Key Parameters And Surrounding Rock Control Of Small Pillar Roadway Protectionwith Roof Cutting And Pressure Releasing In Thick Coal Seam

With the increase of coal seam thickness and mining depth in the coal industry,the reserved width of coal pillars in newly built mines is relatively large,resulting in low coal recovery rate and waste of resources.Moreover,the stress concentration caused by the width of the coal pillar makes the problems of rock burst and roadway deformation more and more serious.Based on the engineering conditions of Longquan Coal Mine,theoretical research,numerical simulation and field industrial test were used to carry out research on the structure and stability of the roadway roof.The influence of key parameters of roof cutting and pressure relief on the stability of the surrounding rock structure is studied,and the results are as follows:(1)A mechanical model of surrounding rock structure of driving along goaf with roof cutting and pressure releasing was established,and the deformation and failure laws of the roof strata were analyzed.Based on the traditional arc triangle block theory,a new model of old roof destruction is proposed.It is considered that:during driving period,the rupture of old roof did not form the traditional arc triangle block B,but the key rock block first broke into the square blockC_L andC_R in the middle,and then these two blocks continued to break equally into shorter rocks under the action of the overlying rock.The calculation of the basic parameters of the key rock blockC_L has nothing to do with the shape of the rock blocks.The horizontal thrust and vertical shear force of square rock blockC_L are calculated as a special case where the bottom angle of the traditional arc triangle block B is 45°.(2)3DEC simulation was used to establish a numerical model of roadway driving along goaf with roof cutting,and the influence of key parameters of roof cutting and pressure releasing on surrounding rock structure was revealed.It is concluded that the pressure relief effect is the best when roof cutting angle is 90°.However,considering the inverted trapezoidal roof cutting structure is more conducive to the stability of advanced roadway roof,75°is selected as roof cutting angle.During change of roof cutting angle from 90°(vertical)to 55°,peak position of abutment pressure is always kept at about 5m.The best effect is when roof cutting length is 20m.When roof cutting length is 5m～20m,the longer the length is,the lower abutment pressure of coal pillar will be.However,when the length is greater than 20m,the roof cutting effect is not significantly improved.In addition,abutment pressure of the roof on the side of coal pillar increases,which is not conducive to the transfer of surrounding rock stress.(3)By analyzing the evolution of abutment pressure with different coal pillar widths before and after roof cutting,reasonable width of coal pillar is determined,and the following results are obtained:The increase in width of coal pillar weakens stress transfer between overlying rock structures,and surrounding rock structure is improved to a certain extent.Before and after roof cutting,the abutment pressure on coal pillar side is still presents the shape of"single peak".The abutment pressure on the small coal pillars(coal pillar width is 4m～8m)presents the shape of"bell",while the abutment pressure on the large coal pillars(>10m)presents the shape of asymmetric"saddle".After roof cutting,the peak value of abutment pressure on both sides of roadway is significantly reduced,among which the reduction range of left side is about 75%,and the reduction range of coal pillar is about 55%.For the same roof cutting method,the roof cutting effect achieved by different coal pillar widths is roughly the same.(4)A surrounding rock control scheme of driving along goaf with roof cutting and pressure releasing was established and applied to field industrial test to investigate the effect of roof cutting and roadway protection,and to verify the rationality and feasibility of research results.It is concluded that roadway achieves self-stabilization in about 15days during driving period;after roof cutting,the floor heave of roadway is reduced by66.2%,the deformation of the two sides is reduced by 41.8%,and the deformation of roof is reduced by 42.9%.Roof cutting effectively reduces the deformation of roadway caused by stress concentration of surrounding rock.During roadway stoping,the moving speed of roof cutting,floor heave and two sides of roadway is characterized by intermittent jump increase,and the speed of floor heave>maximum moving speed of two sides>the maximum subsidence speed of roof.The delamination of roof reached a maximum of 4.2mm on day 11.28 pictures,10 tables,68 references.