Load-Carrying Capacity of Stone Column Encased with Geotextile. Anil Kumar Sahu 1 and Ishan Shankar 2

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1 Load-Carrying Capacity of Stone Column Encased with Geotextile Anil Kumar Sahu 1 and Ishan Shankar 2 1 Professor, Department of Civil Engineering, Delhi Technological University, Delhi, India (sahuanilkr@yahoo.co.in) 2 Student, Department of Civil Engineering, Delhi Technological University, Delhi, India (ishankar1994@gmail.com) ABSTRACT: In recent years stone columns have come under widespread use for increasing the load bearing capacity and reducing settlements in soft clays and loose sands. They have been most commonly used for large foundations wherein the stability of a large area of soil has to be improved, such as tanks, embankments and fills. Conventionally, stone columns are driven into the loose soils and they maintain their stability from lateral confinement due to reaction from the surrounding stiffened soil. However, this may not be possible in very loose sands and additional lateral support may have to be provided to stabilize it and reduce its settlements. This study attempts to improve upon this weakness by wrapping the column in geotextile layers to enhance lateral and vertical reinforcement. In this process, the study investigates the improvement of load bearing capacity of a single stone column encased in geotextile through field tests made on test specimen in a modal tank containing sandy soil, utilizing reaction loading applied through the plate load test. The variation of increase in capacity of a stone column due to variation in its diameter is also studied. The findings may be useful in improving the bearing capacity of stone column-loose sand composites by reinforcing them using geotextile fabric while optimizing the size of stone column to be use. INTRODUCTION Structures constructed on loose sands face widespread problems such as large settlements and lateral instability due to horizontal flow of sand. Also, bearing capacity of such unconfined soils is very low, necessitating the use of certain ground improvement techniques for initiating construction. One of the remedial measures for the same is the insertion of a group of stone columns to improve its geotechnical properties. A stone column (S.C.) is a vertical column constructed from ground level with gravel and sands. The method of installation comprises of partial replacement of sand and then inserting aggregates, while simultaneously compacting Page 1

2 them. This leads to the formation of an aggregate-soil composite which is stiffer and stronger than the original soil. Traditionally, stone columns have been frequently utilized for the stabilization of soft clays and loose silty sands with a large amount of fines. Advantages of using them include reduction in settlements, increased stiffness, reduction of liquefaction potential and an enhanced time rate of settlement. Stone columns derive their load capacity from the lateral confining pressure provided by surrounding soils. When stone columns are installed in very soft clays or silty sands containing a large proportion of fines, the surrounding soil may not provide significant load capacity due to low lateral confinement. McKenna et al. (1975) reported cases where stone column was not restrained by surrounding soft clay which led to excessive bulging and also the soft clay squeezed into the voids of the aggregate. Also, the clay particles squeezed into the voids of the gravel. In such cases the stone column itself needs to be provided with lateral confinement to improve its performance. We propose that similar results will be obtained when such experiments are conducted on loose sands. Encasement with geosynthetic layers provides this lateral confinement along with several other advantages like improved stiffness of column, reduced loss of stones into surrounding soil, preserving drainage and frictional properties of aggregates and as we have determined through our experiments, a steep rise in its load carrying capacity and reduced bulging of the encased column (Raithel et al. 2002, Alexiew et al. 2005, Brokemper et al. 2006, Murugesan and Rajagopal 2006a,b, 2007a). The concept of encasing stone column by wrapping it with geotextile was proposed by Van Impe in the year Murugesan and Rajagopal (2006a,b) have evaluated the behavior of Ordinary stone columns (OSCs) without encasement and geosynthetic encased stone columns (ESCs) through numerical analyses. They have reported that ESCs are stiffer than OSCs and were found to be less dependent on strength of surrounding soil for their load carrying capacity. Malarvizhi and Ilamparuthi (2007) have compared the performance of stone columns with and without geosynthetic encasement and found the ESCs to be more effective. Murugesan and Rajagopal (2007) have also performed laboratory model tests on the stone column installed in a unit cell tank. It was reported that the effect of encasement was found to decrease with an increase in the diameter of the stone column. Only a limited study has been conducted on the variation of stiffness of stone column due to variation in its size and also the variation of the effect of geosynthetic encasement on varying sizes of stone columns. Through our experimentation, we intend to test the improvement in performance of stone columns when they are encased with geotextiles as well as horizontally reinforced by three layers of the same geotextile in order to reduce settlement as compared to conventionally encased stone columns. EXPERIMENTAL PROGRAM In order to determine the load carrying capacity of stone column, plate load tests have been performed in the following cases: 1. Compacted Soil 2. Stone Column in the soil Page 2

3 3. Stone Column encased with geotextile 4. Stone Column encased with geotextile and horizontally reinforced with geotextile in three layers From the plate load test, the following observations were taken: (i) Variation in load-settlement behavior of stone columns (ii) Determination of axial load carrying capacity of stone columns at failure (iii) Variation of load carrying capacity with change in diameter of stone columns (iv) Observation of bulging pattern at failure Properties of Aggregates The properties of aggregates mixture used were determined by the respective tests and are enumerated below. Table 1. Physical Properties of Aggregates of 20 mm and 10 mm nominal size S. No. Properties 20 mm 10 mm 1 Water Absorption 12.00% 14.30% 2 Specific Gravity Aggregate Impact Value 13.80% 15.20% 4 Aggregate Crushing Value 14.80% 16.40% 5 Fineness Modulus Aggregate Abrasion Value 18.00% 19.70% Properties of Sand The properties of loose sand along with the stone column as surrounding soil are listed as follows. Table 2. Geotechnical Properties of sand S. No. Properties Value 1 Specific Gravity Coefficient of Uniformity (Cu) Coefficient of Curvature (Cc) Classification of Soil SP 5 Optimum Moisture Content 12% 6 Maximum Dry Density kn/m 3 7 Cohesion Zero 8 Angle of Internal Friction (φ) 35 o Properties of Woven Geotextile The properties of woven geotextile used to encase and reinforce the stone column are presented as follows. Page 3

4 Table 3. Physical Properties of Woven Geotextile S. No. Properties Value 1 Mass (N/m 2 ) Tensile Strength (kn/m) Max. Elongation (%) CBR Puncture Strength (N) Trapezoidal Tear Strength (N) 847 Gradation of Aggregates Three test combinations of 60:40, 50:50 and 40:60 by weight of 20 mm and 10 mm aggregates respectively were considered and the test was conducted. The fineness modulus of each was calculated, as is mentioned below. Table 4. Properties of Aggregates S. No. Coarse:Fine Fineness Modulus 1 40: : : The ratio of 60:40 (20 mm & 10 mm) by weight was considered suitable for use as it has highest value of fineness modulus. Experimental Procedure The experiment was performed in a unit tank with a stone column cast along with the sand around it. Initially, the load carrying capacity of sandy soil was calculated by conducting plate load test using 150 mm and 200 mm plates, wherein the soil is in the 1 m x 1.5 m x 0.65 m tank using a total number of 1100 blows spread over four layers of 1500 mm each to achieve required density. Later, the stone column was cast and the aggregates in a proportion of 60:40 of 20 mm and 10 mm nominal size by weight were inside a pipe of internal diameter 150 mm and 200 mm in four layers, with 25 blows on each layer to compact it. This column was constructed up to a height of 600 mm for both the sizes (150 mm & 200 mm). Following the same procedure, another column (150 mm & 200 mm) with geotextile encasement throughout its height and in three layers were cast. At depths of 160 mm, 320 mm and 480 mm from the top, a horizontal layer of geotextile was introduced along its cross section to provide vertical stability and to reduce settlements in the stone column, while also improving its axial load carrying capacity. Load Tests on Stone Column The load tests were carried out by providing reaction loading using plate load test. Circular plates of 150 mm and 200 mm diameter were used to apply load on the stone column by the hydraulic jack. Initially a seating load of 1000 N was applied, after which the loads were applied in increments of 2000 N, until the rate of settlement at the applied load was below 0.02 mm per minute. Page 4

5 RESULTS & DISCUSSION The load intensity vs settlement for soil, stone column in soil, stone column with geotextile encasing and stone column with geotextile encasement along with horizontal geotextile layers was plotted simultaneously for diameter 150 mm and 200 mm respectively, and the variation in load-carrying capacity observed as follows. Settlement (mm) Load Intensity (kn/m 2 ) COMPACTED SOIL S.C. IN COMPACTED SOIL S.C. WITH ENCASING OF GEOTEXTILE ENCASED S.C. WITH HORIZONTAL LAYERS OF GEOTEXTILE -60 FIG. 1. Variation of Load Intensity vs Settlement of Stone Column of 150 mm diameter Load Intensity (kn/m 2 ) COMPACTED SOIL S.C. IN COMPACTED SOIL Settlement (mm) S.C. WITH ENCASING OF GEOTEXTILE ENCASED S.C. WITH HORIZONTAL LAYERS OF GEOTEXTILE FIG. 2. Variation of Load Intensity vs Settlement of Stone Column of 200 mm diameter The load intensity at failure for all three specimens of diameter 150 mm & 200 mm and their respective improvement percentage has been tabulated below. Page 5

6 Table 5. Load-carrying capacity of Stone Columns Types of Stone Column (S.C.) Compacted Soil soil soil with geotextile encasing soil with encasing and horizontal layers of geotextile Stone Column of 150 mm diameter Load Intensity at Failure (kn/m 2 ) % Improvement Stone Column of 200 mm diameter Load Intensity at Failure (kn/m 2 ) % Improvement % % % % % % The settlement reduction factor for all four specimens of diameters 150 mm and 200 mm has been tabulated below. Table 6. Settlement Reduction factor of Stone Columns Types of Stone Column (S.C.) Compacted Soil soil soil with geotextile encasing Stone Column of 150 mm diameter Settlement at failure load (kn/m 2 ) % Reduction Stone Column of 200 mm diameter Settlement at failure load (kn/m 2 ) % Reduction % % % % Page 6

7 soil with encasing and horizontal layers of geotextile % % The results of plate load tests carried out on various types of stone columns are tabulated in Table 5 and 6. From the tests conducted on soil, it is observed that on the application of load on the soil, settlement of the plate occurs, which may be due to deformation of the soil within the pressure bulb generated beneath the plate. The size of pressure bulb increases with an increase in the size of plate. Hence the amount of settlement is more in case of larger plate size. The load-carrying capacity of soil reinforced with stone column is higher as compared to that of soil alone in each case of plate size. This may be due to inclusion of high density aggregate in the soil. However there is a decrease in settlement of stone column as compared to that of soil alone at any level of load. The effect of encasing on the stone column was also observed. The load-carrying capacity of encased stone column is more as compared to stone column alone. This may be due to the fact that as the bulging starts, all tensile stresses are being carried by geotextile encasing which does not allow failure at an early stage. However, it was found that the effect of encasement decreased with an increase in the diameter of stone column. This may be due to decrease in stiffness of stone column with an increase in its diameter. In order to improve the stiffness of stone column, the geotextile was placed horizontally in the stone column in three equidistant layers. This resulted in reduced settlements and a higher load carrying capacity at failures in each case. CONCLUSIONS On the basis of plate load tests conducted on various types of stone columns, the following conclusions may be drawn: 1. The load-carrying capacity of the soil increases and settlement decreases with the intrusion of any type of stone column. 2. The failure of stone column without geotextile encasing is by bulging and heaving, which may be a kind of mixed failure. 3. Due to intrusion of encasement in the stone column, the load-carrying capacity is more as compared to that of soil as well as unencased stone column. 4. The load-carrying capacity of encased stone column with horizontal placement of geotextile in multiple layers is further enhanced as compared to encased stone column. Page 7

8 ACKNOWLEDGMENTS The authors appreciate the support of the ACECC and the organizing member ASCE for providing us with the opportunity to present our research at your prestigious conference. We also thank the Vice Chancellor at Delhi Technological University and Head of Department, Civil Engineering at Delhi Technological University for facilitating us in our research. REFERENCES Alexiew, D., Brokemper, D., and Lothspeich, S. (2005). Geotextile Encased Columns GEC: Load capacity, geotextile selection and predesign graphs. Geotech., Spec. Pub No , Brokemper, D., Sobolewski, J., Alexiew, D., and Brok, C. (2006). Design and construction of geotextile encased columns supporting geogrids reinforced landscape embankments: Bastions VijfwalHouten in the Netherlands. Proc., 8 th Int. Conf. on Geosynthetics, Millpress, Rotterdam, The Netherlands, Malarvizhi, S.N. and Ilamparuthi, K. (2004). Load versus settlement of clay bed stabilized with stone and reinforced stone columns. ICGGE 2004, McKenna, J. M., Eyre, W. A., and Wolstenholme, D. R. (1975). Performance of Embankment supported by stone columns in soft ground Geotechnique, 25(1), Murugesan, S., and Rajagopal, K. (2006b). Numerical analyses of geosynthetic encased stone column. Proc., 8th International Conference on Geosynthetics, Yokohama, Japan, Murugesan, S., and Rajagopal, K. (2007a). Model test on geosynthetic encased stone column. Geosythetic International, 24(6), Murugesan, S. and Rajagopal, K. (2010). Studies on the behaviour of single and group of geosynthetic columns. Journal of Geotechnical and Geoenvironmental Engineering, Vol. 136, No.1, January 1, 2010, ASCE Page 8