Experimental and Numerical Studies on Response of the Stone Column in Layered Soil

This paper describes the soil layering effects on response of the stone column and stone column improved ground through a series of small scale laboratory tests and numerical analyses. Two types of layering systems, i.e. soft clay overlying stiff clay and vice versa are considered for the present study. The entire laboratory tests were carried out on 88 mm diameter stone columns installed in a two layered soil systems. Unit cell concept is used to idealize the behaviour of a single column within an infinite group of stone columns. Entire unit cell and only the stone column area were loaded to evaluate the stress versus settlement response of the entire improved ground and that of the stone column. Effects of the top soft and stiff clay layer thickness on the axial stress of the whole improved ground and stone column only are evaluated through laboratory tests. A detailed parametric study using finite element based software Plaxis was also carried out. Elastic-perfectly plastic Mohr–Coulomb failure criterion with drained conditions was used for the soil and stone columns in the numerical analyses. Result shows that the limiting axial stress of the stone column is found to be influenced by the top clay layer thickness up to two times the diameter of the stone column beyond which it remains constant for both the layering systems. The limiting axial stress of the whole improved ground is found to be influenced by the presence of the top layer up to a depth of four times the diameter of the stone column for both the layering systems. The stiffness improvement factor of the improved ground increases with increase in the thickness of the top soft clay layer and attains maximum value for the full depth of soft clay whereas it remains constant for different depth of the top stiff clay. The vertical extent of the bulging increases with increase in the thickness of the top soft clay up to two times the diameter of the stone column for both the layering systems.