Deep mixing method for settlement reduction and bearing capacity enhancement in copper smelter in Gresik

Deep Mixing (Hereinafter “DM”) was applied to soft clay layers in ore storage facilities in a new Copper Smelter in Gresik, East Java, Indonesia for settlement reduction and bearing capacity enforcement. In Indonesia, precast concrete pile is often applied as foundation structures. However, DM was chosen for several facilities because piles require a deeper supporting layer with SPT-N-value of about 50, while DM can be supported by a shallower intermediate layer with SPT-N value of 20-30. In design, it was assumed that facility loading is concentrated on DM columns. Specifications of DM were designed with a focus on the three points : (i) Design compressive strength exceeds a facility loading, (ii) Bearing capacity at the tips of the column is larger than the loading, and (iii) The sum of deformations of the column and the ground under the column is less than a target value. The improvement ratio and design strength were set according to the loading condition. The quality of the constructed columns was sufficient because the in-situ column strengths exceeded the design strength. The ratios of the in-situ column strength to laboratory strength ranged from 0.5 to 2.0. The average deformation modulus was 175 times of the in-situ column strength. Regarding construction work, by arranging machinery to dealing with underground obstacles such as boulders, installation works were able to proceed smoothly. The additional construction work was able to be carried out under appropriate cost with payment conditions determined previously.