Experimental study on the energy characteristics of sandstone in direct shear test under a decreasing normal stress

Excavation and earth surface processes will result in unloading of rock stress, which may induce unloading failure. Rock failure processes accompany energy conversion, including energy accumulation, dissipation, and release. To investigate the characteristics of the energy of rock samples in direct shear tests under unloading normal stress conditions, a new laboratory direct shear test was proposed. Then, a method for calculating the strain energy of a rock sample under a direct shear test was put forward. Results show that elastic strain energy of a rock sample in the direct shear test exists in both normal and shear directions. Increasing shear stress in the direct shear test will induce a release of strain energy in normal direction. The released energy comes from the energy input by shear stress in a direct shear test under constant normal stress, while that comes from both the energy input by shear stress and the elastic strain energy in the normal direction in a direct shear test under unloading normal stress. When a rock sample is damaged, the work done by normal stress is less than zero if the initial shear stress is at a high level, while it is larger than zero if the initial shear stress is at a low level. Part of the work done by shear stress is first stored in the sample as elastic strain energy and then released in the normal direction. This part of energy (namely transformation strain energy) in the loading shear stress process increases gradually as the initial shear stress increases, while it decreases gradually in the unloading normal stress process. The total transformation strain energy has no obvious relation with the initial stresses but fluctuates within a range that can be used to determine whether the rock sample will fail. © 2021, Saudi Society for Geosciences.