Influence of permeability of soft clay on the efficiency of stone columns and geosynthetic-encased stone columns – a numerical study

Abstract

This paper presents the results of a series of numerical studies performed to evaluate the time-dependent behavior of ordinary stone column (OSC) and geosynthetic-encased stone column (GESC) under embankment loading with a special attention on permeability of soft clay. The results from the study indicate that during the embankment construction stage, the excess pore pressure generated in the surrounding soft clay for both OSC- and GESC-treated grounds was found to be the same. However, when the ground was allowed to consolidate, the load-carrying capacity of soft soil was improved because of the presence of stone column. The performance has been improved further when the stone column was encased with geosynthetic layer. The percentage reduction in the ultimate settlement of OSC and GESC when compared to soft clay was found to be 43 and 61%, respectively. A considerable reduction in the time taken for consolidation of GESC-treated ground over OSC-treated ground was also noticed. Effective stress concentration ratio (ESCR) (i.e. ratio of effective vertical stress on stone to that on the soft clay at the same depth) for GESC-treated ground was found to increase gradually during the entire period of consolidation; however, without geosynthetic encasement, it was found to increase up to certain consolidation period beyond which it started decreasing irrespective of soft clay permeability. A reduction in the value of ESCR implies stone column has failed to take the additional load and hence transferred the stress to the surrounding soft clay layer. This signifies the better performance of GESC compared to OSC. The result from the study indicates that the permeability of soft soil has greater influence on the generation and dissipation of excess pore water pressure and hence has a significant influence on the post-construction settlement.