Effect of lateral stress and loading paths on direct shear strength and fracture of granite under true triaxial stress state by a self-developed device

Direct shear is the stress path that rockmass undergoes often and is thus important to the stability evaluation of underground openings in deep tunnels. This study introduces a self-developed true triaxial direct shear device capable of achieving large displacement direct shear, providing a basis for the multi-stage shear experiments. Direct shear experiments were conducted on intact granite samples from the Beishan underground research laboratory under two stress paths respectively with lateral stress greater and smaller than the normal stress. Additionally, the influence of loading and unloading paths on triaxial direct shear behaviors at the residual stage was investigated through multi-stage shear experiments. It shows the peak shear strength experiences an increase of 11.3 similar to 38.1 % when the lateral stress increases from 5 MPa to 35 MPa. Furthermore, the roughness of rock fracture surfaces was also affected importantly by the lateral stress. At multiple scales, increasing lateral stress expands the tensile fracture range, enhancing rock brittleness. When normal stress is 20 MPa, the increase of lateral stress from 5 MPa to 35 MPa reduces the brittle coefficient from approximately 5.40 to 3.80. Additionally, the effect of lateral stress on residual strength is closely tied to the contact state of the fractured rock surface, and in multistage tests, when lateral stress exceeds normal stress, the shear strength during lateral stress unloading consistently surpasses that during loading. The findings quantified the effect of lateral stress on the direct shear properties of rocks and can provide a reference for modifying the rock strength and deformability in the stability evaluation of deep hard rock openings.