Research on top-coal caving mechanism and control technology in extra-thick coal seam with hard dirt band

In order to solve the problems of poor top-coal caving and low recovery rate in fully-mechanized top-coal caving with with hard dirt band (sandwich)，the research methods by using theoretical analysis，numerical simulation and field survey were used to establish a uniformly loaded thin plate mechanical model for sandwich breakage.The numerical simulation compares and analyzes the influence of different intercalated layers on the crushing mechanism and caving behavior of top-coal.Meanwhile，the LS-DYNA3D explicit dynamic analysis program was used to simulate the effect of blasting under different charging methods and different blast hole positions.On this basis，measures such as top-coal’s deep hole pre-splitting blasting and double interval caving process were put forward.The results indicate that the tensile fracture area was located at 3～5 m posterior to the coal wall in the middle of the coal face，and the shear fracture area was located at the face end.The influence degree of dirt band to the top-coal’s cavibility is positively correlated with the number of layers.The blasting effect is better in decoupling charge and when the blastholes’ position is inside the dirt band，top-coal recovery rate is increased to over 85% after implementing top-coal’s weakening measures and optimizing the mining process.The research results have guiding significance for guiding the production practice of fully-mechanized top-coal caving in extra-thick coal seams and improving the top-coal discharge rate.