Movement Law Of The Overlying Strata Above Large Mining Height Longwall Face-end With Shallow Cover And Its Effect On Ground Deformation

A large number of mining-induced damage problems caused by underground coal mining,such as underground dynamic disasters,water and coal bed gas resources destruction and surface subsidence,are closely connected with mining-induced strata movement and mining-induced fractures.These mining damage problems are extremely serious in large mining height longwall face with shallow cover.After the finish of coal mining,a large number of bed separation and fractures are kept below the overlying strata broken blocks above the longwall face-end,and the ground deformation above the longwall face-end is larger than that above the center of longwall face.Therefore,investigating the movement law of the overlying strata obove large mining height longwall face-end with shallow cover and its effect on ground deformation has important theoretical significance and practical guiding value on the research of underground water inrush,gas extraction and mitigating the influence of mining-induced ground deformation on surface structures.With above-mentioned issues in mind,field measurement,physical simulation,theoretical analysis and numerical simulation methods were conducted to investigate the movement law of the overlying strata obove large mining height longwall face-end with shallow cover and its effect on ground deformation.Borehole in-situ test that conducted in the No.52306 longwall face,Daliuta coal mine,shows that the key strata above the longwall working face-end still play the controlling role in the overlying strata breakage.The bed separation below the overlying key strata broken blocks of longwall face-end can always keep a large amount of separation with the mining process,rather than being closed,which is demonstrated by the similar simulation.Field observation result of the surface subsidence shows that the surface subsidence factor in No.52306 longwall face is 0.53.The section coal pillar near the goaf fails gradually in the course of mining,changing the breaking and movement law of the overlying strata above the longwall face-end.This further increases the influence range of surface deformation,but mitigates the deformation degree of ground surface.The failure process of the overlying strata in the form of slab over the longwall face is analyzed theoretically,and the fragmentation degree of the curved triangular block above the longwall face-end is obtained.The mathematical expression of the turning angle of the curved triangular block and that of the bed separation below the curved triangular are deduced.The numerical simulation results show that when the mining height of the longwall face increases,the subsidence and the turning degree of the primary key stratum,and the bed separation below the triangular block also increase simultaneously,and the surface deformation degree will also increase.The numerical simulation also shows that if the section coal pillar fails,the overall subsidence of the overlying trianglular block will increase,but its turning angle decrease,and the bed separation space below the block will also reduce,thereby reducing the maximum inclination of the surface trough.When the distance of the key stratum to the coal seam remain unchanged,the increase of the thickness of the unconsolidated layers will increases the total subsidence of overlying strata,thereby increasing the final surface subsidence and its deformation degree eventually.The field measurement results are in good agreement with the simulation results.The above research related to the movement law of the overlying strata obove large mining height longwall face-end with shallow cover and its effect on ground deformation,can provide significant guidance for the safe and high-efficient mining in extracting the shallow cover coal seam with large mining height,and for reducing mining-induced damage.The research outcome can further enrich the strata movement,mining-induced subsidence and mining-induced fractures,etc.