Dilatancy and Multi-Scale failure characteristics of a foliated rock under triaxial confinement unloading conditions

Controlled by the foliation structure, foliated rocks exhibit mechanical anisotropy and cause squeezing deformation and subsequent supporting structure failure during tunnel excavation. Here, the deformation and failure characteristics of a phyllite under unloading stress paths were experimentally investigated. The strength and elasticity parameters of phyllite exhibited moderate 'shoulder' shaped anisotropy, and specimens failed in three different modes: cross-plane shear-tensile fracture, in-plane shear sliding, and interlayer buckling, depending on the foliation angle and initial confining pressure. Meanwhile, the evolution of dilatancy angle during volume dilatancy also showed significant anisotropy, either increasing or decreasing with the increasing plastic shear strain during post-peak loading. We further discussed the mechanism of the evolution of dilatancy angle based on the multi-scale observation of the failure characteristics. Generally, interlayer buckling improved the dilatancy angle and roughness loss during shear failure resulted in dilatancy angle degradation. These findings provided evidence for a deeper understanding of the failure mechanisms of tunnel rock mass in foliated strata.