Experimental study on creep mechanical properties and failure modes of anthracite in dry and saturated states

Water is crucial to the durability and safety of underground engineering. To investigate the effect of long-term goaf water infiltration on creep characteristics and long-term stability of the No.3 anthracite pillar in the Qinshui coalfield in Shanxi Province, conventional uniaxial compression and uniaxial creep tests were conducted under dry and saturated conditions. In addition, based on the test results, a nonlinear damage viscoelastic-plastic creep model was developed to describe each creep stage in detail. The experimental results reveal the following. (1) When the loading stress exceeds the long-term strength, the anthracite creep in dry and saturated states undergoes decay, steady-state and accelerated creeps; (2) the total instantaneous elastic strain of the anthracite sample increased linearly with the stress level, and the growth rate of the saturated sample exceeded that of the dry sample; (3) the average of the long-term strength of anthracite under dry and saturated conditions is 7.604 MPa and 4.489 MPa respectively, and the corresponding long-term strength to instantaneous strength ratios are 0.73 and 0.68, respectively; (4) the developed nonlinear damage viscoelastic-plastic creep model can simultaneously describe the two-stage creep characteristics of anthracite under low stress and the three-stage creep characteristics under high stress; (5) the failure form of the dry coal sample after the creep test was a shear failure, whereas the failure form of the saturated coal sample was ductile owing to water softening. When anthracite was transformed from the dry to the saturated state, its instantaneous and long-term strengths decreased by ~ 37% and ~ 41%, respectively.