Mechanism of deformation and pressure relief control of dynamic gob-side entry surroundings in fully-mechanized caving mining for extra-thick coal seam

Affected by the superposition of working face mining, strong rock pressure phenomena such as rock burst and large deformation easily occur in the gob-side entry when mining extra-thick coal seams. Taking the extra-thick coal seam mining of Caojiatan Coal Mine in Yushen Mining Area as the engineering background, the appearance mechanism of strong mine pressure in gob-side entry is analyzed and a surrounding rock control technology based on deep-hole directional tension blasting and pressure relief is proposed in our study. The effects of the technology are comprehensively studied by numerical simulation and field experiment. It was found that under the condition of fully-mechanized top-coal caving mining in extra-thick coal seam, the large space long cantilever structure of the gob roof breaks and forms a bench rock beam block with a large length. The instability of the long cantilever structure or the roof key layer will cause strong dynamic pressure, which acts on the coal pillar and further transmits to the surrounding rock of the gob-side entry in the advanced mining area, which is the main reason for the large deformation of the floor heave. Numerical study shows that when the directional roof cutting is performed in the gob-side entry, the surrounding rock stress is transferred away from the entry to both sides. The peak stress value is significantly reduced and the stress range is reduced. The engineering test under different roof cutting conditions was carried out in the field. It was found that the deep-hole roof cutting pressure relief technology based on directional tension blasting can effectively reduce the surrounding rock pressure of the gob-side entry at the end of the ultra-thick coal seam working face. The average load of fore support in the entry was reduced by 11% and the average deformation of floor in the serious section of the entry was reduced by 65% after roof cutting. The combined blasting is beneficial to further reduce the deformation of the entry surroundings and improves the stability of the entry. The research results provide an effective method for controlling the deformation of entry induced by strong mining pressure in ultra-thick coal seam mining.