Analytical theoretical study on ultra-low friction characteristics of coal rock interfaces under stress wave action

Under the influence of geological structure, excavation surface cutting, high stress, and strong mining, deep coal and rock mostly have obvious block-like structural characteristics. When subjected to dynamic stress waves such as roof fracture, these block-like coal rock interfaces are highly likely to induce an ultra-low friction sliding phenomena, leading to slip-type coal burst and other dynamic disasters. In order to explore the ultra-low friction characteristics of coal rock interfaces under the action of stress waves, an analytical theoretical model for the interaction between stress wave propagation and coal rock interfaces was derived based on the theory of stress wave propagation. In the analytical model, the Bandis-Barton model and Coulomb-slip model were used to characterize the normal and tangential behaviors of the coal rock interfaces, respectively. The reliability of the analytical model was verified by comparing its calculation results with existing theoretical models and the same UDEC model. The influence parameters such as the impedance ratio of coal rock layers, coal seam height, and the incident angle of stress waves on the ultra-low friction characteristics of coal rock interfaces under the action of stress waves was discussed. The research results indicate that as the impedance ratio of coal rock layers increases, the change range of relative normal displacement of coal rock interfaces becomes larger, and the coal rock interfaces are more prone to an ultra-low friction or lower friction. The higher the height of the coal seam, the smaller the overall changes in the relative normal displacement and relative tangential displacement of the coal rock interfaces, and the less likely the coal rock interfaces is to induce an ultra-low friction. With the increases of the incident angle of stress waves (10° ～ 40°), the change range of relative normal displacement of the coal rock interfaces decreases, but the tangential stress component of the coal rock interfaces increases. Without horizontal thrust, the possibility of ultra-low friction sliding on the coal rock interfaces is greater as the increase of incident angle of stress waves.