Dynamic response and failure characteristics of combined rocks under confining pressure

Gas explosions or coal and gas outbursts can cause transient destruction of combined coal-rock, and the dynamic mechanical response of combined coal-rock masses plays a key role in accident failure, but we now know little about the dynamic mechanical responses of combined coal-rock. In this article, we selected three rocks (limestone, shale, sandstone) and two coals (bituminous coal and anthracite coal) to form combined coal-rock, and analyze their dynamic mechanical properties by using the SHPB system. We find that the dynamic compressive strength and elastic modulus of combined rock-coal are lower than the average value of single rock and coal, while the ultimate strain and strain rate of combined coal-rock are higher than the average values of single rock and coal. Compressive strength and elastic modulus of the combined body increase gradually with increasing confining pressure, and the strain decreases accordingly. The dynamic stress-strain curve demonstrates an obvious double-peak at high strain rate (85.55 s^-1 and above in the present work), while there is no obvious double-peak of the curve at low strain rate. Dynamic compressive strength of combined coal-rock body increases significantly with increasing confining pressure at low strain rate, but it increases more smoothly at higher strain rate. The elastic modulus also increases with increasing confining pressure, and it seems to be stable as confining pressure increases at low strain rate. The ultimate strain decreases gradually with increasing confining pressure but more gently compared with that at low strain rate. Besides, longitudinal fractures of combined coal-rock bodies include penetrating fractures, partially penetrating fractures, and interrupted fractures stopped at the coal-rock interface. The dynamic mechanical response of combined coal-rock is of guiding significance for maintaining the stability of the roadway and formulating the support measures for the roadway.