Study On Roof Cutting And Pressure Releasing And Falling Law Of Hard Compound Roof

The conditions for the occurrence of coal seams are complex and variable,and hard roof coal seams are often encountered during the mining process.The hard roof has good integrity and high strength.After the coal seam is mined,the roof rock layer of the goaf is overhanged in a large area,and it will not fall naturally in a short time,which poses a serious threat to the mining of the working face.As the working surface continues to advance,the ceiling of the suspended roof suddenly collapses,forming an impact pressure,causing serious damage to the stope and the mine.At the same time,the loading of the rock load on the side sill of the goaf is likely to cause stress concentration in the roadway and the deformation of the roadway is large.Therefore,the pre-splitting blasting and cutting treatment of the hard roof is carried out,and the roof rock stratum structure is changed,so that after the working surface is pushed,the design cut seam is smoothly fallen.Based on the 11-story coal test mining face of Qiuji Coal Mine,this paper analyzes the influence of different cutting seams on the surrounding rock stress evolution of the roadway and the top plate after cutting the roof through theoretical analysis and numerical simulation fallen law.(1)After the coal seam mining,the motion damage of the overburden rock is caused.By simplifying the roof rock stratum into a beam model,the mechanical model of the two ends of the uncut rock beam is established,and the stress state is analyzed to obtain the initial fracture step.The mechanical structure model of rock beam after pre-splitting blasting is established,and the initial fracture step of the rock beam after cutting is derived.Based on the cantilever beam theory,the periodic fracture step is obtained.The pre-fracture mechanics model of the rock beam is established.Under the condition of hard roof,the fracture position of the rock beam appears above the solid coal,which is easy to cause the stress concentration of the surrounding rock of the reserved roadway,which is extremely unfavorable to the stability of the roadway.Therefore,it is necessary to carry out pre-split blasting treatment on the hard roof,and cut off the connection between the roof rock layer of the goaf and the roof rock layer of the reserved roadway to reduce stress concentration.(2)Using FLAC3D to establish a three-dimensional numerical calculation model,compare and analyze the influence of different slits on the stress distribution of surrounding rock of roadway.The models with a cutting height of 6m,8m,and 10m when the cutting angle is 0° are calculated.The calculation results show that when the cutting height is 6 m,there is still a certain relationship between the side roof of the goaf and the roof of the retaining channel,and there is a large stress concentration in the surrounding rock near the roadway.When the cutting height is 8m and 10m The topping pressure relief effect is better,the stress concentration area is far away from the roadway,and there is a obvious pressure relief zone near the roadway,the surrounding rock stress is low,and the roadway stability is good.(3)By comparing and analyzing the cutting height of 8m,the cutting angle is 10°,15°,20° and the cutting height is 8m,the cutting angle is 0°,and the result shows that the slit is deflected to the side of the gob.The direct top bending and sinking process of the goaf does not produce a large compressive frictional resistance with the roof rock layer of the roadway.The top plate of the goaf can be smoothly fell along the pre-cracked slit surface to form the lane gang to realize the lane keeping along the empty.At the same time,the angle of the cutting top is increased,and the length of the cantilever beam structure of the overlying strata is increased,which causes the peak of stress concentration to increase.The stress concentration area shifts to the vicinity of the lane,and the displacement of the roof of the roadway increases,and the deformation of the roadway is large,the stability is poor.(4)UDEC discrete element numerical analysis software is used to establish the model after cutting the top,and the coal mining is simulated by step-by-step excavation.After the cutting is carried out,the roof rock layer collapses with the continuous advancement of the working face.The simulation results show that with the advancement of the working face,the initial stepping distance of the roof rock layer is between 24 and 28.8 m,and the periodic stepping distance is between 14.4 and 19.2 m;the slump height is about 8 m.As the coal seam is continuously produced,the roof rock stratum is continuously degraded,and the rock fissure height above the fall belt is continuously developed,and the maximum height of the overburden rock fissure development is about 34 m.(5)According to the actual work of the 1102 test mining face cutting and unloading along the road,the mine pressure of the reserved roadway and working face is dynamically monitored,including the working resistance monitoring of the working face hydraulic support,the deformation monitoring of the roadway roof,and the anchor(Cable)stress monitoring,etc.Through the mine pressure monitoring,the movement laws of the stope and roadway roof can be grasped in time,and the abnormal pressure of the stope can be monitored and forecasted.Through the on-site hydraulic working resistance monitoring data analysis,the initial pressure step of the stope roof is about 23.6m,and the cycle step is about 16.2m.