Movement of overburden with different mining thickness and response characteristics of surrounding rock under multi-key layer structure

In view of the problem of overburden movement and surrounding rock stress in complex thick coal working face under the key layer structure, the RFPA-strata numerical simulation method was used to study the fracture characteristics of key layers with different mining thickness and the response characteristics of support stress before and after the failure of different key layers under the multi-key layer structure. The results show that:① when the mining height is less than 2.5 m, only the lower key layer is broken and a stable masonry beam structure can be formed. At this time, the lower key layer can bear the weight of itself and the weight of the rock layer above and below the middle key layer.. The coal body only needs to bear the weight of the soft rock under the lower key layer and the additional load. The bearing capacity of the coal body is good, and the peak value of the advanced bearing stress increases gradually with the increase of mining height. ② JP+2When the mining height is greater than 3 m, the middle and lower key layers are broken, and the masonry beamJPLMstructure is formed after the middle key layer is broken. At this time, the middle key layer can bear the weight of itself and the soft rock above and below the high key layer; after the low key layer is broken, the broken rock block fails to articulate with the front incomplete broken rock layer, and the low key layer is a cantilever structure. At this time, the coal body needs to bear the weight and additional load of the rock layer above the coal seam to the middle key layer. A large number of shear failures occurs in the coal-rock mass in the front section, which leads to the reduction of the bearing capacity of the coal-rock mass, and the peak value of the advanced bearing stress gradually decreases with the increase of the mining height. ③ The movement of the key layer affects the distribution characteristics of the supporting stress. After the key layer is completely broken, the subsidence displacement of the lower key layer decreases. The peak value of the advanced bearing stress and the distance from the coal wall decrease with the break of the key layer. ④ When the mining height is greater than 3 m, the fracture of the low key layer mainly affects the distance between the peak point of the advanced bearing stress and the coal wall, and the peak value changes little; the fracture of the middle key layer mainly affects the peak value of the bearing stress, and the distance between the peak point and the coal wall changes are small.