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Due to the ever increasing demand for land space because of increased construction activity world wide, there is an increasing need to improve soft soil grounds which otherwise are deemed to be unsuitable. Ground improvement technique is a potential alternative to mitigate this problem. Amongst the various ground improvement techniques used for improving the in-situ ground conditions, geosynthetics reinforcement and stone column technique are the widely used ones. The three dimensional geocells are the latest adaptation in the avenues of geosynthetic reinforcements. It is a three dimensional, polymeric, honeycomb like structure of cells interconnected at joints. The reinforcement mechanism in the geocells is by all-round confinement of soil within its pockets. The stone columns are formed by compacting stones into the cylindrical hole formed in soft clay bed, generally by vibro-floatation technique. When load is applied the stone column develops end bearing and skin friction resistance as in piles. Their individual application of both these techniques has been studied well. However, combined application of both, has remained unexplored. Under the present study a series of experiments have been carried out to develop an understanding of the behaviour of foundation systems having geocell-sand mattress overlying stone column reinforced clay beds. The model tests were conducted in a test bed-cum-loading frame assembly in the laboratory. The influence of various parameters, such as; length and spacing of stone columns; pocket size, height, density of fill soil in geocell mattress; base geogrid and encasement of stone columns were The test results indicate that, with the provision stone columns, of adequate length and spacing, 3.5 fold increases in bearing capacity of soft clay foundations can be achieved. With geocell mattress of optimum geometry and dense soil infill, seven fold increase in bearing capacity of the clay foundation can be achieved. While, the geocell mattress-stone column composite reinforcement can increase the bearing capacity of clay bed by ten fold. The increase in the load carrying capacity and reduction in settlement, of the composite reinforced foundation bed, increases with increase in length of stone columns till 5dc, beyond which further improvement is marginal. The optimum spacing of stone columns in composite foundation beds is 2.5dsc. With further reduction in spacing, the stone columns do not produce much of additional performance improvement. The critical height of geocell mattress, giving optimum performance of stone columns in the composite foundation bed, is equal to about the diameter of the footing. A layer of planar geogrid at the interface between the geocell mattress and stone column reinforced clay bed can bring an improvement in bearing capacity as high as 30% more than that with the geocell mattress-stone column reinforcement alone. Encasement of stone columns further improves the performance, with the bearing capacity increasing with the increase in the length of encasement. However, with full length encasement of the floating stone columns the performance improvement reduces substantially. Combined application of base geogrid and encasement of stone columns is an added advantage that brings further improvement in load carrying capacity of the composite foundation bed. Dimensional analysis shows that, the ...........
Supervisor: Sujit Kr. Dash