Formation mechanism of water and sand inrush channel in shallow buried bedrock face

The top seam of coal resources in the Shendong mining area is shallow,the coal seam is thick,and the mining intensity is large.The coexistence of thick loose aquifers and thin bedrock has led to some repeated occurrences of water and sand inrushes in the working face.In order to study the causes of water and sand inrushes in shallow buried thin bedrock,the occurrence mechanism of bedrock fissure conduction working face and water-containing loose layer caused by cutting-top compression frame of working face was studied by on-site measurement,theoretical analysis,similar simulation,etc.Overlying rock combination bearing structure model was built.Applicability and rationality of the “main control layer-weak layer” combined bearing structure model in the overburden breaking process of the shallow buried thin bedrock working face in Shendong Mining Area were discussed.Stability of overlying rock composite structure in shallow buried thin bedrock working face of Shendong Mining Area by combined bearing structure model was analyzed by combined bearing structure model,and the criteria for instability of overburden composite structures was deduced,the impact load formation conditions were revealed and the maximum impact load calculation method was given.In order to study the failure laws of overlying rock structures under different “base-to-load ratio” conditions,the similar simulation experiments with base load ratios of 0.625,0.75,0.875,1.125 were carried out.The experimental results shown the influence of base load ratio on the breaking step of the main rock layer:in the first four types of working conditions for the first time,the first break and fall occurred directly.When the base load ratio was greater than 0.875-1.150,the increase in the initial top step and the step thickness decrease.When the base load ratio was greater than 0.750-1.125,the initial stepping distance would increase,and the direct top fall thickness would decrease.When the base load ratio was increased from 0.625 to 1.125,the cycle step was significantly increased,and the overall step was significantly increased,the thickness of the periodic rock formation was significantly reduced comparing to the initial rock formation.The influence of base load ratio on the morphology of overburden structure:when the base load ratio was less than 0.625,the overlying rock would cut through to the ground during the first break.When the base load ratio was greater than 0.75,the block size of the fractured rock in the upper and middle rock formations was i=0.09-0.50,and some rock formations could form stable and simply supported structures,the lower overburden collapsed,the combined bearing structure appeared in the middle and upper part of the overburden.In the process of increasing the base load ratio from 0.625 to 1.125,a single-revolving rock block structure would be formed for the fractured rock blocks of the median rock formation with a block i>0.5,and the cut-off of the rock blocks at this level always exists.Fractured rock blocks in the upper strata with i<0.5 will form a simply supported beam structure,and the overall overburden would form a coordinated bearing structure,the lower overlying rocks collapse and the combined bearing structure appeared in middle and upper rock formations,the simulation results agreed with the theoretical models.The measured resistance of the bracket in the initial mining area of 22402 working face was between 5 000-6 000 kN on average,and obvious dynamic pressure occurred during incoming pressure.The top plate was cut off by pressure and the working face and the aquifer were connected by fracture channels.Therefore,it is possible to effectively prevent the occurrence of water and sand inrushes by increasing stent resistance.