Abstract: Deep mixed (DM) column is installed in field through mixing the injected chemical binder with in-situ soft soil using rotating blades. The injection of chemical binder and rotating mixing disturb the surrounding clay, change the stress state, and cause excess pore water pressure in the surrounding ground. The excess pore water pressure was measured during installation of DM columns in sensitive Ariake clay, Japan. Field measured results showed that significant excess pore water pressure was generated in the surrounding clay and its value was greater than its effective overburden stress to make the surrounding clay under tension state. However, at the initial stage (5 minutes), the excess pore water pressure dissipated very fast. The interaction mechanism between deep mixing column and the surrounding clay during installation can be simulated by using shearing-expanding process of cylindrical cavity. An approach is presented for the calculation of excess pore water pressure around DM column. The proposed method considers the effect of shearing force caused by the rotation of blades during mixing. In the proposed approach, the excess pore water pressure is determined in terms of undrained strength of clay, shearing force, injection pressure and pore water pressure parameter. The proposed method is verified by the laboratory test and field measured data.

Key words: soft Ariake clay, deep mixing column, excess pore water pressure, shearing-expanding process, field investigation, mechanical analysis