DYNAMIC EVOLUTION CHARACTERISTICS AND MECHANISM OF SURROUNDING ROCK FRACTURES DURING THE REPEATED MINING OF CLOSED DISTANCE DEEP COAL SEAM

The disturbance mechanics behavior and mechanics mechanism of repeated mining of coal scam group arc the important theoretical basis for safe mining of deep coal scam group. For the characteristics of interaction among coal seams under the condition of combined mining of coal seam group, in this paper, the spatio-temporal evolution characteristics of 3D mining stress field and the law of strata activity in repeated mining of deep coal seam group were studied by using similar material simulation and numerical simulation methods. The results showed that during the first mining, overburden mining broke the initial stress balance; part of the surrounding rock stress was released and free space was formed; rock mass moved to the direction of goaf, which released the pressure and protected the rock mass; a caving arch was formed in the overburden strata of coal seam whose arch trace was composed of the boundary line of separated strata and the fracture line of rock beam. As the working face continued to advance, the exposed strata in the arch gradually transferred and expanded to the upper strata, and the separated strata also gradually propagated upward, thus forming the periodic failure process of the caving arch. During repeated mining, the upper coal seam underwent the support and compaction process of caving, natural filling, the integrity of the main roof of the underlying coal seam was destroyed, all the interlayer strata collapsed or broke, and were connected with the goaf of the upper coal scam working face, resulting in the fractured rock mass of the goaf directly transferring load downward, and then the periodic weighting of mining was relaxed. The results have certain reference value for the combined mining of coal seams under similar conditions.