Study on Ultimate Strength and Dip Effect of Coal Pillar in Steeply Dipping Coal Seam

The stability of the coal pillar is the basis of safe and efficient mining in the longwall working face of steeply dipping coal seams. Based on a systematic analysis of the deformation and instability of the surrounding rock in the quarry of a steeply dipping coal seam, the structural model of coal pillar for multi-section mining of steeply dipping coal seams is established. The stress distribution of an inclined coal pillar considering the action of shear stress is studied, and a generalized Mohr circle characterizing the stress state of the inclined coal pillar is provided. According to the planar geometric relationship between the stress circle and strength envelope, the calculation equation of strength with an implied inclination angle is proposed, which can help solve the coal pillar safety factor. The influence of coal seam inclination angle, lateral pressure coefficient, width-to-height ratio and other factors on the strength of inclined coal pillar is discussed. The reliability of the theoretical analysis has been tested using on-site engineering examples. The difficulties in controlling the surrounding rocks of the section return airway for inclined coal seam mining are analyzed by using the engineering case in Pansidong coal mine as an example, and the asymmetric surrounding rock coupling support is proven to be a successful support strengthening technology. The research results can provide a reference for the coal pillar strength estimation in steeply dipping coal seams and the control design of surrounding rocks in similar working faces.