Study on mechanical properties of triaxial unloading of granite under pore pressure

Abstract

This research analyzes the strength and deformation characteristics of rock samples under varying pore pressures during the unloading process. By using fine-grained granite from Western China as the research exemplar, laboratory tests of triaxial unloading under pore pressure are conducted by utilizing the unloading confining pressures method without changing the axial pressure. Pore pressure is tested with a design value of 5 MPa. When the confining pressures are released at 35, 25, 15, and 5 MPa, the pore pressures drainage valve are opened, and the mechanical properties of fine-grained granite under pore pressures are analyzed. The following results are obtained. After unloading at different confining pressures, pore pressure has an evident influence on the stress–strain curve of the rock sample. The bearing capacity of the rock sample decreases. The hole pressure has a significant effect on the ring and volume deformation in the unloading process. Consequently, the evident deformation and damage are generated immediately after unloading to 25 MPa. Under the condition of constant pore pressure for a long time, numerous tension fractures are distributed around the main shear fracture owing to the promotion of pore pressure on the fracture opening. Hence, an evident swelling phenomenon occurs in the rock sample.