Longwall chock sudden closure incident below coal pillar of adjacent upper mined coal seam under shallow cover in the Shendong coalfield

Abstract The Shendong coalfield is characterized with shallow cover depth and multi coal seams that are closely distributed. A number of longwall (LW) faces are in operation in the second coal seam. Due to the geological disturbances and different deployments of LW panels, a large number of coal pillars were left in the overlying extracted coal seam. Massive LW chock support failure incidents (e.g. sudden closure and hydraulic cylinder damage) occurred when the LW face of the lower coal seam cut across below the coal pillars of the upper coal seam. Through conducting stability analysis of key blocks above coal pillars, the mechanism involved was revealed and the roles that two key factors (depth of cover of the overlying coal pillar, and inter-burden thickness) played in the process were investigated. It is believed that the rotation and motion of the key blocks of roof strata above coal pillar have the potential to make the overlying stress concentrated at coal pillars. The high compressive stress is very likely to be transferred to the inter-burden and LW chocks. Hence redirecting the stress and/or reducing the capability of stress transfer of coal pillars become the primary consideration for developing a solution. A method of pre-advancing a roadway through the upper coal pillar was proposed to induce it to fail prior to LW face exiting the coal pillar. The proposed solution was subsequently applied to LW22303 of the Bulianta coal mine, Shendong coalfield, and achieved substantial benefits.