Dip effect of stress transfer and structural instability mechanism of coal-rock combination

The key to safe and efficient mining of steeply dipping coal seams is an effective control of surrounding rock. Revealing the dip effect on mining mechanical behavior of coal-rock combination are the basis for the multi-scale surrounding rock collaborative control in steeply dipping coal seams. According to mechanical characteristics of coal-rock combination such as stress transfer between layers, principal stress deflection and non-equilibrium deformation and failure at different inclination angles, the dip effect of principal stress distribution and deformation and failure law in coal-rock combination is analyzed by research methods of rock mechanics experiment, numerical simulation and theoretical analysis combined with feedback.The results show that: when the angle changes in the range of 0°–60°, the evolution characteristics of the size and direction of the principal stress of the coal-rock combination at the interface are divided into two categories; When the dip angle is less than the critical angle α₀, the first principal stress decreases with the increase of the dip angle, the third principal stress exhibits a monotonic evolution trend of “coal decrease + rock increase”, and the principal stress direction is a deflection state of “coal clockwise + rock counterclockwise”. When the angle is larger than the critical angle α₀, the first principal stress of the coal-rock combination increases with the increase of the angle, the variation of the third principal stress increases, and the principal stress direction is in the deflection state of “coal counterclockwise + rock clockwise”. When the coal-rock combination is affected by the dip angle, the mechanical characteristics of the two states are different, so that the combined failure mode of coal-rock combination changes from “the rock mass on the upper side of the interface + non-interface part of the coal body” to “the lower side of the interface + non-interface part of the coal body”. That is, corresponding to the transformation of the shear deformation failure law of the composite body to the slip failure law at the interface. Therefore, the strength of coal-rock combi-nation decreases with the increase of dip angle. The research results reveal the non-equilibrium transfer law of stress in coal-rock combination and the dip effect of its failure mechanism, which has certain theoretical reference significance for the safe and efficient mining of steeply dipping coal seams.