Abnormal Subsidence Mechanism Of Caving Mining Face With Thick Unconsolidated Layers And Collapse Columns

Coal still dominates China’s primary energy consumption structure,and surface subsidence area ranks the first worldwide.The mechanism and law of stratum movement deformation has been a hot topic for scholars at home and abroad.The surface subsidence laws vary from changeable geological and mining conditions of different mining areas.The empirical analogy method often results in significant deviations under specific geological and mining conditions.In this paper,the problem of surface anomaly subsidence under the conditions of thick unconsolidated layer,large mining height and mining collapse column in Huoerxinhe coal mine is combined with mining by theoretical analysis,mechanics calculation,similar simulation,numerical simulation and field measurement.The theoretical research methods of geology,mine pressure,mining subsidence and soil mechanics,etc.,studied the influencing factors of surface subsidence of 3205 working face in Huoerxinhe coal mine.Based on the mechanism of overburden failure and unconsolidated layer cohesive deformation,three kinds of surface subsidence models are constructed and the corresponding applicable conditions and characteristics are given.The "four belts" mechanical model of the collapse column is constructed and the fracture under the condition of mining through the collapse column is studied.Field,stress field and its influence on surface subsidence;through mechanical analysis and simulation experiments,the mechanism and law of soil layer affecting surface subsidence are studied.The main research contents and conclusions of the thesis are as follows:（1）Characteristics and main controlling factors of surface subsidence areaIn the direction of advancement of the 3205 working face,the ultra-full mining is achieved,and in the direction of inclination,it is not a full mining extraction.After the actual measurement,the area of the sinking basin is larger than the expected result,and the horizontal deformation value is larger than the expected result.The sinking value is smaller than the expected result,and the local area sinks a large amount.The large thickness of the unconsolidated layer,high-strength mining（large mining height and fully mechanized caving）and the mining through the collapse column are the main controlling factors for the surface subsidence.（2）Formation subsidence morphology based on "rock mining ratio" and "soil-rock ratio"For nearly flat coal seams,thickness,bedrock thickness and in-situ stress（mainly horizontal principal stress）affect the movement deformation of bedrock into "saddle type"or "inverted funnel shape".It is proposed that the larger the rock mining ratio and the ratio of soil-rock are,the shifting deformation mode of bedrock tends to "saddle type".The larger the ratio of rock mining and the ratio of soil rock are,the moving deformation mode of bedrock tends to "inverted funnel type".（3）For the thick unconsolidated layer,three kinds of surface subsidence models are constructed and their characteristics are analyzed.Based on the ratio of rock mining ratio λ,the mining subsidence of thick unconsolidated layer is divided into three models,namely "small-big funnel model","hyperbolic model" and "Y model".The expressions of surface subsidence range D of the three models are as follows:1 λ>a/（bM+c）meets the "small-big funnel model":D=2[cotθH基+cot(45°+ψ/2(H±]+d2入≤ a/（bM+c）in line with "hyperbolic":D=2a(（1+H²/b²）^1/2)3λ≤<a/（bM+c）and the shear resistance of the bedrock is in line with the Y model:D=2 cot（45°+ψ/2）H±+dThe mining subsidence of the 3205 working face in Huoerxinhe coal mine is in accordance with the "small-big funnel model",and the surface subsidence range D=（214.4+d）m is calculated,which is about 308.8m larger than the surface subsidence range calculated by the "hyperbolic model".（4）The mechanical model of the "four belts" of the collapse column is constructed,considering the geologic characteristics of the collapse column and its surrounding rock.The affected area of the collapse column is divided into the column low stress zone,the column edge stress reduction zone,the peripheral stress rise zone and the original rock stress stabilization zone in the horizontal direction.When the working face mines through the collapse column,the front support pressure of the working face is coupled with the"four belts" stress of the collapse column to form a superposed abutment pressure.The evolution of the support pressure state of the working face of the collapsed column can be divided into five stages:coupling generation,coupling enhancement,column absorption,secondary coupling and coupling dispersion.A high pressure bearing area will appear in the working face before and after mining though the column.Due to the stress peak property and the coupling path difference,the high value of the abutment pressure is not symmetrically distributed around the collapse column.（5）Based on the formation characteristics of the collapse column and the geological characteristics of the surrounding rock,the collapse column surrounding rock fracture model is constructed.The column and the surrounding rock are divided into the column cluttering area,the column edge fracture opening area,the peripheral fracture development area and the original rock stable area from the inside to the outside.Based on the collapse column surrounding rock fracture model and the mining fracture "O" ring theory,the coupling law of the overburden fracture field is analyzed.There are mining fissures and surrounding rock fractures in the overburden.The mining fractures are dominated by the fractures in the O-ring and the fractures in the lower part of the re-compaction.The coupling of the fracture under the influence of mining is accompanied by new changes in the fracture of the surrounding rock of the collapse column.The combined effect of the two causes the failure of the overburden to present new features.（6）Similar simulation and numerical simulation of surrounding rock stress,fracture and surface subsidence characteristics of collapsed columnThe simulation results show that the overburden of the working face of the collapsed column is concentrated and destroyed in the surrounding rock of the column.The working face is 60m to 20m before the collapse column and 40m after the collapse column belongs to the high bearing pressure zone.The surrounding rock of the column top can produce transverse delamination fractures and longitudinal fractures due to tensile failure,and they can penetrate each other.The deviation of the surrounding rock and the column makes the rock layer sink easily,and under the vertical tensile stress,the new crack is mainly caused by the transverse delamination fractures.Longitudinal fractures are mostly formed during the period of mining through the collapse column and concentrated in the area closer to the stope,due to the erosion of the shear zone of the working face and the full rotation of the formation.Two types of new fractures can penetrate the fractures of the two large fracture fields.The research shows that there is a large deformation subsidence at the top of the collapse column after the mining,which can partly affect the surface movement deformation.（7）The fluid-solid coupling model of sand layer and clay layer under the influence of mining is established.v=2qx/Eb（1-3x/b）y-γnsinθ/2Ey2+CAmong them,γ_n is the gravity of the clay,and C is the constant obtained by the integral.After the mining,the sand layer is bounded by the boundary of the goaf to the surface projection point,and is divided into a stretching section and a compression section.Under the two physical forces of gravity and seepage force,it is relatively dislocated with the overlying clay layer.When the hydraulic gradient is not reached,the sand layer is linearly displaced in the sand layer along the thickness direction,and the upper and lower displacement values of the sand layer reach the maximum bq/E;when the hydraulic force is forced to fall,the soil movement phenomenon occurs,and the maximum movement value is 4q/Ey.（8）Aquifer sand layer mining activation mechanismA special test device was developed to study the law of the movement of the upper clay layer after the shallow sand was affected by the mining subsidence and the slope.The effects of natural angle of repose and critical hydraulic gradient on the fluidity of saturated sand and clay were studied.The effects of sand particle size,clay layer thickness and model inclination on the uplift and horizontal displacement of the clay layer were studied.It indicates that the sand layer can locally increase the surface movement deformation.The above research results reasonably explain the surface subsidence mechanism of the 3205 working face in Huoerxinhe coal mine.At the same time,the research results enrich the mining subsidence theory in China and have reference significance for engineering problems under similar geological conditions.