Numerical modelling of micro and macroscopic hydromechanical behaviour of unsaturated soils

<p>The unsaturated soil is a three-phase mixture consisting of solid (soil particles), liquid (water), and gas (air). The pore-scale air-water distribution keeps changing with the deformation of solid structure and the fluid pressure and air-water interfacial tension introduce additional forces to the particles, which makes the hydromechanical behaviour of the unsaturated soil of unsaturated soil is outstandingly complicated, compared with saturated or dry soils. Therefore, it is pivotal to develop numerical models that can reproduce the key features of the micro and macroscopic hydromechanical behaviour of unsaturated soils. In this study, the pore morphology method (PM) was revised and extended to reproduce the air-water distribution in unsaturated soils considering the cavitation mechanism and the locally variable contact angles. A micro-mechanical model based on discrete element method (DEM) was proposed to study the microscopic and macroscopic behavior of unsaturated soils under different suctions by developing a novel pore-scale numerical method for simulating the liquid-solid interfaces. The proposed micro-mechanical model was further used to investigate the micro and macroscopic hydromechanical behaviour of unsaturated soil in triaxial tests, direct shear tests and interface shear tests. The typical macroscopic responses such as stress-strain relationship, volume change, degree of saturation change were reproduced by the proposed models. The suction contribution to effective stress and the effective stress parameter estimated by the proposed models agreed well the experimental results. The microscopic responses (pore water distribution, capillary forces, fabric anisotropy and contact forces) of unsaturated soils in various stress conditions were investigated and discussed.</p>