Interaction mechanism between deep mixing column and surrounding clay during installation

Abstract

Laboratory and field investigation has shown that the shear strength of the surrounding clay increased in the vicinity of a deep mixed (DM) column in the short period following installation. Field observations indicated that significant excess pore pressure, which may be higher than the hydraulic fracture pressure, was generated during installation.The behaviour between a DM column interacting with the surrounding clay during installation can be simulated as the shearing-expanding process of a cylindrical cavity. A simple approach is proposed to calculate the excess pore pressure around a DM column during installation. The proposed method of calculation considers the effect of a shearing force caused by rotating blades during mixing. In the proposed approach, the excess pore pressure is expressed in terms of undrained strength of clay, shearing force, injection pressure, and a pore pressure parameter. An approach is developed to analyze clay fracturing using a tensile failure mechanism. Analytical results indicate that the rotation of the mixing blades has a significant effect on the generation of excess pore pressure and clay fracturing in a close region around the column. The proposed approach has been verified against laboratory model tests and in situ DM column installation in clay ground at two construction sites. Clay fracturing around a DM column was observed in the laboratory and is analyzed based on field data. This paper reveals that clay fracturing is the major factor behind the strength increase and property changes of the surrounding clay in a short period.Key words: soft clay, deep mixing column, shearing-expanding process, excess pore pressure, clay fracturing, laboratory test, field tests.