Experimental study on the influence of lithology and rock-coal height ratio on mechanical properties and impact effect of combined body

Uniaxial compression experiments were carried out on a combination of 12 different lithology and different coal-rock height ratios to study the effects of lithology and rock-coal height ratio on the mechanical properties and impact effects of the composite. The mechanical constitutive models of coal, rock-coal-rock combination were used to analyze the instability process of the combined body and the evolution of physical parameters such as displacement, acceleration, stiffness, energy of various parts during the loading process. The criterion for the stability and instability of the combined body with the stiffness of post-peak of coal and the stiffness of rock unloading as the basic parameter was proposed. The results show that with the decrease of rock-coal height ratio, the failure types of the combined body are clastic complete failure, Y-type semi-complete failure, local incomplete failure (clastic, Y-type and local refer to the failure form of the specimens, complete failure, semi-complete failure, and incomplete failure refer to the failure degree of the specimens), and the compressive strength and elastic modulus of the combined body are reduced, and the impact energy index of the combined body is increased; the harder the rock components in the combined body are, the greater the compressive strength, elastic modulus and impact energy index of the combined body are; the failure state of the combined body is determined by the post-peak stiffness lambda of the coal, and the contrast relationship between the stiffness and the rock stiffnesses k(1), k(2) of the roof and floor. After the peak is unloaded to the point S where the change rate of the stiffness of the coal curve meets , the combined body changes from stable failure to unstable failure. The release of the roof and floor during the failure of the combined body is proposed. The ratio alpha of the energy released by the roof and floor to the coal consumption energy is used as an evaluation index of outburst proneness under coal and rock combination, and the criterion is proposed.