Discrete element study on mechanical properties of layered sand-cobble strata under unloading stress path of shield construction

Aiming at the deformation problem of sand-cobble composite strata induced by tunnelling, the stress path of soil during tunnel excavation was analyzed by numerical simulation. Two stress path schemes (σ1 reduction-σ3 reduction, σ1 reduction-σ3 stability) were proposed. The triaxial test model of composite stratum is constructed. The DEM simulation results demonstrated significant differences in the macroscopic mechanical response of soil under different stress paths. The simultaneous unloading of axial pressure and confining pressure will accelerate soil softening, while the decrease of confining pressure unloading rate can improve the toughness of soil. With the increase of the thickness of cobble stratum, the bearing capacity and toughness of the soil show an increasing trend, and the increase of cobble particles enhances the overall stability of the soil. In addition, the increase of stress ratio also leads to the improvement of soil strength and stability, so that the soil can better resist the softening effect caused by stress release during unloading.