Performance of Stone Columns in Cohesive Soil

Abstract

Stone columns are widely used and generally considered to be one of the most cost-effective and environmental-friendly soil improvement technique for highways and embankments. They are also used as drainage to reduce the consolidation period, which accordingly increases the bearing capacity, reduces settlement, and reduces the liquefaction potential. Current design theories used to estimate the bearing capacity of a group of stone columns are based on the unit cell or homogenized material concepts, which neglect the effect of the column interactions and installation technique. This thesis therefore presents an experimental investigation, together with numerical modelling, to examine the performance of a single stone column and group of stone columns subjected to vertical loading. An analytical model is developed to capture the effect of an arrangement of stone columns and the mode of failure within a column and the surrounding soft clay material. A single stone column and a group of stone columns were investigated in a large-scale experimental set-up. The testing program was divided into four steps: (a) filling the testing tank with the clay, (b) installing the stone columns in the clay bed, (c) extracting samples of the reinforced soil (a block of stone columns surrounded by the soft clay loading), and (d) testing the samples in a triaxial apparatus. The results showed that the mode of failure of the reinforced soil depends on the column spacing and the strength of the column materials and the surrounding soil. Numerically, a 3-D finite element model was developed to examine the influence of the governing parameters on the bearing capacity of the group. The model was validated against experimental results from this study and results available in the literature. The numerical model was used to simulate the actual driving process during installation of the columns. The model was then used to predict the actual failure plane under a rigid footing reinforced by stone columns for a given geometry/soil condition. An analytical model was developed utilizing the actual failure plane deduced from the numerical model to develop a theory to predict the bearing capacity of the reinforced soil. The theory developed was validated against the results obtained from the numerical model and results reported in the literature.