Study On Mechanism Of Mining Subsidence Affected By Fault

Surface will move and deform when original stress state of overlying rock strata is destroyed caused by mining.surface movement and deformation laws is significantly different from ordinary geological conditions When the fault is existing and activation influenced by mining.At present,the research on fault activation mechanism mainly focuses on fault-induced rock-burst,water-inrush from fault,fault-induced earthquake,etc.At the same time,the research on fault activation-induced surface abnormal subsidence mainly focuses on the description and analysis of a certain mine subsidence phenomenon,and rarely involves fault activation mechanism.Based on the geological and mining conditions of FengFeng mining district,the similar and numerical simulation,combining theoretical analysis and field testing,the subsidence model influenced by fault is established around surface abnormal subsidence,influence of fault on mining stress transfer and fault activation mechanism when mining in footwall and hanging-wall.The results are got as follows.(1)The analogy simulation tests is utilized to study surface subsidence and stress transferring laws influenced by fault and the results shows that the maximum subsidence deviates to open-off cut when the working face advancing length is small for the reason that caving angle of overlying rock at the side of open cut is greater than that at stopping line and it becomes converse when fault is activating.Since the horizontal distance between fault and working face is equal after hanging-wall and footwall mining,the hanging-wall mining only produces cracks on the surface at the fault outcrop,while it produces step drop at the outcrop.The phenomenon indicates that fault is easier to activate when mining in footwall than mining in hanging-wall.At the same time,the stable area of mining in hanging-wall and footwall is obtained by theoretical analysis.According to the regional distribution,fault is easier to activate when mining in footwall and the possibility of fault activation increases as fault inclination increases,while it's converse for footwall mining.The advancing abutment pressure is formed in front of the working face when mining in hanging-wall and footwall,and the peak value of advancing abutment pressure gradually increases with the advancing of the working face.When working face advancing length reaches a certain degree,the peak value fluctuates near a value.At the same time,the advancing abutment pressure peak value in hanging-wall mining is greater than that in footwall mining.Fault has an obvious blocking effect on the overburden deformation and force transferring.Surface movement and deformation mainly occurs in the mining wall,while the movement and deformation on the other wall is very small.Simultaneously,the advancing abutment pressure drops sharply at the fault.No matter in hanging-wall and footwall mining,the firstly fractured rock is at the fault outcrop and fault activation starts from fault outcrop.(2)The numerical simulation model is built to analyze surface subsidence laws of different coal height,coal seam buried depth,fault throw,fault zone width,fault angle,protective coal pillar width,whether water being in fault zone,fault zone rock cohesion,fault zone rock friction angle.The results show that fault is easier to activate when fault throw,fault zone width,coal height and coal seam buried depth is larger.While,it's converse for fault zone rock cohesion,fault zone rock friction angle and protective coal pillar width.At the same time,when there being water in fault zone,water promotes fault activation.Fault has blocking effect on overburden deformation and force transferring and the blocking effect is more obvious when working face is closer to the fault.Comparing to mining in hanging-wall,the blocking effect begins to show when working face advancing length is small,which indicates that fault is easier to activate when mining in footwall than in hanging-wall.When mining in hanging-wall,the normal stress of rock mass at fault outcrop gradually decreases with the advance of working face,while the shear stress increases and the ratio of shear stress to normal stress increases.The normal stress and shear stress of fault rock mass with large burial depth increases with the advance of the working surface and the ratio of shear stress to normal stress also increases.When mining in footwall,the normal stress and shear stress of rock mass at the fault outcrop increases with the advance of the working face,while the ratio of shear stress to normal stress decrease.The shear stress and normal stress of deep buried fault zone rock mass decreases gradually,and the ratio of shear stress to normal stress firstly increases,and then decreases when the working face advances a certain length.Compared with mining in hanging-wall,the shear stress of fault zone rock mass in footwall is larger and the normal stress is smaller.The ratio of shear stress to normal stress is 3~5 times that of hanging-wall mining,and fault is more easily activated when mining in footwall.Based on grey correlation analysis,the influence of the above factors on fault activation is ranked as fault zone width,fault throw,fault angle,coal seam buried depth,coal height,protective coal pillar,fault zone rock mass friction angle,fault zone rock mass cohesion.(3)The fault effect on mining zone transmitting and the initiation process of fault activation are analyzed theoretically.At the same time,two different fault activation modes are proposed.The results show that when there is no fault in overburden strata,surface subsidence curve is skewed and the maximum subsidence value deviate to open-off cut for the reason that the caving angle is different at open-off cut and stopping line.The influencing factors are rock hulking coefficient below key strata,coal seam height,rock height below key strata,gob length,the broking length of key strata,loose layer thickness.Sum of surface subsidence space,overburden separation space and the pores between the caved rocks is equal to the mining space,which satisfies the conservation of space.At the same time,the existence of faults has an increasing effect on the spatial transfer of mining.Based on proposed fault activation criterion of the step subsidence is larger than 10mm,fault activation mechanism is analyzed and two modes of fault activation is put out,which especially are fault plane producing separation space and coal pillar compressing.The relationship between the main influence angle and fault dip angle control the fault plane separation space and thus plays a controlling role in the surface movement and deformation laws.When advancing abutment pressure reaches the fault,it will cause fault activation of coal pillar compressing mode.Comparing to there being no fault,the peak of advancing abutment pressure is larger,but the distance from peak value to working face is no change.Advancing abutment pressure increase with the working face advancing and it will not change and move with working face when the earth surface reaches full subsidence.Advancing abutment pressure increases with working face length and when the working face length reaches l_m,it still increase with smaller magnitude.The decreasing of horizontal and vertical stress caused by mining is the power of fault activation,the transmitting of fault plane separation to looser layer,which results in step subsidence at fault outcrop,is the results of fault activation.When the influence angle intersects with the fault outcrop,fault is fully activated.(4)Based on probability integral method,starting from surface point deformation caused by coal seam unit mining,the prediction of inclined coal seams,of which the influencing radius r is always changing,is transformed to prediction of equivalent plane with constant r through the mutual transformation between double integrals and line integrals.Prediction model influenced by fault is built by superimposing the surface movement and deformation caused by working face and fault plane separation at the points along the fault plane with the interval of 5 meters.Meanwhile,the prediction model is developed to make theory model numerical and graphical based on the platform of Microsoft.NET by using C#program language.For example of a working face with 600m strike length,300m dipping length,coal seam angle of 0degree,coal height of 5m,buried depth of 700m and a fault with angle of 70 degree when mining in hanging-wall,the surface subsidence value of different fault angle,coal height,coal seam buried depth,protective coal pillar width is got by using the developed prediction grogram.The results show that the step subsidence increases with the fault angle as well as mining height and coal seam buried depth.While,it's converse for protective coal pillar width.(5)By using the prediction program,the surface subsidence caused by mining of15233 working face in JiuLong Colliery,FengFeng Mining District is calculated.The calculation indicates that fault will activate after the mining,which resulting in abnormal subsidence at fault outcrop with step.The result is similar to the building damage position of field investigation and it verifies the correctness of theory model.