Nonlinear Unloading Model for Hard Rock Under High Geo-Stress

Abstract

The strength feature of excavated rock mass has the nature of non-linearity under high geo-stress action. To simulate the brittle failure of underground engineering, a new mechanical model was put forward based on true triaxial unloading test. At first, triaxial unloading mechanical test for hard rock was performed under low, middle, very high confining stress. The axial failure stress of unloading test is quite different from that of loading test, the strength envelop of unloading is nonlinear and S-typed. It is an important discovery in the unloading test. Then based on the concept that brittle damage is also a progressive process, a new unloading model is presented to simulate the brittle failure of hard rock by using nonlinear Hoek-Brown failure criterion. The parameter m increases and s decreases in the unloading model, m and s are the functions of plastic strain. At last, the new unloading model was applied to Mine-by experimental tunnel, and the test result shows that the new model can reflect the brittle and tensile failure of rock mass. The case study proves the nonlinear brittle model is reasonable.