Research On Coal Wall Slip Spalling And Its Stability Control In Fully Mechanized Face With Large Mining Height

Due to the advantages of strong applicability and simple technology,the fully mechanized mining with large mining height has already become the main method of mining thick coal seam in China.However,with the increase of mining depth and mining height,the problem of coal wall spalling in fully mechanized mining face with large mining height is more prominent,which seriously restricts the safety and and high-efficiency production of the working face.The inclined straight line type and circular arc type are the main types of coal wall spalling in large mining height working face,accounting for more than80%of the total.The existing results mostly use the sliding body theory to study the failure mechanism and influencing factors of the two types of sidewall.However,in the research process,there are some problems,such as the distribution law of the safety coefficient of sidewall sliding is unknown,the most dangerous sliding surface is difficult to search,and the maximum sliding depth is difficult to accurately predict.In view of this,taking the Hongqinghe3^-1101 working face as the background,this paper adopts the methods of literature search,field observation,theoretical derivation,numerical simulation and laboratory experiment to study the safety coefficient change rule of inclined straight line type and circular arc type sliding surface of coal wall,the determination of the most dangerous sliding surface and the limit equilibrium sliding surface of coal wall,the influence rule of the initial support force,height of guard board and horizontal thrust of guard board on coal wall stability of working face,the mechanism of weakening roof to control coal wall spalling.The main work and the main conclusions of the thesis are as follows:(1)Considering the stress characteristics of the coal wall of the working face,the mechanical model of the inclined straight line type slip coal wall spalling is established by the precise solution of the limit equilibrium analysis method,and the mathematical model of the coal wall safety factor of this type slip surface is derived.In addition,the change rule of safety coefficient of coal wall straight line type sliding surface with the position of sliding surface is studied,and the method to determine the limit sliding depth and the most dangerous sliding depth of coal wall straight line type is put forward.The accuracy of prediction method of wall depth and mathematical model of safety coefficient is verified by field measurement.The results show that along the direction of coal wall depth,the safety factor of the slip surface decreases first and then increases with the increase of the depth of the slip surface;along the direction of coal wall height,with the starting point of sliding surface moving from floor to roof,the safety factor of coal wall sliding surface first decreases and then increases,and the position of minimum safety factor(i.e.the position of the most dangerous sliding surface)always appears at 0.31 times mining height from the floor,not affected by mining factors.According to the change rule of the safety factor of the sliding surface,the minimum safety factor of the inclined straight line sliding surface of 3^-1101 working face is 0.9277,and the corresponding most dangerous sliding depth is 0.9 m;the maximum sliding depth is2.12 m,which is closer to the maximum cutting depth of 2.08 m measured on site compared with the existing pressure shear algorithm and sliding algorithm.The minimum safety factor of coal wall increases linearly and exponentially with cohesion and internal friction angle,and decreases logarithmically with burial depth and mining height.According to the increase,it can be concluded that cohesion and burial depth have significant influence on the minimum safety factor of straight line type slip surface.(2)Considering the stress characteristics of the coal wall of the working face,the mechanical model of circular arc type coal wall spalling is established by simplified Bishop slice method limit equilibrium analysis,and the mathematical model of safety factor of circular slip surface of coal wall is derived.The change rule of safety factor of circular slip surface of coal wall is studied,and the method to determine the limit slip depth and the most dangerous slip depth of circular slip surface of coal wall is put forward.The accuracy of the prediction method of circular slip depth and the mathematical model of safety factor are verified by comparing with the field measurement results.The results show that along the direction of coal wall depth,the safety factor of coal wall slip surface decreases first and then increases with the increase of slip surface depth.According to the change law of safety factor,the maximum slip coal wall spalling depth of 3^-1101 working face in Hongqinghe coal mine is 2.2 m,compared with 2.25 m measured on site,the result is closer to the measured on site than the maximum spalling depth obtained by the previous method.The minimum safety coefficient of circle arc slip surface of coal wall increases linearly with the increase of cohesion,increases quadratic with the increase of internal friction angle,decreases logarithmically with the increase of coal seam buried depth,decreases linearly with the increase of coal seam dip angle when mining in an upward inclined way;increases linearly with the increase of coal seam dip angle when mining in an downward inclined way.(3)Using FLAC^3D numerical simulation software,the influence rule of initial support force and horizontal thrust of guard board of hydraulic support on the stability of coal wall is simulated and analyzed,and the mechanism of the support support strength and initial support force to maintain the stability of coal wall is revealed.The conclusions are as follows:when the initial support force of hydraulic support is increased from 0 k N to 12000 k N,the peak value of support stress is reduced by 16.8%,the area of coal wall damage is reduced by48.6%,and the horizontal deformation of the coal wall is reduced by 21.33%.It shows that the increase of the initial support force of the support can reduce the roof subsidence,change the distribution of the advance support pressure,and reduce the area of coal wall damage and the horizontal deformation.The horizontal deformation of the coal wall can be effectively reduced by increasing the height and horizontal thrust of the guard board,but the bearing pressure and the failure area of the coal wall have little change,which shows that the improvement of the height and horizontal thrust of the guard board is through the intervention of the lateral deformation of the coal wall,making it return to the three-way stress state,so as to improve the stability of the coal wall.Lifting the initial supporting force to reduce the load on the coal wall,that is,maintaining the stability of the coal wall through pressure relief;improving the support capacity of the guard board to maintain the stability of the coal wall through reducing the horizontal deformation of the coal wall.(4)The method of combining FLAC^3D numerical simulation with strength reduction theory is proposed to calculate the coal wall safety factor,and the relationship between the stability of coal wall and the initial support force of hydraulic support,height and horizontal thrust of the guard board are quantitatively studied.The results show that under any given initial support force,the minimum safety factor of coal wall is positively correlated with the height and horizontal thrust of the guard board,and the height and horizontal thrust of each group correspond to a minimum safety factor of coal wall.Therefore,it can provide quantitative index for the selection of hydraulic support and the optimization of guard board.(5)Based on Winkler elastic foundation beam theory and basic roof fracture structure"masonry beam"theory,the mechanism of improving spalling by direct roof and basic roof water injection is revealed respectively,and the good effect of roof water injection method is verified by numerical simulation.The conclusions are as follows:According to the elastic foundation beam theory,the stress model of coal wall and roof is established.as the elastic modulus of the direct roof decreases after water injection,it can be concluded that the softening of the direct roof will transfer the peak abutment pressure above the coal wall to the inside of the coal wall,and the abutment pressure will decrease as a whole,so as to improve the stability of coal wall.Additionaly,in the initial mining stage,the basic roof changes from the stress state of the fixed support beam to the stress state of the simple support beam after the water injection pre splitting of the open cut,the first fracture step will be greatly shortened,and the supporting pressure of the coal wall will also be reduced;in the normal advancing stage,the pre splitting of the basic roof in the roadway can reduce the fracture step and rotation of the basic roof,so as to reduce the pressure exerted by the roof on the coal wall of the working face and achieve the purpose of controlling the coal wall spalling.Compared with the"roof untreated",when the"roof water injection treatment"method is used,the peak supporting pressure is reduced by 17.82%,the distance between the peak position and the coal wall surface is increased by 40%,the area of coal wall damage is reduced by 26.92%,and the minimum safety factor of coal wall slip surface is increased by 19.67%.