Bending Deformation Characteristics Of Floor Rock Under The Double Action Of "increasing Load And Weakening" Of Water Absorption

Coal mining under goaf is a common phenomenon,and most goafs contain water.At present,the influence of goaf water on the mining of the lower coal seam and the influence of the mining of the lower coal seam on the goaf water only consider the load of the upper coal seam caving belt and the damage of the goaf floor.In fact,the occurrence of water in goaf and the weakening of mechanical properties of floor after water absorption will affect the safety of coal mining,which is not involved in the current research.Different lithology,fragmentation and water absorption of goaf will affect the load of goaf floor.At the same time,there are a large number of broken coal and rock mass in the old goaf.When the water in the goaf is absorbed,the load on the floor rock layer in the goaf increases significantly.In addition,the water absorption of the floor rock layer leads to the double effects of water absorption,load increase and mechanical property weakening,which aggravates the bending deformation of the floor rock layer and affects the safety production of the lower coal seam.In this paper,the characteristics of water absorption and load increase of broken coal and rock in goaf,the experimental study of water absorption and load increase on the bending deformation of floor rock mass,and the experimental study of water absorption and load increase and mechanical property weakening of floor rock mass are carried out.In this paper,the goaf of the upper group of coal mine is taken as the research object.Through the indoor water absorption test,theoretical analysis,mechanical strength test and simulation,the problem is studied and the following research results are obtained:(1)Two groups of tests were designed to analyze the relationship between the water absorption and load increase and the bending deformation of the floor rock mass.In the first group,there is no ponding in the goaf,and the accumulated material exists in the goaf.In the test,when the simulation floor is laid with the non accumulated water powder coal,the change characteristics of the bending curvature are obtained,the change laws are analyzed,and the factors affecting the bending characteristics are analyzed.In the second group,the coal slurry water mixture formed after wetting and capillary water absorption is saturated and absorbed by the coal powder accumulated in the goaf.According to this characteristic,the design test is carried out to analyze the change characteristics of bending curvature when the saturated water absorption coal powder is laid on the simulated floor,analyze its change rules,and analyze the factors affecting its bending characteristics.(2)Through the test,the relationship between the water absorption and the bending deformation of the floor rock mass is analyzed,and two groups of tests are designed for analysis.In the first group,there is no ponding in the goaf,and the accumulated material exists in the goaf.In the test,when the simulation floor is laid with the non accumulated water powder coal,the change characteristics of the bending curvature are obtained,the change laws are analyzed,and the factors affecting the bending characteristics are analyzed.In the second group,the coal slurry water mixture formed after wetting and capillary water absorption is saturated and absorbed by the coal powder accumulated in the goaf.According to this characteristic,the design test is carried out to analyze the change characteristics of bending curvature when the saturated water absorption coal powder is laid on the simulated floor,analyze its change rules,and analyze the factors affecting its bending characteristics.(3)According to the mechanical test criteria,the standard coal and rock samples are selected and prepared,and the tensile strength and compressive strength of the samples are measured through the indoor mechanical test.At the same time,taking a group of standard samples of saturated water absorption to test the mechanical strength,it is found that the mechanical strength of mudstone and sandstone after saturated water absorption is reduced to different degrees,the compressive strength of mudstone is reduced by 31.4%,the tensile strength is reduced by 24%,and the compressive strength of sandstone is reduced by 51.9%,and the tensile strength is reduced by 61.3%.(4)According to the measured experimental data,the simulation is carried out,and the absolute value of bending deflection is used to represent the bending deformation.Two sets of simulation with different parameters are established,and two common parameters of mudstone and sandstone are used to establish the comparative test.The simulation results show that the absolute value of the bending deflection of the floor mudstone and sandstone increases by 124.1% and 49.74% respectively under the action of "increasing load weakening" of water absorption and the action of water absorption of coal and rock in the upper goaf.The research results have important practical significance for the safe mining of the coal seam under the water logged goaf.