Indian Geotechnical Conference IGC2016 15-17 December 2016, IIT Madras, Chennai, India 1 AN INNOVATIVE APPROACH TO INCREASE THE BEARING CAPACITY OF STONE COLUMNS Manita Das 1 Ashim Kanti Dey 2 1 Ph.D Student, 2 Professor, Department of Civil Engineering, National Institute of Technology, Silchar, Assam – 788010 Email address: 1 manitta403@gmail.com, 2 ashim_kanti@yahoo.co.in ABSTRACT: Stone columns are being used to improve the bearing capacity and reduce the settlement of a weak or soft soil. Under a vertical load the top of the stone column bulges and the capacity of the stone column is derived from the depth of bulging. The higher the depth of bulging the greater is the load carrying capacity. As such researchers have tried skirted stone columns. The capacity of the stone column also increases with decrease in diameter of bulging. Geosynthetic encased stone columns have been used to achieve this goal. Both the methods have their inherent merits and demerits. In the present study an innovative technique of using a soil-cement stabilized bed over the stone column has been tried to increase the capacity of the stone columns. The technique is easy, economic and does not require any skilled labour. The technique reduces the extent of bulging and increases the depth of bulging. In the present study the thickness of the soil cement stabilized bed is varied and the effect of thickness of the bed on improvement of capacity of stone column is studied. It is observed that the capacity can be increased by 5 times and the amount of bulging can be reduced by 25 percent. KEYWORDS: Stone column, bulging failure, stabilized bed, cement. 1 INTRODUCTION Due to the scarcity of land and explosion of population, the marshy land is now being used for construction. The marshy land usually comprises of soft to very soft clayey soil. Piles are normally being used in this land to carry the load of a super structure. For a low rise construction, the cost of piles may be very high and people often use some ground improvement technique. Installation of stone columns is one of the very promising techniques of improvement of soft soil, but its use is limited for residential buildings because of high risk of settlement. It is reported that the stone columns may reduce the expected settlement upto 50%. The bearing capacity of stone column normally depends on the depth and amount of the bulging. If somehow the depth of bulging can be increased, then the bearing capacity of stone column is also increased. Many rersearchers (Murugesan and Rajagopal (2006), Chungsik Yoo (2010), Pulko et al. (2011), Keykhosropur et al. (2012), Fattah and Majeed (2012), Deb and Mohapatra (2013), and so on) used geosynthetic or geogrid encased stone columns (GESC) to reduce the amount of bulging as well as to increase the depth of bulging. Some researchers (Deb et al. (2008), Afshar and Mahmoud Ghazavi (2014)) used geosynthetic material in layers as reinforcement. Stone column jacketing with tubular mesh was also used as a reinforcement to strengthen the soft soil (Black et al., 2007)). All these techniques are not widely used because of some inherent deficiencies. In the present study, an innovative technique of using a cement stabilized loamy soil bed over the stone columns has been tried. The loamy soil has both cohesion and angle of internal friction. Upon mixing with five percent cement by weight and a specified quantity of water this soil gives a very hard mass upon compaction. This hard mass is placed below the footing where the induced stress intensity is very high. Obviously the load intensity on stone columns becomes minimum and thus the extent of bulging is also minimum. The thickness of the soil-cement stabilized bed plays a major role in load carrying capacity of the stone column. In the present study effect of thickness of the stabilized soil bed has been studied. Results show that cement stabilized soil bed can increase the load carrying capacity by more than 5 times. Moreover, bulging of the stone column is also reduced by twenty five percent. 2 EXPERIMENTAL INVESTIGATION 2.1 Material used In the present study three types of locally available materials, namely, clay for preparation of clay bed, stones of size 2 – 6 mm for preparation of stone columns, red-loamy soil popularly known as tilla soil for preparation of stabilized soil cement bed were used. Ordinary Portland cement was used to make the cement-stabilized bed. The properties of the clay are shown in table 1. For determination of shear strength, of the clayey soil, a series of unconfined compressive strength (UCS) tests were conducted on cylindrical