Study on instability and fracture of overlying hard rock strata in 8105 working face of Tongxin Coal Mine
Taking Tongxin Coal Mine as the engineering background,this paper uses a theoretical analysis method and on-site monitoring method to study the instability and fracture of hard strata overlying 8105 working faces.To describe the instability characteristics of hard strata,a structural model of hard strata is established to describe the structure formed by the fracture of low,middle,and high hard strata.The structure is divided into a cantilever structure,masonry structure,and pressure arch structure.Based on the key layer theory,thin plate theory,plastic failure theory of beam,instability model of hard rock,and calculation formula of cooperative load of hard rock,the instability criterion of hard rock is put forward,and the theoretical calculation formula of instability of hard rock is deduced.The failure modes of different hard strata are also different,which depend on the length of the hard strata inclined to hang along the working face and the length of the hard strata along the working face.When the low and middle hard strata are broken for the first time,2x≤a,which belongs to the instability mode a,and when the high hard strata are broken for the first time,2y≤b,which accords with the model b.Combined with the rock instability physical and mechanical parameters corresponding to the borehole columnar diagram of the working face,the initial instability scales of low,middle,and high hard strata are 35.4 m,74.9 m,and 122.1 m,respectively.Similarly,when periodic instability occurs in low and middle hard strata,it is model a,and when high hard strata occur periodic instability,it is model b,and the scale of periodic instability is 13.5 m,63 m,and 109.9 m,respectively.Therefore,the instability criterion of hard rock is used to predict the coming pressure of the working face,and it is theoretically analyzed that the instability and breaking of low hard rock lead to 25 times the small cycle pressure.The middle hard rock layer causes large cycle pressure 9 times.The instability and breaking of high hard rock strata causes strong ore pressure 5 times.Through the comparison of the actual data,the pressure accuracy caused by the instability and fracture of low,middle,and high hard rock strata is 56％,89％,and 100％respectively.According to the microseismic stress monitoring data,the statistical diagrams of the occurrence heights of different hard strata and microseismic events are drawn.It is found that the instability of hard strata couples with the overlying soft rock,forming compound mine pressure,which makes the height of microseismic events gradually increase,while the low hard strata do not break,and the occurrence height of microseismic events is almost the same.Therefore,the instability of different hard strata has a certain influence on the height of microseismic events.
safety engineeringore pressure appearanceinstability and fracture of hard rock stratafracture line theorycriterion of hard rock instability
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