Apparent-Depth Effects of the Dynamic Failure of Thick Hard Rock Strata on the Underlying Coal Mass During Underground Mining

Thick hard rock strata (THRS) are widely distributed among coal-bearing strata in China. The dynamic THRS failure may induce the coal-rock dynamic hazards during underground coal mining. In this study, the size, stress and energy characteristics of dynamic THRS failure were analyzed; the Yangliu coal mine, Huaibei coal basin, China, was taken as a case study. Then the apparent-depth effects of the dynamic THRS failure on the underlying coal mining were researched. The results revealed that the rock roof would be caved, fractured and deformed and even develop the bed separation during underlying coal mining. A large quantity of elastic energy is stored in the bent and compressed THRS, the amount and scope of the vertical stress concentration in front of the working face increases significantly before THRS breaking. THRS generally exhibit the square-form structure failure (SSF), after which the elastic and kinetic energy is attenuated in power form with the energy spreading distance. The shock force and energy effects of the dynamic THRS failure on the underlying coal mass were greater than those observed for the common lithological rock strata. Thus the mining coal mass below THRS bears similar mechanical conditions as the deeper coal mass and is of greater apparent depth, which is called the apparent-depth effects of the dynamic THRS failure. The apparent-depth effects of the dynamic THRS failure significantly influenced the coal-rock dynamic hazards according to in situ measurements of the mine earthquake energy, in situ stress, roof pressure and gas emission parameters. As a result, reasonable measures should be taken to avoid stress concentration and energy accumulation and to reduce the apparent depth of the mining-disturbed coal mass for preventing coal-rock dynamic hazards. The research results provide significant value for the control of coal-rock dynamic hazards below THRS.