Experimental study on triaxial creep characteristics of the anthracite coal under low confining pressure

The creep characteristics of coal pillars left over from old mining directly affect the long-term stability of underground engineering. To study the creep characteristics and long-term stability of the No. 3 anthracite pillar in Qinshui Coal Field, Shanxi Province, China, conventional triaxial compression and triaxial creep tests were conducted under four low confining pressures (0.5, 1.0, 1.5 and 2.0 MPa), and the creep laws of the anthracite samples under different confining pressures were explored. Moreover, based on the test data, a nonlinear viscoelastic-plastic creep model was established which could better describe each creep stage. The test results show that: 1) when the axial stress is higher than the yield stress, the creep of anthracite undergoes three stages: initial creep, steady creep, and accelerated creep; 2) the instantaneous elastic strain of anthracite increases linearly with the axial stress level, and the smaller the confining pressure was, the faster the instantaneous elastic strain increased; 3) Compared with the instantaneous strength, the long-term strength of anthracite decreased by approximately 20%. The creep properties reduced the shear strength of anthracite, and compared with the short-term shear strength, the internal friction angle and cohesion decreased by 8% and 14%, respectively.