CFD-DEM simulation of microbially treated sands under undrained consolidated cyclic triaxial tests

The microbially induced calcite precipitation is a promising technology to improve ground, and the treated soil can be regarded as the structural one. In this study, firstly, based on the three-dimensional (3D) contact model for granulates incorporating rolling and twisting resistances and 3D bonds failure criteria, and considering both the slight plastic deformation of particles during collisions and the rate-dependency, a cyclic bonded contact model is established. A time-dependent relationship is then proposed to describe the denitrification reaction in reinforced sand. Next, the mechanical responses of microbially treated sands at different cementation and bubble contents are investigated by the coupled CFD-DEM in undrained-consolidated cyclic triaxial tests. The effects of biological bond and biological bubbles on the liquefaction resistance of sands are analyzed in link with the mechanism from macroscopic and microscopic scales. The results show that the coexistence of cementation and bubble does not increase the liquefaction resistance as expected in the form of "1+1=2". The presence of cementation enhances the liquefaction resistance of unsaturated sands evidenced by the decrease of excess pore water pressure ratio and axial strain, and the increase of coordination number. However, the presence of bubbles reduces the liquefaction resistance of cemented sands, where the number of cycles to the initial liquefaction decreases, the axial strain increases significantly in the tensile direction, and the coordination number decreases significantly.