Discrete element modelling of the macro/micro-mechanical behaviour of unsaturated soil in direct shear tests including wetting process

Soil suction, varying with environmental conditions, has an important role in the mechanical behaviour of unsaturated soil. In this study, a series of three-dimensional (3D) simulations of direct shear tests under various suctions are performed by using the discrete element method (DEM) to investigate the influence of suction on the macro/micro-mechanical behaviour of unsaturated soil. A series of shearing-wetting-shearing tests under constant shear stress levels are simulated to disclose how the suction reduction affects the shear process of soils. Silty soil is employed, and a shear rate of 3.75%/min is applied for the shearing process. The simulated macroscopic results are compared with laboratory test results in the literature. Through the simulations, the contact force networks, coordination number and fabric anisotropy are presented and analyzed for both scenarios: (1) direct shear under constant suctions and (2) wetting under constant shear stresses. The links between macro-mechanical behaviour and micro-mechanical variables are discussed.