Abstract: Mechanical properties of rocks are significantly affected by various initial unloading levels, which are caused by deep rock excavation. In this study, conventional triaxial compression tests and triaxial unloading tests at different initial unloading levels were carried out on sandstone. The strength, deformation and dilatancy characteristics of rock were studied under loading and unloading of high stress. The testing results showed that with the same initial confining pressure, the higher the initial unloading level was, the lower the confining pressure was reduced, and the shorter the process of unloading confining pressure was. Under different initial unloading levels, the uniform decreasing increment of confining pressure increased with the increase of initial confining pressure. It indicates that under high-stress state, rock specimens can only be damaged when much greater unloading confining pressure was applied. With the increase of initial unloading levels, the cohesion c increased exponentially, whereas internal friction angle decreased in a polynomial relationship. The lateral strain increment was higher than that of axial strain increment in both two stages of unloading confining pressure, which exhibits an obvious lateral expansion. It was found that when the initial unloading level was lower, the maximum dilatancy angle was larger and the dilatancy effect was more apparent. The dilatancy index showed a power function relationship with the confining pressure, whereas it showed a quadratic function relationship with the initial unloading level. Moreover, the expression of Mohr-Coulomb yielding criterion for sandstone was modified under different initial unloading levels, based on the obtained experimental results.

Key words: rock mechanics, unloading confining pressure test, initial unloading levels, dilatancy angle, expansion index