Proceedings of Indian Geotechnical Conference December 15-17,2011, Kochi (Paper No. H-059.) AN EXPERIMENTAL STUDY OF STONE COLUMNS IN SOFT SOILS Kausar Ali, Research Scholar, Department of Civil Engineering, I.I.T. Delhi, India, email: kausarali786@rediffmail.com J.T. Shahu, Associate Professor, Department of Civil Engineering, I.I.T. Delhi, India, email: shahu@civil.iitd.ac.in K.G. Sharma, Professor, Department of Civil Engineering, I.I.T. Delhi, India, email: kgsharma@civil.iitd.ac.in ABSTRACT: The use of stone columns is becoming more popular as an ideal ground reinforcement technique for supporting flexible structures such as embankments, oil storage tanks, etc., resting on soft soils. Due to very low lateral confinement provided by the soft soil, stone columns undergo excessive bulging when loaded, resulting into only a small improvement in the bearing capacity. The strength of the composite soil may further be increased by reinforcing the stone columns by some suitable geosynthetic. In the present study, laboratory model tests have been carried out on floating and fully penetrating single stone columns with and without geosynthetics to find out the effect of reinforcement and l/d ratio on the bearing capacity of the composite soil. Bulging of stone columns and stress distribution in soil mass have also been investigated. The tests indicate that the encasement increases the ultimate bearing capacity substantially. Keywords: Soft Clay, Stone Column, Ground Improvement, Geosynthetic. INTRODUCTION The prime requirement for the development of any country is the sufficient infrastructure of buildings, roads, tunnels, bridges and other civil engineering works. Since India has a vast area of soft soils along its long coast which are highly compressible with low bearing capacity. For the construction of high rise buildings and other important structures, the soft soil is bye passed by providing the pile foundation resting on firm strata. But for low rise buildings and other flexible structures such as liquid storage tanks, rail/road embankments, factories etc. that can tolerate some settlements, ground improvement techniques are normally considered as economical. Stone columns is one of the most suited technique and used worldwide. Stone columns subjected to compressive loads fail in different modes, such as bulging, general shear failure and sliding. For stone columns having lengths greater than critical length (i.e., about four times the diameter of the column), it is recognized that the bulging failure governs the load carrying capacity whether they bear on stiff layer or penetrate partially into the medium stiff soil [1]. An elastic approach [2] may be used to predict the load sharing and resulting settlement of ground improved by stone columns assuming free strain condition. It is well established that the stone columns derive their load carrying capacity from the lateral earth pressure against bulging from the surrounding soils [3]. When the stone columns are installed in extremely soft soils, the lateral confinement offered by the surrounding soil may not be adequate. Consequently, the stone columns installed in such soils will not be able to develop the required load-bearing capacity. Many cases have been reported where the stone column was not restrained by the surrounding soft clay which led to excessive bulging and squeezing of the soft clay into the voids of the aggregate [4]. In such situations, the bearing capacity of the stone column can be improved by reinforcing these with a suitable geosynthetic. Many researchers [5, 6, 7, 8] have studied the behaviour of reinforced stone columns. Most of the work done so far is limited to fully penetrating columns; therefore, in this study fully penetrating as well as floating columns have been investigated. EXPERIMENTAL PROGRAMME Model dimensions, Tests schedule, Instrumentation and Materials The model tests were performed in a perspex cylindrical tank of 300 mm diameter, 600 mm depth and 10 mm wall thickness. To compare the relative performance of ordinary and reinforced stone columns, nine model tests were conducted on composite soil as shown in Table 1. Table 1 Summary of the model tests conducted Test No. d (mm) L (mm) Remarks 1 - - Plain clay 2 30 300 Ordinary Column 3 30 300 Encased Column 4 50 300 Ordinary Column 5 50 300 Column with Horizontal strips 6 50 300 Encased Column 7 50 550 Ordinary Column 8 50 550 Column with Horizontal strips 9 50 550 Encased Column d = diameter of the stone column. L = length of the stone column. Most of the tests were conducted on 50 mm diameter columns. Two tests were, however, also conducted on 30 375