Optimal Design of Stone Column Using Numerical Analysis

Abstract

In the present work, the design of a stone column is done using the Finite element package PLAXIS. For the analysis, infinite number of stone columns were considered under a circular rigid raft considering equilateral triangular pattern of stone columns. A unit cell approach was considered for the design from the infinite number of stone columns having effective diameter and area replacement ratio. Two cases were considered-in the first case, the analysis was done on a single unit cell stone column and loading was done on the stone column using prescribed vertical displacement and in the second case, entire area was loaded on a single unit cell stone column and comparison was done with group of stone column under entire area loading. Parametric study was conducted to study the effect of the parameters governing the design of stone columns. From the parametric study, the optimal length and spacing were found out for varying modular ratios and friction angle. In this analysis, columns used were of floating nature. The diameter of the stone column was kept constant and taken as 1m whereas the length of the column varied as 2m, 4m, 5m, 10m, 15m and 20m to have Slenderness ratio (L/d) of 2, 4, 5, 10, 15 and 20. The undrained cohesion of clay was varied as 10kPa, 20kPa, 30kPa and 40kPa whereas the friction angle varied as 40, 45 and 50. The spacing to diameter ratio (S/d) was taken as 1.74, 2.13, 3.00 and 4.26 so as to have area replacement ratio of 5%, 10%, 20% and 30%. It is observed that bearing stress is highest for higher area replacement ratio and high value of undrained cohesion. Stiffness improvement factor and performance ratio were determined for stone column under entire area loading. Using the Statistical Package for Social Sciences, the bearing stress equation has been developed. A single stone column when loaded alone fails due to bulging and the column having length more than four times the diameter fails by bulging and less than that fails by punching.