Fracture Development And Stress Relief With Space Influenced By Mining Of Coal Seams Group

Coal is one of the most important energy sources in modern China society.Gas has always threatened the safety of coal mine production for a long time as a source of disaster,gas explosion,coal and gas outburst occur sometimes,with the increase of mining depth,such problem are more severe.The extraction of protective layer is the most effective regional gas control and extraction measures proved by a large number of theoretical and field application practice.Therefore,it is very important to grasp the mechanism of pressure relief of the protective layer,stress relief scope,and its influencing factors to optimize gas drainage and improve the gas concentration and eliminate the hidden danger of gas.In this dissertation,the fracture development and stress relief with space influenced by mining of short-distance coal seams are studied,this topic has been deeply studied by means of theoretical analysis,numerical simulation,physical simulation experiment and engineering practice.The failure mechanism of the fractured coal and fracture development are analyzed from the mesoscopic and macroscopic aspect by means of elastoplastic mechanics and fracture mechanics based on the summary of the pores and cracks in the coal and rock mass,then the development mechanism of overburden fracture and vertical fissure is obtained;the stress re-establishment regularity in the gob in longwall mining is analyzed by theoretical analysis and physical simulation,the complexity of stress distribution in gob area and its main influencing factors are pointed out,and the stress re-establishment distance formula of gob is derived;the stress field and displacement field distribution in coal seam group under different mining conditions are obtained by pressure relief coefficient and relief angle based on stress distribution in the gob;the fracture development during single coal seam mining and double-pressure-relief mining are conducted through UDEC numerical simulation and physical simulation experiments.The main conclusions are as follows:1.The mining-induced failure characteristics and meso-fracture evolution mechanism of rock mass(1)The geologic tectonic movement causes a large number of pores and cracks in the interior during and after the process of coal formation,according to the pore diameter inside the coal body,it can be divided into five levels: micropores,pores,mesopores,macropores,visible holes and fracture,according to the size and shape of coal fractures,it can be divided into four categories: micro-cracks,small cracks,cracks and large fissures.(2)According to the Mohr-Coulomb strength criterion,the strength and failure mode of a cylinder with a single penetrating are analyzed,when ???w,the compressive strength of the rock mass is equal to the compressive strength of the intact rock,the rock mass can only occur shear failure like intact rock.When ???w,ko?kw,and ?co??cw,the failure mode of the rock mass will be different with different confining pressure,it will be transformed from slipping failure along the fissure surface to shear failure like intact rock;When ???w,ko?kw,and ?co??cw,the rock mass will occur slipping failure along the fissure surface,the rock mass strength is determined by fissure plane;When ???w,ko?kw,and ?co??cw,if the confining pressure ?3??3cr,the compressive strength of fissure plane is larger than that of intact rock,when the axial pressure increases to a certain extent,the fractured rock mass will occur shear failure,if the confining pressure ?3??3cr,the compressive strength of fissure plane is larger than that of intact rock,when the axial pressure increases to a certain extent,the rock mass will occur slipping failure along the fissure surface,that means the failure mode of rock mass is transformed from shear failure like intact rock to slipping failure along the fissure surface during the increase of confining pressure.When ???w,ko?kw,and ?co??cw,the fractured rock mass will occur shear failure,the rock mass strength is determined by the intact rock strength.(3)In the pressurized stage,the original crack in the coal and rock mass undergoes the development process of shear slip—self-similar propagation—bending extension—shear extension;In the unloading stage,the stress condition of the fracture in the process of axial unloading is obtained and the stress intensity factor at the crack tip is obtained.During the unloading process of the maximum principal stress,the axial stress unload to the confining pressure,the reverse slip deformation of the fracture must occur,and the open deformation of the crack need stress condition;during the process of axial stress unload to zero,both the crack open and extension need stress condition.2.The inter-layer structure and macroscopic fracture evolution short-distance coal seams(1)The types of inter-layer structures between short-distance coal seam are divided into four types based on key stratum: the inter-layer with none inferior key stratum,the inter-layer with single inferior key stratum,the inter-layer with two inferior key strata,the inter-layer with several inferior key strata.(2)It is concluded that the vertical breakage of the roof breakage should meet the condition of the breaking deformation strength and the deformation coordination condition.The opening angle of the vertical fracture is related to the second derivative of the subsidence curve.Based on the maximum strain theory of rock beam,the span condition satisfying the fault when the rock beam is regarded as simple beam is deduced.(3)When there are several inferior stratum in the inter-layer,the position and the number of inferior will affect the dynamic development and distribution of overburden fracture,the inferior stratum will not affect the development of fractures to the height,while two inferior stratum with same thickness and properties will decrease the fracture development range.3.The mechanism of ascending in coal seam group and the fracture development of surrounding rock mass with single coal seam mining(1)The feasibility of the lower 8# coal seam as a protective layer is verified by the method of ratio distinguishing method and three belts judgment,furthermore,the maximum damage height of the roof is taken out to verify the feasibility by considering the vertical scale of the gob through elastic-plastic mechanics.(2)The stress concentration factor increase first and gradually stabilized as a constant with the advance of the working face.Before the excavation,the vertical stress of the coal mass is larger than the horizontal principal stress,after the excavation,both the size and direction of the principal stress change.The major principle stress of the roof near the coal face is horizontal stress,and the vertical stress is almost zero.The hydraulic shield support has little effect on the vertical stress distribution of the roof.The principle stress distribution of the floor is divided into two regions.In the range of 1m in the floor,the major principle stress is vertical stress,and the horizontal stress is small,with the increase of the depth into the floor,the major principal stress gradually changes from vertical stress to horizontal stress,and then goes back to vertical stress.In the front of the working face,the trend of the principal stress is approximately "curved",and the arc is increasing from the middle of the coal face to roof and floor in front of the working face.And the major principal stress also changes from the vertical stress to the horizontal stress.The vertical principal stress back in the gob is larger than that in front of the gob,and gradually returns to original stress;the stress distribution of the surrounding rock around the gob is similar to "arch",and the arch height increase first,then decreases slightly with the working face advancing,and finally stabilized at a certain height.(3)The influence of the elastic modulus of the crushed rock mass on the stress field,displacement field and fissure field of the surrounding rock is studied.The same deformation characteristics are exhibited with different gob behavior and face advance,the surrounding rock mass of roof and floor can be divided into four zones: compression deformation zone,pressure relief expansion deformation zone,deformation recovery zone,no deformation zone,but the size and scope is different with different gob behavior,the smaller the elastic modulus of the crushed rock mass in the gob,the smaller the recovery distance of the waste area,the larger the stress concentration factor in front of coal face,the damage range of the coal body,the lager the vertical displacement of the roof and the expansion and compression area of the surrounding rock at the same position of the roof,the more conducive to the release of elastic potential of coal and rock,so that will be favorable to the activation of overburden gas desorption and flow,and increasing the coal and rock permeability,which is conducive to the layout of the gas drainage.4.Stress distribution regularity and stress relief with space in short-distance coal seams mining(1)The stress distribution character is studied.Based on the calculation model of the overlying rock mass load,the lateral extended bearing load model and the stress recovery distance fitting formula of the goaf based on the surface subsidence,Taking the 84306 working face of 8 # coal seam in Changping Coal Mine as an example,when the mining height is 3m,the mining depth is 530 m,the stress re-establishment distance is 103 m,169m and 140 m respectively.The stress re-establishment distance in the gob is positively correlated with the mining height,the breakage coefficient of the rock mass and the mining depth,and it is negatively correlated with uniaxial strength of gob roof,it also has a nonlinear relationship with mining depth;the increase of mining height mainly affects the breakage coefficient of rock mass in the gob,and its influence on stress recovery distance in gob is smaller than that of roof rock strength.(2)The spatial stress and displacement distribution of overlying strata after lower coal seam working are obtained,according to the distribution of vertical stress and vertical displacement of the roof of the protective coal seam,the roof is divided into five areas along the face,and from the front of the face to the gob,the five areas are: the original stress area,the compression area,the expansion area,the stress recovery area and the re-compacted area.The formation and development characteristics of the gas channel in each area are analyzed.(3)The stress distribution at different heights and different locations of the roof of the lower coal seam is different,the farther away from the coal seam,the lower decrement between the vertical stress and the original stress,the difference between the three-dimensional stress(SXX,SYY,SZZ)is getting smaller and smaller,and the larger three-dimensional stress gradient and stress asymmetry will be more likely to cause the formation and expansion of cracks in coal and rock that will be destroy finally,more closer to the face of the protective layer,the more cracks and more damage caused by mining.(4)The special pressure relief effect of upper coal seam measured by the pressure relief coefficient and the unloading angle is obtained in the process of lower coal seam advancement.With the advancement of face of the protective layer,the degree and the range of pressure relief of protective layer are gradually increased,with collapse of the immediate roof and main roof,the gravity of overburden is shifted to the coal wall and interconnection and it lead to stress concentration.With the continuous subsidence of roof in mined-out area,the compaction of broken rock in mined-out area is gradually increased,and the supporting effect of roof is gradually increased,so that the degree of pressure relief of protective layer in the middle direction of rear area of goaf has a certain reduced,take the centerline of the goaf as the center line,forepart and afterpart is asymmetrical,the maximum pressure relief position of protective layer is located in the direction of face.From the different positions of the coal pillar,the pressure relief angle of the interconnection side and the coal wall along the coal seam are increased and then decreased near the coal pillar,the angle along the direction of coal seam advancement is different.From the different positions of the coal wall of face,and the pressure relief angle of coal pillar increased first and then decreased with the distance from the face,the angle along the inclined direction of coal seam in mined-out area is different.5.The following conclusions are obtained by physical simulation experiments in laboratory:(1)The initial caving interval of immediate roof and the initial weighting interval of main roof of the upper coal seam working are less than lower coal seam,when the face movement of lower coal seam is 40 m,the large immediate roof area collapse,when the movement is 50 m,the fracture of main roof forms the first weighting,the initial caving interval of immediate roof and the initial weighting interval of main roof of the upper coal seam are 30 m and 45 m.(2)The collapse angle of face side is generally reduced increased-decreased-increased cycle change process during face-advancing of upper coal seam,the periodic weighting intervals are 10 m,20m,25 m,etc,which cause violent and unconspicuous periodic weighting phenomenon,and it generally shows that the collapse angle decreases when the unconspicuous periodic weighting phenomenon occur and increases when the violent periodic weighting phenomenon occur,the periodic weighting intervals and weighting strength both increase because of key stratum's broken.(3)The degree and the range of pressure relief of upper coal seam are gradually increased in the process of lower coal seam working,the peak of vertical stress is moving forward in front of face,the vertical stress distribution in upper coal seam is in the order of "V"(the main roof does not collapse),"U"(the main roof first collapse)and the "W"(the main roof cyclical collapse)type.(4)The roof periodic caving and subsidence with the advancement of lower coal seam,the degree and the range of pressure relief of gob side first increase and then tend to stabilization,the caving crushed rock mass in gob area is gradually compacted,the floor of gob gradually restored the level of original stress,the floor undergoes a process of pressurization-decompression-pressurization – recovery by stress monitoring,the recovery distance of the stress in gob is 120 m appromixmately.(5)At any point of upper coal seam has experienced these stages during the lower coal seam working: compression,expansion deformation,expansion deformation increases,expansion deformation decreases and the expansion stability,after the deformation stability of overburden in the goaf,the upper coal seam can be divided into five areas: compression deformation zone,pressure relief expansion zone of transition,pressure relief expansion zone of stability,pressure relief expansion zone of transition,compression deformation zone.The cracks in upper coal seam also experienced the dynamic process of development-extension-compaction closure.And it is consistent with the numerical simulation results.(6)The statistics of the quantity,length and inclination of overburden fracture during the process of coal seam mining are obtained by Matlab.The total number of fissures increases by the "S" curve with the face advance,and the fitting curve bttween the fracture quantity and mining length are obtained.The inclination angle is mainly horizontal and vertical,and the number of horizontal fissures in the overlying strata increases first and then decreases,the rose scheme of fracture angle distribution for 80-140 m ahead of setting room is observed with mining length increasing.(7)The double-pressure relief mining of coal seams shows the different distribution and evolution of overburden fractures with single coal seam,the overburden fractures undergo a complex dynamic process of generation,expansion,compaction,tensioning,and compaction,the upper coal seam working makes the secondary development of the overburden fractures,and the range and degree of pressure relief are increased,the fractures formed by the upper and lower coal seams working are interconnected to form a three-dimensional gas migration channel.Especially interconnection and the face of the coal face side of the vertical broken permanent fractures through each other,which form the macro gas channel.