Stress and pore pressure changes due to sand compaction pile installation in soft clay

Abstract

This paper presents centrifuge model data and describes a semi-empirical approach to predict short-term total radial stress and pore pressure changes around sand compaction piles installed in soft marine clays. Comparison of the measured changes in radial stress and pore pressure with plane strain cavity expansion theory shows that the latter gives a reasonably good estimate at large depth for the entire installation process and for the peak stress measured during casing jack-in, but not for the residual stress after casing jack-in. However, deviation from plane strain cavity expansion theory was noted at shallow depths. To account for the effect of the ground surface, a plane stress cavity expansion theory was proposed for the shallow zones, which assumes constant vertical overburden stress. The establishment of the two limits formed by the plane strain and plane stress theories allows semi-empirical relations to be established by fitting to the data. These semi-empirical relations allow the post-installation and peak jack-in stress and pore pressure to be reasonably estimated. However, post-jack-in values remain significantly overestimated. These findings imply that, in order to mobilise significant set-up of stress in the improved ground, there must be substantial further cavity expansion during the sand injection stage of SCP. Cumulative total stress increment at a given location due to the installation of multiple piles in a grid may be reasonably estimated by superimposing the increments due to the installation of each pile. On the other hand, pore pressure build-up is shown to be less readily superimposed, possibly because the shear-induced component of excess pore pressure does not increase linearly and infinitely with deviator stress.