Investigation On Mechanical Fractured Characteristics And Water Outburst Risk Of Mining Floor Above Confined Aquifer

The mining of coal resources in China is seriously threatened by confined aquifer.In order to inveatigate the floor stress redistribution,mining failure characteristics and water-inrush risk caused by coal seam mining above confined aquifer,theoretical analysis,numerical simulation,in-situ measurement and similar simulation test were used to carry out relevant research contents.The main results are as follows:(1)Considering the interaction of mining-induced and hydraulic pressure,the theorically mechanical models of minig floor were established to accurately calculate the stress distribution characteristics and damage range.The fractured zones of the floor along the strike and inclination are approximately to"spoon-shaped" and "inverted saddle-shaped",and the maximum fractured depths are 13.5 m and 12 m,respectively.The maximum concentrated area of vertical stress occurs near the elastic-plastic boundary,and the concentration and influence range are obviously larger than that of horizontal stress.The shear stress is inversely symmetrical and spiral,forming a"positive-negative" sharp stress change zone and easy to cause compression-shear or tension-shear failure of the floor.(2)The coupled hydromechanical numerical modelling of mining above confined aquifer with FLAC3D was constructed to investigate the mining-induced fractured zone and stress redistribution of floor strata.The in-situ test was carried out by using the"double-end plugging leak detection device".The profile of failure zone obtained by numerical simulation closely resembles the theorical model,and the failure boundary is shear failure mode,which is consistent with the theoretical calculation.The maximum failure depths of numerical simulation and field measurement are 15.875 m and 13.75 m,respectively,which are approximately equal to the theoretical value of 13.5 m.After mining,pore water is progressively elevated along the floor,and water inrushes are prone to occur along the oblique lower side of the mining face and start-cutting.(3)Considering the synergistic action of stress reduction zone,mining failure zone load and hydraulic pressure,etc.,the mechanical analysis model of floor key water-proof stratum was established to investigate its stability and water-inrush risk.The key aquifer is easily destroyed in advance from the intersection of the maximum bending straight line with the boundary(long side)of the coal wall side and the two sides(short side)of the working face respectively.Water inrush occurs preferentially from the rock mass below the coal wall side along the working face strike,which is consistent with the results obtained by numerical simulation.(4)Similar simulation test of water inrush from mining floor was carried out to obtain the stress,displacement,crack propagation and seepage evolution.Shear cracks and vertical cracks occur in the open-cut hole and the coal wall side floor,accompanied by layered cracks and micro-cracks.With the increase of depth,mining disturbance weakens and the propagation of disturbance lags behind.The pore seepage mainly forms along the oblique lower part of the coal wall and inflows into the working face,forming water inrush phenomenon.