LAUSR

Mechanical behavior and deformation characteristics of rock under unloading conditions are of particular significance for analyzing excavation-induced instability problems of underground rock masses. In this paper, triaxial compression tests with progressive unloading of confining pressure were conducted upon sandstone specimens under different combinations of initial confining pressures and pore pressures. It is shown from the tests that the magnitude of initial confining pressure and pore pressure influences significantly the rock strength, energy conversion, crack propagation, and dilatancy during unloading process. The strength under unloading conditions is in compliance with the linear Mogi-Coulomb criterion. The energy conversion method can be employed to determine the crack damage thresholds during the unloading process. The analysis on cracking process suggests that crack damage thresholds increase linearly with initial confining pressure, but decrease with pore pressure. Under unloading conditions, material yield and brittle failure occur more easily with the increase in pore pressure. Furthermore, given the initial confining pressure, a higher level of pore pressure can accelerate volumetric expansion in the specimens.