Analysis of Crack-Characteristic Stress and Energy Characteristics of Sandstone under Triaxial Unloading Confining Pressure

The deformation and failure of underground engineering are usually caused by unloading. In this work, triaxial unloading confining pressure tests carried out to simulate the process rock mass unloading, analyze crack-characteristic stress, study energy evolution under pre-peak post-peak characteristics combined with theory. results show that, when increases from 5 MPa 20 MPa, crack closure stress σcc, initiation σci, dilatancy σcd, peak σp 6.34 times, 2.75 1.93 1.66 times higher than original, respectively. By comparing increase in it can be found that has a large effect on while dilatation have relatively little influence. From perspective evolution, axial absorption U1 exponentially, elastic Ue is similar U1, circumferential consumption U3 dissipation Ud small. After reaching growth rate decreases slightly, rapidly, rapidly but only as small fraction total energy, grows almost exponentially becomes main part energy. Under each state, characteristic parameters gradually pressure, which manifested slope linear fitting formula parameters. release releasable after divided into three stages “slow–fast–slow”, shows an obvious effect.