Some Laboratory and Numerical Studies on the Behaviour of Stone Columns Installed in Mumbai Marine Clay

Mumbai marine clay is problematic in nature for substructures and it needs to be strengthening before making it available for any construction activity. Out of many available geotechnical solutions, stone columns are quite handy in significantly improving the bearing capacity of marine clay, which also supplements by dissipating excess pore water pressure and reducing the settlements of clay beds under external loads. For the investigation on behavior of stone columns, marine clay was collected from the Uran site near Mumbai, India. Slurry consolidated clay beds were prepared and gravity loading mechanism was adopted to consolidate the clay beds in the laboratory. Laboratory tests were performed to ascertain the uniformity and repeatability of the clay beds by slurry consolidation. Augers of 50, 75 and 100 mm diameter were used to bore the holes in the clay bed and aggregates were used for stone column preparation by replacement method. The static displacement controlled tests were performed on the stone column reinforced clay bed. In the present study, the loading is applied on stone column alone, as it leads to the ultimate axial capacity of stone column improved ground. Clay bed reinforced with stone column exhibits improved load-settlement response, compared to unreinforced clay beds. The improvement was very significant at higher settlements. The failure load of reinforced clay bed is around 6 times more than the unreinforced clay bed. In order to further understand the behaviour of stone column reinforced ground, numerical modeling of stone column reinforced clay bed is conducted using FLAC^3D. Using the validated numerical model of the stone column, the effect of variation of the diameter of stone column on its performance was studied. From the numerical analysis and physical tests conducted in laboratory, it is observed that the stone columns with smaller diameter when subjected to vertical loading carried higher bearing pressures when compared to that of the larger diameter stone columns, which may be due to the greater confining and larger bulge formed at a depth of 2–3 times the diameter of stone columns.