Evolution characteristics of mining stress of bearing arch and interval strata in longwall mining of steeply dipping coal seam groups

During the mining process of steeply dipping coal seam groups, the evolution law of mining stress in the interval state is extremely complicated due to the influence of repeated mining. Revealing the mining stress transfer path and three-dimensional stress state evolution characteristics of interval strata are the cornerstones for realizing safe and efficient mining of coal seam groups. In this paper, on the basis of using the physical simulation experiment to study the temporal and spatial evolution law of the deformation and failure of the surrounding rock in the mining of steeply dipping coal seams, the numerical simulation is used to analyze the mining stress transfer evolution law. The evolution characteristics of the three-dimensional stress state of the interval strata are revealed, and the evolution law of the gradual change in magnitude and direction deflection of the first principal stress is quantitatively characterized. The results show that during the mining of steeply dipping coal seam groups, the interval strata experienced the disturbance process of “original stress-pressure relief of upper mining-unbalanced constraint of gangue-pressure relief of lower mining”,and finally produced asymmetric deformation and failure. The instability of interval state will cause the bearing arches formed during the mining process of the upper and lower working faces to evolve into a “large range” of bearing arch that envelop the two working faces. The upper arch leg of the bearing arch is located in the tailgate of the lower working face, and the lower arch leg is located in the headgate of the upper working face. The “large range” stress transfer arch which controls the deformation and fracture of strata also presents similar evolution characteristics. The influence of mining on the low interval strata is severe and the stress release is large; the position where the median strata pressure and tensile state change is shifted to the lower part along the dip; the position where the pressure and tension state of high strata changes is located in the hallow surface, and the strata change from three-directional pressure to one or two-directional pressure state. The interval strata can be divided into five regions from bottom to top along the inclination of the working face: the upper coal pressurization, the upper coal pressurization-the lower coal unloading, the repeated unloading, the upper coal unloading-the lower coal pressurization, and the lower coal pressurization. In the upper coal unloading-lower coal pressurization section, the x and y axis components of the first principal stress show a decreasing-increasing-decreasing trend, and the z axis component shows a decreasing-increasing trend. In the repeated unloading section, the stress release and the direction deflection is largest. In the upper coal pressurization-lower coal unloading section, the first principal stress x and y axis components show an evolutionary trend of increase-decrease, and the z axis component continues to decrease.