Effect on Top-Coal Mass Failure under Load–Unload Induced by Shield Support

Prior to being released, the top coal of a fully mechanized caving face typically experiences the effects of mining-induced pressure and disturbances from the hydraulic support’s canopy. To investigate the promoting effect of canopy disturbances in the support-controlled area on the damage of top coal during fully mechanized top-coal caving mining, block instability theory and discrete block numerical simulation methods were employed. The instability modes during the initial support and cyclic disturbance stages were analyzed. The fracture evolution and displacement distribution curves were studied for coal bodies with strengths of 3.1 MPa, 15.0 MPa, and 29.5 MPa under the mining influence and support strengths of 0.5 MPa, 1.0 MPa, and 1.5 MPa during the initial support and cyclic disturbance processes. The results showed that during the initial support stage, the instability of the block-structured coal body exhibited four modes as follows: inter-block breakage, inter-block delamination, intra-block fracture expansion, and inter-block sliding. During the cyclic disturbance stage, the coal body instability showed three modes as follows: inter-block sliding, intra-block fracture expansion, and inter-block rotation. The disturbance from the support had little impact on the fragmentation of both low-strength and high-strength coal bodies. However, small support forces combined with multiple cycles of disturbance had an enhancing effect on the instability of medium-strength block-structured top coal.