Study on damage of coal based on permeability and load-unload response ratio under tiered cyclic loading

The samples collected from Banxi coal mine in Chongqing, Southwest China, have been used to study on damage of coal based on permeability change rate (PCR) and load-unload response ratio (LURR) under cyclic loading by using thermal-hydrological-mechanical (THM) coupling experimental equipment. Research results show that (1) the LURR is applied to describe the samples response to cyclic loads based on loading/unloading elastic modulus. The LURR is larger at the initial cyclic, with the cyclic loading and unloading, it decreases sharply first, and then slowly decreases to 1 value, and then slowly rises away from 1 value. The increase of LURR after stabilization can be used as the precursory information to judge the failure and instability of coal and rock. (2) A new damage characterization method based on PCR can well characterize the three stages of coal-rock damage under cyclic loading, that is, first accelerates damage and then remains smooth, at last also accelerates damage. The evolution of damage variable based on PCR is similar to the general trend of damage variable based on LURR. As gas monitoring is a routine monitoring indicator for coal mines, so it is a practical, innovative, and economical method to characterize the damage variables based on PCR.