BEHAVIOUR OF GEOTEXTILE REINFORCED STONE COLUMNS MANITA DAS, A.K.DEY ABSTRACT

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1 BEHAVIOUR OF GEOTEXTILE REINFORCED STONE COLUMNS MANITA DAS, A.K.DEY ABSTRACT Stone columns are being used to improve the bearing capacity and reduce the settlement of a weak or soft soil. The improvement can be enhanced by encapsulating the columns with tensile resistant materials. The improvement depends on the confinement offered by the surrounding soil, the reinforcing material and the granular column material. The benefit of using the stone columns in weak soil has been proved to be an efficient method to improve the load-carrying capacity of the soil. The bearing capacity of a stone column mainly depends on circumferential confinement provided by the surrounding soil. Circumferential confinement is normally achieved by using a casing. In this study, the confinement is tried to achieve through placement of horizontal layers of geo-textiles placed at different depths. Laboratory model tests were performed on stone columns of diameter 50 mm and length 500mm, i.e. ten times the diameter. Since lateral bulging occurs up to a depth of 1.5 to 2 times of the diameter of a stone column, hence, horizontal layers of geo-textiles were provided at different depths up to 15 cm from the top. Tests were also performed on un-reinforced stone columns for comparison study. The numerical study is also conducted here. The test setup consists of a test tank with dimensions 1 m x 1 m and height 1 m. The test bed consists of saturated clay consolidated under its own weight. Moisture content of the clay layer at the time of tests was observed to be 44% with undrained shear strength of 20 kpa. This water content was tried to maintain for all the tests. The inner side walls of the test box were coated with grease coated plastic sheet to reduce the friction between the clay and tank wall. The clay bed thickness was kept as 0.9 m for all the tests. In addition, tests were also performed on groups of stone columns, reinforced and unreinforced, arranged in a triangular pattern, spaced at three times the diameter. Results show that the bearing capacity of a reinforced stone column is higher than that of a normal stone column. Moreover, bulging of a reinforced stone column is less than that of a normal stone column. Numerical analysis based on Plaxis 2D is also conducted to study the scale effects. Important conclusions are drawn from the experimental and analytical results. The same tests are conducted in numerical method and finally the results are compared. The results are expressed in terms of load versus settlement curve. From the load versus settlement curve, the bearing capacity is found out by double tangent method. The comparison graph shows almost same result of experimental test with numerical result. The load ratio versus settlement graphs are also observed both for single stone column and group stone columns. Keywords: Stone column, bulging, geosynthetic, reinforcing element.

2 MANITA DAS & A.K.DEY

3 BEHAVIOUR OF GEOTEXTILE REINFORCED STONE COLUMNS MANITA DAS, M.Tech Student, N.I.T SILCHAR, India, A.K.DEY, Professor, N.I.T SILCHAR, India, ABSTRACT:Stone columns are being used to improve the bearing capacity and reduce the settlement of a weak or soft soil. The improvement can be enhanced by encapsulating the columns with tensile resistant materials. The improvement depends on the confinement offered by the surrounding soil, the reinforcing material and the granular column material. The benefit of using the stone columns in weak soil has been proved to be an efficient method to improve the load-carrying capacity of the soil. The bearing capacity of a stone column mainly depends on circumferential confinement provided by the surrounding soil. Circumferential confinement is normally achieved by using a casing. In this study, the confinement is tried to achieve through placement of horizontal layers of geotextiles placed at different depths. Laboratory model tests were performed on stone columns of diameter 50 mm and length 500mm, i.e. ten times the diameter. Since lateral bulging occurs up to a depth of 1.5 to 2 times of the diameter of a stone column, hence, horizontal layers of geo-textiles were provided at different depths up to 15 cm from the top. Tests were also performed on un-reinforced stone columns for comparison study. INTRODUCTION The increasing infrastructure growth in urban and metropolitan areas has resulted in a drastic rise in land prices and lack of suitable sites for development. As a result, construction is now carried out on marshy land or barren land with poor load carrying capacity and high compressibility. Many site specific ground improvement techniques are now being adopted to improve this type of poor soil and improvement by stone columns is one of the widely used techniques. They also can be used in loose sand deposits to increase the density. They are capable of dissipating excess pore water pressure in the insitu soil and thereby reducing the void ratio in the zone of influence. Whereas the first use of stone columns was in Germany in 1950, the first use in India was in early 1970s. Performance of a stone column foundation was studied in 1985[1]. Vibroreplacement method was used for installation pf stone columns to support a large wastewater treatment plant. Numerical analysis of stone column supported foundations was studied by Pande and Scheiger [2]. In this paper, settlement and failure load of rafts resting on stone column reinforced soft clays are analyzed. Improvement of soft clay characteristics due to stone column installation studied in 2007 [3]. This method was proposed for evaluating the improvement of the Young modulus of soft clay in which a vibro compacted stone column was installed. Stone column was studied in 2011 [4] to determine the soil improvement factor. Many researchers [5,6,7,8,9,10,11,12,13] worked on geosynthetic encased stone columns. Among all the papers, three studies [5,11,12,13] are analytical based. In all these papers, a series of numerical studies were performed on the contribution of geosynthetic encasement in enhancing the performance of stone columns in very soft clay deposits. The papers [6,7,8,9,10] studies are based on experimental work. From all the studies it can be seen that the ultimate load carried by soft soil increases by using OSCs. The ultimate load and stiffness of the treated soil can be further increase by the use of vertical reinforcing material. When the length and strength of reinforcing encasement increase, the ultimate capacity and stiffness of stone columns increase. The major disadvantage of a stone column is its bulging tendency under a compressive load due to non-rigid structural form. It is observed that the

4 MANITA DAS & A.K.DEY maximum reduction in expected settlement due to installation of stone columns is around 50%. Thus, stone columns are not normally provided below ordinary buildings. In case, bulging of stone columns can be reduced, the overall settlement can also be reduced horizontal layers of geo-textiles were provided at different depths up to 15 cm from the top. Tests were also performed on unreinforced stone columns for comparison study. To improve the performance of soft clay foundations using stone columns and geocell-sand mattress one study [14] was performed experimentally in A series of experiments were carried out to develop an understanding of the performance improvement of soft clay foundation beds using stone columngeocell sand mattress as reinforcement. In this study from the experimental results it can be obtained that the composite reinforcement i.e. geocell mattress over stone column is very effective one to increase the bearing capacity and reduce the settlement. PROGRAMME OF THE PRESENT WORK In this study, one clay bed test, two single column test and two group column tests are performed. The diameter of the stone column is taken as 50mm and height of the stone column is taken as 500 mm taking L/D=10, where L is the length and D = diameter of the column. This is because a minimum L/D = 4 is required for control of bulging failure mode. In group column test triangular pattern is considered. The centre to centre spacing between the columns is taken as 3 times of the column diameter i.e. 150mm. According to previous researchers [10] bulging failure occurs upto a depth of 1.5D to 2D from the top of the column. Hence, only the top portion of the stone column needs more lateral confinement in order to reduce the bulging, hence, horizontal layers of geo-textiles were provided up to a depth of 15 cm. The depths of placement of geo-textiles were kept as 0.0cm, 1.5 cm, 3.0 cm, 5.0 cm 10 cm and 15 cm from the base of the footing. In all cases stone columns are placed at the centre of the clay bed and load test is performed and bearing capacity and settlement values are found out and the test results are compared.during the tests water content of the clay bed is kept constant. Although many researchers have studied the bearing capacity and expected settlement of the treated ground, very few have discussed on the reduction of bulging or settlement of the treated soil. Use of horizontal layers of geotextiles as a measure to reduce the the bulging of the stone columns is a new proposition. EXPERIMENTAL INVESTIGATION Properties of Materials Properties of clay, stone and geotextile are obtained in the laboratory and shown in Tables 1-3. Table 1 Properties of clay- Specific gravity 2.43 Bulk unit weight 1.72 gm/cc Liquid limit 63.5% Plastic limit 35.07% Unified system CH classification Table 2 Properties of stone Stone size 2-6 mm c 0 Angle of internal friction Table 3 Properties of geotextile- Parameter Quantity Size 20cm*4cm Tensile strength 20 KN/m EXPERIMENTAL SETUP A test setup is designed for the current research work. This setup consists of a large test box with plan dimensions of 1m*1m*1m.for preparation of clay bed, water content is taken as 40% corresponding to kpa. The clay bed thickness of 900 mm is used to counteract the effect of stone column with bottom of the tank.

5 Fig. 2 Plan of single stone column Fig. 1 Experimental set up PREPARATION OF CLAY BED Clay bed was prepared in a large test box with plan dimension of 1m*1m.The clay bed thickness is taken as 900 mm. The clay bed is prepared in layers each of which is 50 mm thick. To prepare the clay bed at a moisture content of 40% which will give a soft consistency to the soil with expected undrained shear strength 15 to 20 kpa, initially natural water content of the clay was determined and the amount of additional water was added to the clay to achieve 40% water content in a large plastic box. The surface of the box was sealed with a nylon sheet for five days to achieve uniform water content within the clayey soil mass. The inner face walls of the test box are coated by a thin layer of grease to reduce the friction between the clay and tank wall for each layer. The clay was placed in the tank with measured weight to reach a certain bulk unit weight of 1.72gm/cc. Fig 1 shows a typical test set up. Fig. 3 Plan of group stone columns CONSTRUCTION OF REINFORCED AND UNREINFORCED STONE COLUMN In the current test, stone columns diameter is of 50 mm constructed by the replacement method. The column is constructed at center of large test box. The plan dimension of tank is selected such that results of test will not be affected by boundaries of the tank. An auger of diameter 50 mm is pushed through the clay bed upto required height and then pulled out slowly. To construct the stone column, the free drop height was 100 mm with 15 blows. High quality of granular material was selected so that breakage of the stone column material could be neglected and new stone column material used in each new test, not the material used for the previous tests. Figs 2 and 3 show

6 MANITA DAS & A.K.DEY construction of single stone column and group of stone columns respectively. TEST PROCEDURE The test procedure involves application of the load and determination of load-displacement behavior of the clay treated with stone columns. For each test, steel plate was used as footing. For the single column tests, the diameter of the plate is taken as 10 cm and thickness is 10 mm and for the group column test, diameter is 40 cm and thickness is 1.5 cm. For each test, one hydraulic jack of capacity 1 tonne is used for giving the load. One proving ring of capacity 500 kg and two dial gauges on the both sides of the proving ring are used for the experimental set up as shown in Fig.1. The settlement value is obtained by taking the average of two dial gauge values and the load vs settlement graph is drawn. RESULTS AND DISCUSSION Deformation and Failure Mode After completion of tests, the deformed shape of the columns was observed. In the single column test, it is observed that the bulging failure mode governed. The bulging failure occurred at a depth of 1.5D to 2D from the stone column head in single stone column test as shown in Fig 4. Fig.4 Cross-section of single stone column without geosynthetic. Load-Settlement Behavior To improve the bearing capacity, various methods viz. use of cement slurry and iron casing are tried in this project. From the load settlement graph, it can be seen that the iron casing is more effective than the cement slurry. The load settlement behaviour of single column with cement slurry and iron skirting as reinforcement is shown in Fig 5. Fig 5 Load vs settlement graph for single stone column with cement slurry and casing as reinforcement.

7 But iron casing is not so beneficial from the economic point of view, so a new method by using geosynthetic material as reinforcement in layers is adopted whose bearing capacity is equal with the iron skirting and also cost effective. The load settlement behavior of single and group stone columns by using geosynthetic as reinforcement are shown in Figs 6 and 7. Fig.8 Load ratio vs settlement graph for single column. Fig 6 Load vs settlement graph for single stone column with and without geosynthetic and for clay bed. Fig.9 Load ratio vs settlement graph for group stone column. Fig 7 Load vs settlement graph for group stone column with and without reinforcement. A term Load Ratio is defined as the ratio of load capacity of stone column to load capacity of clay bed for the same settlement. The load ratio vs settlement graphs of single and group stone column are shown in Figs. 8 and 9. NUMERICAL METHOD Numerical Analysis is done by Plaxis 2D. PLAXIS 2D is a finite element package intended for the two dimensional analysis of deformation and stability in geotechnical engineering. It is equipped with features to deal with various aspects of geotechnical structures and construction processes using robust and theoretically sound computational procedure. The objective is to evaluate the ultimate bearing capacity of a circular footing of diameter 11cm for single column and 40 cm for group column. Numerical results A total of 26 model tests are performed. For all the tests water content is kept constant. The footing is placed at centre of the column with the help of plumb bob. For group stone columns the triangular pattern is considered whose centre to centre

8 MANITA DAS & A.K.DEY spacing is taken as 3 times the diameter of the stone column. Total 5 tests, one on unreinforced clay bed, two on single stone column and two on group columns are made. Six increments of load are applied for each test to obtain the deformation. Typical load-settlement curves are obtained. The ultimate bearing capacity is calculated from the load-settlement curve by using double tangent method. The load-settlement curve and the deformed mesh of different tests are shown in Figs Single stone column without geotextile Fig. 12 Load- deformation curve for single stone column without geotextile. Fig. 10 Load- Deformation curve for unreinforced clay bed. Fig. 13 Deformed mesh (total displacement) Single stone column with geotextile- Fig. 11 Deformed mesh (total displacement). Fig. 14 Load- deformation curve with geotextile.

9 Group stone column with geotextile Fig. 15 Deformed mesh (total displacement). Group stone column without geotextile Fig. 18 Load- deformation curve for group stone columns with geotextile. Fig. 16 Load- deformation curve for group stone columns without geotextile. Fig. 19 Deformed mesh (total displacement) COMPARISON OF TEST RESULTS OF EXPERIMENTAL WITH NUMERICAL The comparision graphs of load-settlement are shown in Figs Fig. 17 Deformed mesh (total displacement)

10 MANITA DAS & A.K.DEY Fig. 20 Comparison of load settlement curve of unreinforced clay bed. Fig. 23 Comparison of load settlement curve of group stone column without geotextile. Fig. 21 Comparison of load settlement curve of single stone column without geotextile. Fig. 22 Comparison of load settlement curve of single stone column with geotextile. Fig. 24 Comparison of load vs settlement curve of group stone column with geotextile. CONCLUSIONS In this investigation, laboratory tests have been performed on single and group stone columns with diameter of 50 mm. Reinforced stone column test results are compared with those obtained from tests on unreinforced stone columns. The experimental results also are compared with the numerical results. Based on results from experiments on single and group of stone columns, the following concluding remarks are extracted: a. Iron casing is more effective for increase in load carrying capacity of a stone column than injection of cement slurry. Horizontal layers of Geotextile reinforcement is equally effective with iron casing. b. The ultimate load carried by soft soil increases by 2.5 times with the use of stone columns. The ultimate load and stiffness of the stone column can be further increased by 3 times with the use

11 geotextiles placed in 5 layers at depths 0.0D, 0.3D, 0.6D, 1.0D, 2.0D and 3.0D from the top. c. From the group of stone column tests, it is observed that the ultimate capacity of soft soil is increased by 9 times with the use of stone columns and 27 times with the use of geotextile reinforced stone column. d. From the single stone column results, it is observed that settlement of stone column is decreased by 1/7 times with the use of geotextile reinforcement. e. From the group stone column results, it is observed that settlement of stone column is decreased by 1/2 times with the use of geotextile reinforcement. f. From the single stone column test results, it is observed that the lateral bulging decreases in reinforced stone column by 1/2 times compared with ordinary stone columns due to additional lateral confinement provided by geosynthetic material. g. From the group stone column test results, it is observed that the lateral bulging decreases in reinforced stone column by 3/4th times compared with ordinary stone columns due to additional lateral confinement provided by geosynthetic material. REFERENCES 1. Mitchel, J. (1985), Performance of a stone column foundation ASCE. 2. Pande and Scheiger (1986), Numerical analysis of stone column supported foundation Computers and Geotechnics, 2, Guetif Z., Bouassida M., Debats J.M.(2007) Improved soft clay characteristics due to stone column installation, Computers and Geotechnics, 34, Pivarc J.(2011), Stone columns determination of the soil improvement factor, Vol. XIX, No. 3, Khabbazian M., Kaliakin V., Meehan C.(2011), Performance of quasilinear elastic constitutive models in simulation of geosynthetic encased columns, Computers and Geotechnics, 38, Wu C and Hong Y.(2009), Laboratory tests on geosynthetic-encapsulated sand columns Geotextiles and Geomembranes, 27, Gniel J and Bouazza A. (2009) Improvement of soft soils using geogrid encased stone columns, Geotextiles and Geomembranes 27, Gniel J and Bouazza A. (2010), Construction of geogrid encased stone columns: A new proposal based on laboratory testing Geotextiles and Geomembranes, 28, Zhang Y, Chan D, Wang Y. (2012), Consolidation of composite foundation improved by geosynthetic-encased stone Columns, Geotextiles and Geomembranes, Ghazavi M and Afshar J. (2013) Bearing capacity of geosynthetic encased stone columns, Geotextiles and Geomembranes, Lo S., Zhang R., Mak J.(2010), Geosyntheticencased stone columns in soft clay: A numerical study, Geotextiles and Geomembranes, 28, Pulko B., Majes B., Logar J. (2013), Geosynthetic-encased stone columns: Analytical calculation model, Geotextiles and Geomembranes, 29, Imam R, Keykhosropur L, Soroush A. (2012), 3D numerical analyses of geosynthetic encased stone columns, Geotextiles and Geo membranes, 35, Dash S and Bora M (2013), Improved performance of soft clay foundations using stone columns and geocell-sand mattress, Geotextiles and Geo membranes, 41,