Failure characteristics of Beishan granite under unloading confining pressures

To better understand the effects of unloading confining pressure on failure mechanism, several trial tests on loading axial stresses and unloading confining pressures with different unloading rates are performed, and acoustic emission events are recorded at the same time. In the experiment, five unloading confining pressure rates are set at 0.1, 0.5, 1.0, 1.5 MPa/min and 2.0 MPa/min, and the axial stress loading rate is 10 times the unloading confining pressure rate. Five confining pressures are set at 5, 10, 15, 20 and 30 MPa under every unloading rate. The variation laws of the acoustic emission events with the unloading rates are analyzed, the hierarchical clustering is used to divide the acoustic emission events into tensile failure and shear failure, and the effects of unloading rate, confining pressure, unloading time on the tensile and shear failures are investigated. The results show: (1) The acoustic emission decreases in the form of power function with the increase of the unloading rate, and the effects of the unloading rate on the acoustic emission are more obvious when the confining pressure increases; (2) The ratio of tensile to shear fractures increases with the increasing unloading rate, but it decreases with the increasing confining and unloading time; (3) b-values decrease with the increase of the confining pressure unloading rate and increase when the confining pressure increases, and the influences of the unloading rate and confining pressure on the b-values of the tension fractures are more obvious than those on b values of shear fractures; (4) The local stress state is analyzed by using the Maxwell model, and the mechanism of variation of tensile and shear fractures is further studied.