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.

卸荷条件下岩石的力学行为和变形特性对于分析因开挖引起的地下岩体失稳问题特别重要。在本文中，在初始约束压力和孔隙压力的不同组合下，对砂岩试样进行了三轴压缩试验，并逐步施加约束压力。从测试中可以看出，卸荷过程中初始围压和孔隙压力的大小会显着影响岩石强度，能量转换，裂纹扩展和膨胀率。卸载条件下的强度符合线性Mogi-Coulomb准则。能量转换方法可以用来确定卸载过程中的裂纹损伤阈值。对开裂过程的分析表明，裂纹损伤阈值随初始围压而线性增加，但随孔隙压力而降低。在卸载条件下，随着孔隙压力的增加，材料屈服和脆性破坏更容易发生。此外，在初始围压的情况下，较高的孔隙压力会加速样品的体积膨胀。