Research On Reasonable Width And Support Technology Of Small Coal Pillar Mechanized Top Caving Mining Of Extra Thick Coal Seam

The fully mechanized top coal caving mining in extra thick coal seam has a large mining space at one time,a wide range of mining influence,and the mine pressure has a prominent manifestation law,which puts forward a new topic for the theory and technology of roadway and stope surrounding rock control.Among them,the reasonable width of coal pillar is an important content of mine pressure control theory and technology in extra thick coal seam mining.If the coal pillar is too small,it will increase the difficulty of mine pressure control and even affect the normal production.Based on the 8102 working face of a coal company of Jinneng holding group,combined with the mechanical parameters of the coal seam and roof of the working face,this paper discusses the reasonable width and support technology of small coal pillar in fully mechanized top coal caving mining of extra thick coal seam in 8102 working face by using the methods of engineering investigation,theoretical analysis,numerical simulation and field measurement.The distribution characteristics of overlying strata in the stope are analyzed by using the key strata theory of strata control,and the location and failure form of the key strata are identified.It is concluded that the cantilever beam structure is formed at the coal wall when the low sub key strata are backward,and the masonry beam structure is formed when the main key strata are far away from the coal seam,which becomes the main bearing body to bear all the load of the overlying strata.Based on the different fracture positions of the key blocks in the masonry beam structure of the key layer of the overlying strata,the disturbance of the overlying strata to the coal pillar and roadway is analyzed.When the fracture position of arc triangle block B in triangle arch hinge structure is on the right side of coal pillar,the disturbance of coal pillar and roadway surrounding rock caused by mining and the difficulty of control are the lowest.By analyzing the load of coal pillar,it is concluded that the stress distribution of wide coal pillar is saddle shape and that of narrow coal pillar is horseshoe shape;On this basis,the failure mechanism of coal pillar and five failure modes of coal pillar affected by different geological factors are studied.Limit equilibrium theory is used to calculate the width of plastic zone formed by coal pillar in the mining space and on both sides of the roadway.The width of plastic zone is4.0m and 1.4m respectively.According to that the width of elastic zone of coal pillar is30%-50% of the width of plastic zone on both sides of coal pillar,the value range of elastic bearing zone is 1.62-2.7m.To sum up,considering that the coal pillar should have a certain bearing capacity,the width of small coal pillar is determined as 8m.Through numerical simulation,the stress distribution and displacement evolution characteristics of surrounding rock under coal pillars of 5m,6m,7m,8m,9m,10 m,11m and 12 m in 8102 working face are studied,and the reasonable width of coal pillar is 8m.Under the condition of 8m coal pillar,the stress distribution and deformation law of surrounding rock in each stage from undisturbed to mining are analyzed to understand the stress evolution process of surrounding rock.Combined with the determined width of coal pillar,the distribution of stress field of bolt(cable)is compared with the change of pretension,length and spacing,and the diffusion law of bolt(cable)prestress in surrounding rock is studied.The combined support mode of "bolt + anchor cable + W / JW steel belt + short steel beam + metal mesh+ shotcreting" is determined for the small coal pillar roadway,and the reinforcement scheme is designed for the broken section of the roadway.Through the arrangement of surface displacement observation station,roof separation instrument and anchor(cable)dynamometer,the surface displacement of roadway and the stress of anchor(cable)are mastered,and the rationality of coal pillar reservation and surrounding rock support effect are verified.According to the measured results of the roadway,after adopting the optimized supporting form,the deformation of the roadway is within the reasonable control range,and the supporting effect is good.