BEHAVIOUR OF STONE COLUMN IN LAYERED SOILS USING GEOTEXTILE REINFORCEMENT

International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 8, August 2017, pp. 453 462, Article ID: IJCIET_08_08_046 Available online at http://http://ww www.iaeme.com/ijciet/issues.asp?jtype=ijciet&v VType=8&IType=8 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 IAEME Publication Scopus Indexed BEHAVIOUR OF STONE COLUMN IN LAYERED SOILS USING GEOTEXTILE REINFORCEMENT S. Siva Gowri Prasad Assistant Professor, Department of Civil Engineering, GMR Institute of Technology, Rajam, Andhra Pradesh, India Ch. Vasavi P.G Scholar, Department of Civil Engineering, GMR Institute of Technology, Rajam, Andhra Pradesh, India K. Praveen Sai P.G Scholar, Department of Civil Engineering, GMR Institute of Technology, Rajam, Andhra Pradesh, India ABSTRACT Stone column is one of the ground enhancement techniques and is suitable for upgrading of soft soils by increasing the load carrying capacity and reducing the large surface settlements. The load carrying capacity of stone column depends upon the undrained shear strength of the surrounding soil and lateral resistance offered by it. The strength of the composite soil may further be increased by reinforcing the stone columns with geotextile. In the present study, to investigate the behavior of stone column in layered soil consisting of weak soft soil (marine clay) overlying silty clay, different laboratory tests were conducted in floating condition for varied soft soil thickness of D, 2D, 3D and 4D where D is the diameter of the stone column. The stone column was reinforced with geotextile to a length of 3L/4 to find out the effect of encasement. Laboratory tests were carried out on columns of 50mm diameter and load was applied to an area double the area of the stone column (100mm) to represent an area replacement ratio of 25%. Tests were conducted to evaluate the load versus settlement response and found that the load carrying capacity of the stone column increases and the vertical extent of the bulging decreases with increase in the thickness of the soft soil (D,2D,3D,and 4D) by using the encasement length of 3L/4. Key words: Stone column, Layered soils, Sand, Marine clay, Silica manganese slag, Geotextile reinforcement. Cite this Article: S. Siva Gowri Prasad, Ch. Vasavi and K. Praveen Sai, Behaviour of Stone Column in Layered Soils Using Geotextile Reinforcement. International Journal of Civil Engineering and Technology, 8(8), 2017, pp. 453 462. http://www.iaeme.com/ijcie ET/issues.asp?JType=IJCIET&VType=8&ITy ype=8 http://www.iaeme.com/ijciet/index.asp 453 editor@iaeme.com

S. Siva Gowri Prasad, Ch. Vasavi and K. Praveen Sai 1. INTRODUCTION Due to ever increasing infrastructure development, space constraints particularly in metropolitan cities we are forced to construct the structures on weak soils like soft clays, which cover a vast area all along the coast. To improve this soft soil of very low shear strength and high compressibility, different techniques for ground alteration and improvement are available around the world. They include dewatering, vibro compaction, preloading of soil with and without vertical drains, vacuum consolidation, grouting, deep mixing, densification, soil reinforcement etc. (1). Amongst these techniques for improving in situ ground conditions, strengthening the ground with stone columns or granular piles is one of the most popular and cost effective technique (2). Stone columns increase the bearing capacity of the foundation, slopes stability and reduces total and differential settlements, accelerates the consolidation time and decreases the liquefaction potential. The stone column and its surrounding soil acts as a composite material and the stiffness of this composite ground is much higher than the soft soil (3). Applications of stone columns include, supporting the earthen embankments, liquid storage tanks, raft foundations and other low rise structures (4). Encased stone column have gained popularity as compared to the conventional stone column. The concept of encasing the stone columns was first intended by Van Impe in 1985. The advantage of encasement is it reduces the settlement of composite foundation and increases the load carrying capacity (5). Siva Gowri Prasad et al.(6) investigated the behaviour of the floating column with geotextile with different encasement lengths of D, 2D, 3D & 4D by using Silica-Manganese Slag and Sand as the stone column material. The tests indicate that the load carrying capacity increases by replacing the Stone aggregates with Silica-Manganese Slag and also increases with the increase of encasement length and settlements have been decreased. Ambily et al. (7) carried out the experiments to study the behaviour of stone column by changing the spacing, shear strength of soft clay and moisture content. The test results indicate that the failure is by bulging of the column with maximum bulging at 0.5 to 1 times diameter of the column below the top. Tandel et al. (8) reported that ordinary stone column is one of the ground improvement techniques for deep soil strata and this technique may not be suitable for soft soil having undrained shear strength less than or equal to 15 kpa due to excessive bulging. Ali et al. (9) conducted the laboratory model tests on floating and end-bearing single columns with and without reinforcements to evaluate the relative improvement in the failure stress of the composite ground due to different configurations of the reinforcement and finally concluded that the end-bearing columns perform better than floating columns for each reinforcement configuration. Asskar and Hossein (10) studied the uses of finite element program Plaxis on to improve the soft clay bed reinforced by geosynthetic-encased stone columns to understand the influence of per-strain of the encasement during installation on the performance of stone columns. It is concluded that the formation of pre-straining and the initial tension force is developed in the encasement which improves the lateral confining pressure and enhances the bearing capacity of the stone column. Sidhi Sabnis et al. (5) studied on the techniques for installation of encased stone column and failure modes of encased stone columns. Studies showed that the stone column improves the bearing capacity of the soil and geosynthetic further improves the properties and reduces the bulging. Increasing the geo synthetic stiffness makes the column stiffer and increase in lateral confinement. Shivashankar et al. (11) carried out the experimental studies to investigate the behaviour of stone columns in layered soils, tests were carried out with two types of loadings where one is on entire area loading and another is on the stone column only. The test results indicate that the performance of stone columns in layered soils where the top layer is weak is found to be poor and hence it needs proper reinforcement to reduce the excessive bulging and to improve its performance. Mohanty et al. (12) studied soil layering effects on response of the stone http://www.iaeme.com/ijciet/index.asp 454 editor@iaeme.com

Behaviour of Stone Column in Layered Soils Using Geotextile Reinforcement column with two types of layering systems. A detailed parametric study using finite element is also done after experimental studies and found that the vertical extent of the bulging increases with increase in the thickness of the top soft clay up to 2D of the stone column for both the layering systems. Pradip Das et al. (13) have done the experiments on stone columns to improve the load carrying capacity of sandy silt with clay in naturally consolidated state. Load tests are performed through compression testing machine with geotextile encasement on layered soil bed. The load carrying capacity of treated soil improves with the increase in diameter of stone column where as it reduces for layered soils. In the present study, the performance of stone columns in layered soil, which consists of weak soft clay overlying relatively stronger clayey silt, is being studied for various thicknesses of top weak layer. Series of laboratory model tests were performed in a circular unit cell tank with the stone column reinforced with geotextile to a length of 3L/4 to find out the effect of encasement, effect of top soft layer thickness on the strength, stiffness and bulging characteristics of stone columns in a layered soil bed. 2. MATERIALS & PROPERTIES The materials used in the study are Marine clay, clayey silt, Silica manganese slag, Sand and Geotextile. Laboratory tests have been conducted to determine the properties of these materials and are given below. Marine clay is collected from Visakhapatnam port, India. The particles passing through 4.75mm IS Sieve is used for this study. Clayey silt sample is collected from Rajam, Srikakulam (Dist.) Andhra Pradesh, India. The soil sample is collected at a depth of 2m from the ground surface. The particles passing through 4.75mm IS Sieve is used in this study. The properties of marine clay and clayey silt are given in Table 1. Table 1 Properties of Marine clay and clayey silt S.NO Property of soil Marine clay Value Clayey silt 1 Gravel 0 2 % 2 Sand 15 34 % 3 Silt & clay 85 64% 4 Liquid limit (W ) 61.5% 45% 5 Plastic limit (W ) 20.78 20.8% 6 Plasticity Index (I ) 40.72 24.2% 7 Optimum Moisture 27% 17.2% 8 Maximum Dry Density 1.47 g/cc 17.52 kn/m 3 9 Specific gravity 2.5 2.65 10 Shear strength 12.5kPa 22 kpa Silica-Manganese slag is used as stone column material which is produced during the primary stage of steel production which is acquired from Sri Mahalaxmi Smelters (PVT) Limited Vijayanagaram (Dt), India. Particles passing through 10mm and retained on 4.75 mm are used as stone column material. The properties of Silica-Manganese slag are given in Table 2. http://www.iaeme.com/ijciet/index.asp 455 editor@iaeme.com

S. Siva Gowri Prasad, Ch. Vasavi and K. Praveen Sai Table 2 Physical properties of Silica-Manganese slag Property Value Specific Gravity 2.82 Water absorption (%) 0.7 Density (g/cc) 1.54 The sand is used as blanket of 20mm thickness on layered soils which is acquired from Nagavali river, Srikakulam (Dt), India. This sand is sieved through 4.75mm sieve and is used in this study. Geotextile is used as the reinforcing material for encasing the stone column and is collected from Ayyappa Geo-textile installers, Vishakhapatnam, India. This sheet is stitched to form the tube for encasing the stone column. Mass of the geotextile is 100g/m 2 and Tensile strength is 4.5kN/m. 3. EXPERIMENTAL INVESTIGATION All experiments were conducted on 50 mm diameter stone columns surrounded by layered soil in cylindrical tanks of 300 mm height and diameter 200mm and having an area replacement ratio of 25%. In this study two types of soils are used such as soft clay overlying clayey silt. The moisture content required for the soft clay (44%) to get the desired shear strength of 12.5kPa was determined by conducting several vane shear tests on cylindrical specimen of 76mm height and 38mm depth. For the clayey silt layer, the water content required (20%) for desired shear strength of 22kPa was determined by conducting UCS tests on cylindrical specimen of 76mm height and 38mm depth. Tests were conducted in layered soil beds for varying top weak layer thicknesses of D, 2D, 3D and 4D. Three series of tests were conducted in this study in which the first series of tests were conducted on homogenous soil beds consisting of clayey silt and soft soil. Second series of tests were conducted on layered soil beds with unreinforced stone column. These tests are used to study the load carrying capacity of the stone column. Third series of tests were conducted on layered soil beds with geotextile encased stone column and these tests were performed to study the effect of encasement on stone column behaviour. Shivashankar et al. conducted tests on stone columns in layered soils having top soft layer and concluded that bulging is observed at top 2-3 diameters of the column and this can be reduced by providing the reinforcement in this zone. In order to avoid bulging of the column in this zone, the experiments have been conducted by providing the reinforcement in the zone of soft soil i.e. at a reinforcement length of 3L/4 from the top. The Figure 1 shows the test setup used in this study. Figure 1 Test setup with loading http://www.iaeme.com/ijciet/index.asp 456 editor@iaeme.com

Behaviour of Stone Column in Layered Soils Using Geotextile Reinforcement Test Description Table 3 Experimental program Homogeneous soil bed Weak soft Clayey silt soil Layered soil bed (top soft clay layer thickness (t) in terms of diameter of stone column, D) Total No. of tests 1D 2D 3D 4D Only soil/soils 6 Layered soils with stone column Layered soils with stone column and reinforcement (3L/4) - - 4 - - 4 3.1. Preparation of soft clay bed The air dried and pulverized clay sample which is passing through 4.75 mm IS sieve is taken and is mixed with water content of 44%. The clay is thoroughly mixed to a consistent paste and is filled in the tank in 50 mm thick layers to the desired height of 300mm by hand compaction such that no air voids are left in the soil. Before soil is filled in to the mould and grease is applied to the inner surface so as to reduce the friction between the mould and soil. After compaction of clay bed, it was covered with wet gunny cloth and left for 24 hours for moisture equalization and was tested. 3.2. Preparation of layered soil beds A series of Model experiments were performed on layered soils and soil improved with ordinary stone column (OSC) and encased stone column (ESC). The mould is taken and grease is applied to the inner surface so as to reduce the friction between the mould and soil. Layered soil bed is prepared with clayey silt and soft soil. Soft soil layer having thickness equal to multiples of diameter of stone column i.e. D, 2D, 3D and 4D have been placed over the bottom layers of clayey silt with predetermined weight and then each layer is compacted with wooden tamper to achieve 50mm height. After preparation of layered soil bed, it was covered with wet gunny cloth and left for 24 hours for moisture equalization and was tested. 3.3. Preparation of layered soil beds with stone column Grease is applied to the inner surface of the mould before filling the soil so as to reduce the friction between the mould and soil. The bottom layer is prepared with clayey silt to thickness twice the diameter of the stone column (100mm) and the PVC pipe of 5cm diameter and 1mm thick was placed at the centre of the bed. Grease is applied to the outer surface of the pipe and kept at the center of the soil bed and clayey silt is filled in the mould in 50 mm thick layers so as to achieve an overall thickness of 300mm by replacing the top soft clay layers with D, 2D, 3D, 4D thickness. To prepare stone column, the aggregates are filled in the pipe layer wise and compacted with a steel hammer having weight of 900gm by giving 10 blows for each layer. Compaction is done carefully to avoid disturbances to the surrounding soil which creates bulging of the soil. After compaction of each layer, the pipe is lifted such that there will be 5mm overlap between the two layers and withdrawing the casing pipe simultaneously for every 50mm of depth along the length of column. After completion of layered soil bed, it was covered wet gunny cloth and left for 24 hours for moisture equalization before testing. 3.4. Stone column with reinforcement To construct the stone column for encasement length of L, the process of construction of stone column is same as explained for unreinforced stone column, whereas before placing the PVC pipe, it was encased with the geotextile and placed at the centre of the clay bed. To http://www.iaeme.com/ijciet/index.asp 457 editor@iaeme.com

S. Siva Gowri Prasad, Ch. Vasavi and K. Praveen Sai construct the stone column for encasement length of 3L/4, the process of compaction of the stone column is continued up to the remaining unreinforced length of L/4 and the pipe is taken out. The pipe is encased with the geo-textile for a length of 3L/4 and the pipe is placed in position and then the stone column is casted in steps of 50mm layers by using the above procedure. The same procedure is followed for the other reinforcement lengths. After completion of layered soil bed with encased stone column, it was covered wet gunny cloth and left for 24 hours for moisture equalization before testing. 3.5. Test Procedure Sand blanket of 20 mm thick was laid on the surface of soil bed/stone column to be tested. To apply load, a circular disc of 12mm thick and having diameter of 100mm which is double the diameter of stone column is placed at centre of the bed over the stone column. The load deformation behaviour of the stone column is studied by applying compression loading up to 20mm settlement at a rate of settlement of 0.24mm/min through a conventional loading frame and the settlements were taken at every 1mm interval. 3.6. Post-test analysis After completion of the test, to study the deformation properties of the stone column, the aggregates were taken out which are forming the stone column. A thin paste of plaster of Paris is poured into the hole which is left after removal of aggregates to get deformed shape of the column and is kept for 24 hours for hardening. The soil outside the stone column is removed and the hardened column is taken out. 4. RESULTS AND DISCUSSIONS 4.1. Behaviour of untreated ground The Figure 2 shows the typical load versus settlement behaviour of un-treated layered soils. The load carrying capacity of the clayey silt is higher than that of soft clay. Where as in case of layered soils, load carrying capacity of the soil bed decreases with increase of weak layer (soft clay) thickness. The load carrying capacities of the layered soils with soft clay thickness of D, 2D, 3D and 4D are 35%, 20%, 13% and 6% higher than the plain clay bed. This can be seen in Figure 2. Settlement in mm 0 5 10 15 20 25 load in kn 0 0.2 0.4 0.6 0.8 soft clay soft clay layer thickness of D soft clay layer thickness of 2D soft clay layer thickness of 3D soft clay layer thickness of 4D clayey silt Figure 2 Load-Settlement curves of soft clay, clayey silt and plain layered soils having soft clay layer thickness of D, 2D, 3D, and 4D http://www.iaeme.com/ijciet/index.asp 458 editor@iaeme.com

Behaviour of Stone Column in Layered Soils Using Geotextile Reinforcement 4.2. Behaviour of layered soils with stone column Tests were conducted to evaluate the load versus settlement response of the stone columns compared to that of untreated layered soils. The load was applied to an area double the area of the stone column (100mm) to represent an area replacement ratio of 25%. Figure 3 shows typical load-settlement behaviour of improved ground with 50mm diameter stone column for different top soft layer thickness of D, 2D, 3D and 4D. The load carrying capacities of the soil bed reinforced with stone column are increased by 30, 37, 43, and 46% as compared to that of untreated layered soils. setttlement in mm 0 5 10 15 20 25 load in kn 0 0.2 0.4 0.6 0.8 1 soft clay layer thickness of D soft clay layer thickness of 2D soft clay layer thickness of 3D soft clay layer thickness of 4D Figure 3 Load-Settlement curves of layered soils with Stone columns having top soft clay layer thickness of D, 2D, 3D and 4D 4.3. Behaviour of layered soils with stone column for an encasement length of 3L/4 The load carrying capacity of the geotextile encased stone column in layered soils with soft clay as top layer having thickness of D, 2D, 3D and 4D were increased by 45, 55, 56, and 61% when compared to that of the untreated layered soils. The load carrying capacity of the geotextile encased stone column in layered soils with soft clay as top layer having thickness of D, 2D, 3D and 4D were increased by 11%, 12%, 9%, and 10% respectively, when compared to that of the unreinforced stone column. settlement in mm 0 5 10 15 20 25 load in kn 0 0.2 0.4 0.6 0.8 1 soft soil thickness of D soft soil thickness of 2D soft soil thickness of 3D soft soil thickness of 4D Figure 4 Load-Settlement curves of stone columns of layered soils with 3L/4 encasement having top soft clay layer thickness of D, 2D, 3D and 4D http://www.iaeme.com/ijciet/index.asp 459 editor@iaeme.com

S. Siva Gowri Prasad, Ch. Vasavi and K. Praveen Sai 4.4. Bulging Analysis The maximum bulging was found to occur at a depth of D from the top. The total length of the stone column subjected to bulging was observed to be 2 3 times the diameter of the column shows in Figure 5. In case of stone columns installed in layered soils, the entire bulging was observed in the top weak layer only. This may be due to the poor lateral confinement offered by the top weak soil. The maximum bulging of 5.8, 6.2, 6.8 and 7.4mm were found for the unreinforced stone column for the soil bed having soft clay thickness of D, 2D, 3D and 4D respectively. When the stone column is reinforced with 3L/4 encasement length for different top layer thickness of D, 2D, 3D & 4D, the maximum bulging of 1mm, 1.2mm, 1.4 and 1.4mm were found respectively. Figures 5(a) and 5(b) shows the bulging of plain stone columns in layered soil bed and the bulging of encased stone columns with top layer thickness of D, 2D, 3D, and 4D respectively. Figure 5(a) Figure 5(b) Figure 5 (a) Bulging of unreinforced stone columns in layered soil bed with top layer thickness from D, 2D, 3D and 4D Figure 5 (b) Bulging of encased stone columns in layered soil bed with top layer thickness from D, 2D, 3D and 4D 0 Bulging(cm) 0 0.5 1 1.5 2 soft clay layer thickness of D soft clay layer thickness of 2D 5 soft clay layer thickness of 3D Depth of column(cm) 10 15 20 25 soft clay layer thickness of 4D encased column with soft clay thickness of D encased column with soft clay thickness of 2D encased column with soft clay thickness of 3D encased column with soft clay thickness of 4D Figure 7 Deformations of unreinforced and reinforced stone columns http://www.iaeme.com/ijciet/index.asp 460 editor@iaeme.com

Behaviour of Stone Column in Layered Soils Using Geotextile Reinforcement 5. CONCLUSIONS The experimental analyses have been carried out to study the behavior of stone column on the layered soils consisting of top weak layer thickness of D, 2D, 3D and 4D. Based on the test results the following conclusions were made In case of homogeneous grounds, the improvement of soft soil with stone columns largely depends on the strength of the surrounding soil. Improvement in the relatively stronger clayey silt is higher compared to that in soft clay. Due to the relatively low confinement of soft clay. In case of the untreated layered soils, the load carrying capacity of the soil bed decreases with increasing the top weak layer (soft clay) thickness. When layered soil bed is improved with stone column, the load carrying capacity is increased when compared to the untreated soil which is in the range of 30-46%. It can be concluded that the load carrying capacity is improved when the stone column is encased with 3L/4 reinforcement with varying soft clay layer thickness as D, 2D, 3D and 4D, and it also reduces the bulging of stone column. REFERENCES [1] Beena K.S. (2010) Ground Improvement Using Stone Column. International Conferences on Recent Advances in Geotechnical, Kerala pp.1-6. [2] Alamgir M., Miura N., Poorooshasb H.B, Madhav M.R (1996) Deformation analysis of soft ground reinforced by columnar inclusions. Comput Geotech Vol.18, No.4, pp.267-290 [3] Mohammadreza Jaberi Nasab, Adel Asakereh (2015) A Review of Methods of Increasing the Efficiency of Stone Columns to Ground Improvement. International Journal of Science and Engineering Investigations Vol.4, No.46, pp.75-78 [4] Hughes J.M.O., Withers N.J., Greenwood D.A. (1975) A field trial of the reinforcing effect of a stone column in soil. Geotechnique Vol.25, No.1, pp.31 44 [5] Sidhi Sabnis, Smita Aldonkar (2017) Soil stabilization using stone column. International Conference on geotechniques for infrastructure projects, Thiruvananthapuram [6] Siva Gowri Prasad S., Satyanarayana P.V.V., Anil Kumar B. (2016) Improvement of Marine Clay Performance Using Silica-Manganese Slag Stone Column Reinforced with Geotextile. International Journal of Engineering Research and Development Vol.12, No.10, pp.16-21 [7] Ambily A.P., Gandhi S.R. (2007) Behavior of stone columns based on experimental and FEM analysis. Jounal of Geotech Geoenviron Eng (ASCE) 133(4):405 441 [8] Tandel Y.K., Solanki C.H., Desai A. K (2012) reinforced stone column: remedial of ordinary stone column. International Journal of Advances in Engineering & Technology Vol.3, No.2, pp.340-348 [9] Ali K., Shahu J.T., Sharma K.G. (2014) Model Tests on Stone Columns Reinforced with Lateral Circular Discs. International Journal of Civil Engineering Research. Vol.5, No.2, pp.97-104 [10] Asskar Janalizadeh Choobbasti, Hossein Pichka (2012) Improvement of soft clay using installation of geosynthetic-encased stone columns: numerical study. Arab Journal of Geoscience N0.10 [11] Shivashankar, R., Dheerendra Babu M. R., Nayak S., Rajathkumar V. (2011) Experimental studies on behaviour of stone columnsin layered soils. Geotechnical Geological Engineering Vol.29, pp.749-757. http://www.iaeme.com/ijciet/index.asp 461 editor@iaeme.com

S. Siva Gowri Prasad, Ch. Vasavi and K. Praveen Sai [12] Mohanty P., Samanta M. (2015) Experimental and Numerical Studies on Response of the Stone Column in Layered Soil. International journal of Geosynthetics and Ground Engineering pp.1-14 [13] Pradip Das, Dr. Sujit Kumar Pal (2013) A Study of the Behavior of Stone Column in Local Soft and Loose Layered Soil. EJGE Vol.18, pp.1777-17786 [14] A. Siva Teja, K. Keshav Prasad, B. G. Rahul and K. Hemantha Raja, Performance Evolution of Bitumunous Mixes Using Coir Geotextiles. International Journal of Civil Engineering and Technology, 8(4), 2017, pp. 1068 1073 http://www.iaeme.com/ijciet/index.asp 462 editor@iaeme.com