et al. (2004), Ambily and Gandhi (2007), Black et al. (2007), Andreou et al. (2008). The vibro-replacement method used in the reinforcement by stone columns leads to the improvement of initial mechanical properties of soil (Guetif et al, 2007). This improvement has been also verified after recorded data from in situ tests (SPT, CPT, vane shear) and laboratory tests as well (Frikha, 2010). This paper studies the influence of the method of column installation in soft clay in term of modified initial soil behavior. An experimental approach is suggested to simulate the installation of stone column by horizontal expansion occurring in cylindrical cavity due to applied Proceedings of the International Conference on Ground Improvement and Ground Control Edited by Buddhima Indraratna, Cholachat Rujikiatkamjorn and Jayan S. Vinod Copyright © 2012 by Research Publishing Services. All rights reserved. ISBN: 978-981-07-3559-3 :: doi:10.3850/978-981-07-3559-3 02-0219 763 764 Proceedings of the International Conference on Ground Improvement and Ground Control internal pressure. First, the preparation of hollow cylindrical kaolin clay specimens is presented. The procedure of the expansion of cylindrical cavity and column installation are described as well as for the consolidated undrained triaxial tests which aim to analyze the modification of soft clay proprieties. Experimental results are interpreted and confronted with those obtained previously. 2. PREPARATION OF CLAY SPECIMEN A speswhite kaolin, extracted from South Western England, having ultra fine particles is used to represent the soft clay. The grain size distribution of kaolin specimens shows 80% of particle dimension is less than 2 µm. Properties of kaolin clay are: specific gravity = 2.6, liquid limit = 57%, plasticity index = 24% and compression index = 0.136. The kaolin specimen is prepared by using the slurry consolidation method. The powder of virgin kaolin is mixed at a water content of 86% that is about 1.5 times its liquid limit. The obtained slurry is shacked by an electrical mixer, during 30 mn, and, then, it is placed, at least 24 hours, in a vacuum bell jar to eliminate the trapped air. The slurry sample is subjected to an initial K0 consolidation (Frikha et al., 2008). The consolidometer is a hollow cylinder of 100 mm internal diameter and 300 mm height. A vertical thin tube of 20 mm diameter, covered by a filter paper is placed in the axis of consolidometer to form the cavity. The consolidometer is then filled by the slurry and the kaolin sample is formed. The remolded sample was drained vertically by two porous stones placed at the top and bottom of consolidometer. A horizontal drainage was assured by four bands of filter paper with 40 mm in width placed at the internal surface of hollow cylinder. A silicone grease was spread on the internal contact surface between the remolded soil and the tube to reduce the roughness’ effect that allows an easy unmold of the specimen. By using a spoon the mould was carefully filled by the slurry which is gently tamped to expulse the penetrated air. Once the predetermined height of specimen is reached, the consolidation loading cap is placed and the loading starts slowly to provide the desired consolidation stress (Figure 1). The consolidation of slurred soil is considered completed when the recorded settlement stops to evolve. This procedure was adopted for all tests during which two levels of consolidation stress were tested: 98 kPa and 170 kPa. 3. EXPANSION OF CYLINDRICAL CAVITY The height of hollow specimen unmolded from the consolidometer varies between 18 and 20 cm. After its installation into the hollow cylindrical cell the specimen is subjected to a full saturation so that the minimum recorded Skempton’s coefficient is 95% (Figure 2). The applied isotropic consolidation stresses were: σint = 100, 200 and 300 kPa. The consolidation phase is followed by the horizontal expansion of cavity under controlled increase of water volume, and the corresponding internal pressure as well, by using a pressure volume controller GDS equipment that is a general-purpose water pressure source and volume change gauge. The expansion tests are executed in undrained condition characterized by a prescribed deformation rate of 0.055% per minute. According to the in-situ installation of stone columns five levels of cavity expansion ratio are applied: V/V0 = 1, 1.25, 1.50, 1.75 and 2.00 where V0 and V denote respectively the initial and post expansion volume of cavity. These expansion ratios correspond to a duration of 0, 7.5, 15, 22.5 and Experimental Study of Stone Columns Installation in Kaolin Clay 765 1: Vertical displacement sensor 2: Force sensor 3: Exterior and interior neoprene membranes 4: Cavity 5: Kaolin 6: Hollow screw 7: Porous stones 8: Pressure transducers Figure 1. K0 consolidometer. Figure 2. Cross section of the hollow cylinder. 30 mn respectively. The expansion starts by an increase of internal pressure applied along the cavity border to reach the initial volume of cavity V0 , and then the internal pressure is increased at the prescribed rates while the applied confining pressure is kept constant. 4. TRIAXIAL TESTS At the end of cavity expansion, the hollow specimens were subjected, according to AFNOR standard NF P 94–074, to consolidated undrained (CU) triaxial tests where the same rate of axial displacement of 0.03 mm/mn during the shear phase was applied. The experimental program comprised fifteen triaxial tests summarized in Table 1. Each triaxial CU + u test comprises three steps: ∗ Step 1: The isotropic consolidation is applied for three confining stress: 100, 200 and 300 kPa. ∗ Step 2: The lateral expansion of cavity is run, as detailed in section 3, by allowing the drainage of specimen and recording its volume variation and excess pore pressure. This step ends when no volume variation is observed and excess pore pressure is negligible. ∗ Step 3: The shear loading is applied in undrained condition, at constant controlled strain rate by a linear variable differential transformer, with recorded excess pore pressure till Table 1. Summary of triaxial CU + u tests data. Test Number Test qualifier 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 K K K K K K K K K K K K K K K CU100 CU100 CU100 CU100 CU100 CU200 CU200 CU200 CU200 CU200 CU300 CU300 CU300 CU300 CU300 Cavity 1.00V0 Cavity1.25V0 Cavity1.50V0 Cavity1.75V0 Cavity2.00V0 Cavity1.00V0 Cavity1.25V0 Cavity1.50V0 Cavity1.75V0 Cavity2.00V0 Cavity1.00V0 Cavity1.25V0 Cavity1.50V0 Cavity1.75V0 Cavity2.00V0 σ3 (kPa) σ0 (kPa) Repetition V/V0 100 100 100 100 100 200 200 200 200 200 300 300 300 300 300 98 98 98 98 98 170 170 170 170 170 170 170 170 170 170 2 3 3 3 2 2 2 2 2 2 2 1 1 1 2 1.00 1.25 1.50 1.75 2.00 1.00 1.25 1.50 1.75 2.00 1.00 1.25 1.50 1.75 2.00 766 Proceedings of the International Conference on Ground Improvement and Ground Control failure of the specimen; both the internal and external pressures are kept constant. The deviatoric stress is deduced from the recorded axial force. The behaviour of the speswhite kaolin was examined in post cavity expansion, for five deformation levels, with total dissipation of excess pore pressure. Table 1 indicates the majority of tests have been performed twice or three times to confirm the recorded results, to verify the repeatability of tests and to validate the experimental procedure and used equipments. The repetition of some tests was performed with the same procedures and devices and gave a good convergence of results with a relative error of recorded data lower than 5%. Averaged values of recorded undrained shear strength and Young’s modulus were considered. 5. INTERPRETATION OF RESULTS Results of triaxial tests are presented with focus on the influence of cavity expansion ratio and undrained shear strength and Young modulus of reinforced soil. 90 5.1. Influence of Cavity Expansion Ratio 140 80 120 70 Deviatoric stress q (kPa) Deviatoric stress q (kPa) Figures 3a and 3b illustrate the evolution of deviatoric stress versus the axial strain for different ratios of expanded cavity and for two consolidation stress values 100 and 200 kPa. Figure 4 shows a quasi-linear variation of the peak deviatoric stress Axial strain (%) Axial strain (%) in function of cavity expansion ratio and consolidation stress equal 100 and Figure 3. Stress-strain curves during CU + u triaxial 200 kPa. tests (σ3 = 100 kPa and 200 kPa). Figure 5 clearly indicates that the normalized peak deviatoric stress (qmax /σ3 ) increases as the cavity expansion ratio V/V0 does. Besides, the variation of (qmax /σ3 ) versus the cavity expansion ratio is also more significant when the consolidation stress is the lowest. For σ3 = 100 kPa, (qmax /σ3 ) increases from 0.63 to 0.87 although, for σ3 = 300 kPa, it increases from 0.53 to 0.62 when the cavity expansion ratio (V/V0 ) varies between 1.0 and 2.0. 100 80 60 60 50 40 30 40 20 ȱK_CU100_Cavity1.00V0 ȱK_CU100_Cavity1.25V0 ȱK_CU100_Cavity1.50V0 ȱK_CU100_Cavity1.75V0 20 ȱK_CU100_Cavity2.00V0 10 ȱK_CU200_Cavity1.00V 0 ȱK_CU200_Cavity1.25V 0 ȱK_CU200_Cavity1.50V 0 ȱK_CU200_Cavity1.75V 0 ȱK_CU200_Cavity2.00V 0 0 0 0 5 10 Ha 15 20 0 5 10 15 20 ȱ Ha 5.2. Undrained Shear Strength and Young Modulus From consolidated undrained triaxial tests, the undrained shear strength is defined as the half of peak deviatoric stress (cU = qmax /2). The evolution of the undrained shear strength cU (V )/cU (V0 ) in function of the cavity expansion ratio significantly increases as the cavity expansion ratio V/V0 increases (Figure 6). For the maximum cavity expansion ratio V = 2V0 , V = current volume of expanded cavity (V0 is its initial value), the undrained shear strength cU respectively increases by 1.37, 1.25 and 1.18 times for consolidation stresses of 100, 200 and 300 kPa. The normalized shear strength cU (V )/cU (V0 ) is improved more significantly at lower consolidation stress. The undrained Young modulus is determined from the initial quasi-linear observed Experimental Study of Stone Columns Installation in Kaolin Clay 200 767 0,9 175 0,8 125 qm ax/V qm ax(kPa) 150 100 75 0,7 0,6 50 0,5 25 V3=100 kPa 0 1,00 V3=200 kPa 1,25 1,50 V3=300 kPa 1,75 V3=100 kPa 1,00 1,25 V/V0 V3=100 kPa 30 V3=200 kPa (MPa) cU (V)/cU (V 0) 2,00 40 V3=300 kPa ȱEU 1,2 20 10 1,1 ȱȱV =100ȱkPa ȱȱV =200ȱkPa ȱȱV =300ȱkPa 0 1,25 1,50 1,75 1,00 2,00 1,25 1,50 1,75 2,00 V/V0 V/V 0 Figure 6. Gain in undrained shear strength versus cavity expansion ratio. Figure 7. Undrained Young modulus of kaolin EU versus cavity expansion ratio. 500 2,0 σ 3=100 kPa σ3=200 kPa σ3=300 kPa 400 EU/cU EU (V)/EU (V0) V3=300 kPa 1,75 Figure 5. Normalized peak deviatoric stress versus cavity expansion ratio. 1,4 1,0 1,00 1,50 V/V0 Figure 4. Peak deviatoric stress versus cavity expansion ratio. 1,3 V3=200 kPa 0,4 2,00 1,5 300 σ3=100 kPa 1,0 1,00 σ3=200 kPa σ3=300 kPa 200 1,25 1,50 1,75 2,00 V/V0 Figure 8. Normalised undrained Young modulus versus cavity expansion ratio. 1,00 1,25 1,50 1,75 2,00 V/V0 Figure 9. Ratio EU /cU versus cavity expansion ratio. behavior of soil in the range of small strains. The undrained secant Young modulus is defined by: EU = ∆σ/∆ε for a strain level of 0.01 (1%). Figure 7 shows up a significant improvement of Young modulus when the cavity expansion ratio increases and the consolidation stress does too. Such result has been predicted by Guetif et al (2007) who carried out a numerical simulation of stone column installation in soft clay. It is also observed that, for consolidation stress σ3 = 200 and 300 kPa, the normalized undrained Young modulus EU (V )/EU (V0 ) remains quasi-constant when the cavity expansion ratio is higher than 1.5. However, at a maximum cavity expansion ratio (V/V0 = 2), the undrained Young’s Modulus EU is respectively increased by 1.74, 1.55 and 1.52 times the consolidation stress of 100, 200 and 300 kPa (Figure 8). The ratio between 768 Proceedings of the International Conference on Ground Improvement and Ground Control the undrained Young modulus and undrained cohesion (EU /cU ), as a function of cavity expansion ratio, increases when the ratio (V/V0 ) is lower than 1.50 (Figure 9). It is also noted that the ratio (EU /cU ) is the highest when the consolidation stress is the lowest. 6. CONCLUSIONS The improvement of remolded soft kaolin by laterally expanded stone column has been investigated in laboratory. Such reinforcement has been simulated by a controlled lateral expansion applied, at different rates, which follows an initial isotropic consolidation. All the hollow soft kaolin specimens were subjected to undrained shear loading till failure with recorded excess pore pressure. The main findings from the carried out experimental program are outlined below. The expansion of cylindrical cavity in remolded soft kaolin improves both its undrained Young modulus and shear strength. The increase of undrained shear strength essentially depends on the consolidation stress and the cavity expansion ratio. The increase of undrained shear strength is more pronounced at lower stress of consolidation and higher cavity expansion ratio. The increase of undrained Young modulus only depends on the ratio of cavity expansion when it is lower than 1.5. For higher values of the ratio of cavity expansion the undrained Young modulus depends, in addition, on the stress of consolidation. As observed for the undrained Young modulus, the ratio (EU /cU ) increases when the ratio (V/V0 ) is less than 1.5 especially when the stress of consolidation is high. It is noted that the ratio (EU /cU ) increases when the consolidation stress decreases. The dissipation of excess pore pressure after the installation of stone column turned out to be significant to enhance the bearing capacity and to reduce the settlement of the reinforced soil. REFERENCES 1. AFNOR, NF P 94-074 (1994). Sols: reconnaissance et essais. Essais à l’appareil triaxial de révolution. Appareillage, Préparation des éprouvettes et Essais (UU), (CU+u) et (CD). French Standard, October 1994. 2. Ambily A.P. and Gandhi, S. (2007). Behavior of Stone Columns Based on Experimental and FEM Analysis, J. Geotech. Geoenviron. Eng. 133(4): 405–415. 3. Andreou P., Frikha W., Frank R., Canou J., Papadopoulos V. and Dupla J.C. (2008). 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