Failure Analysis of Thin Bedrock and Clay Roof in Underground Coal Mining: Case Study in Longdong Coal Mine

It was found that movement in the overburden after mining was complicated under the condition of thin bedrock and clay roof (TBCR). In order to explain the small-big periodic weighting phenomenon occurring on the 7,128 working face of the Longdong coal mine, a mechanical model was established. This model is based on voussoir beams (VB) and cantilever beams and considers the bearing capacity of deep-buried clay layer that has poor fluidity and good consolidation. The TBCR model's motion rule and failure mechanism were obtained through elastoplastic mechanics calculation and force analysis and verified by field data. The movement of the bedrock and clay layer in the model are not synchronized but are divided into two stages: the primary stage and the interaction stage. The fracture length of the clay layer is two to three times that of the bedrock, which prevented the TBCR from moving as a whole. The interaction between the bedrock and clay layer allowed the VB structure formed by the fallen bedrock to slide easily, thus causing shear failure of the clay layer. The numerical simulation performed using the discrete-element software 3DEC proved that the abutment pressure had a small-big period weighting phenomenon, and the stress concentration factor of the coal seam mining under TBCR conditions was greater than that of the thick bedrock.