Research On The Influence Of Key Layer On The Failure Characteristics And Crack Evolution Of Overburden Rock Under Repeated Minin

The occurrence conditions of coal seams in Guizhou are mostly complex,and the thickness of coal seams is thin and the gas content is large.Most of them are coal seam groups,coal seams and roof and floor are soft,and the surface of the mining area is mostly mountainous terrain.It is difficult to predict the movement of overlying strata and fracture development under repeated mining.In this paper,the key strata are taken as the breakthrough point,and the Anshun Coal Mine in Guizhou Province is taken as the research background.By means of field measurement,theoretical analysis,uniaxial test,similar simulation analysis and numerical simulation analysis,the fracture development law and deformation and failure evolution characteristics of rock-like specimens under unidirectional stress are analyzed.The influence of prefabricated fracture dip angle,fracture number and fracture shape on the strength of rock-like specimens is revealed,so as to determine the change of overburden strength during mining.By analyzing the movement characteristics and fracture development of overlying strata under different key strata and different key strata thickness,the influence of key strata on the movement and fracture evolution of overlying strata under repeated mining is obtained.The results show that :1.Through the analysis of the fracture development law and deformation and failure evolution characteristics of rock-like specimens under uniaxial stress,it is concluded that during the change of fracture dip angle from 0°→ 45° and 90° → 45°,the reduction degree of uniaxial compressive strength of rock increases from 12.47 %to 28.83 % and 16.66 % to 28.83 % respectively.The average peak strength of single fracture specimens is 80.24 % of the peak strength of complete specimens.The average peak strength of double fracture specimens is 69.07 % of the peak strength of complete specimens.The average peak strength of three fracture specimens is 56.94 % of the peak strength of complete specimens.2.Through the analysis of the roof structure of the only key strata in the coal mine in the southwest mountainous area,the mechanical model of the roof structure of the ’ inclined step ’ in the coal mine in the southwest mountainous area is established,and the stability analysis of the key block of the ’ inclined step ’ structure is carried out.The minimum support force of the key block without sliding phenomenon is obtained,and the minimum support force required for different layers of the key layer is further calculated in turn.It is found that the higher the key layer,the smaller the minimum support force required.3.The physical similarity test shows that the key strata have no or little effect on the initial weighting step,the number of periodic weighting and the average periodic weighting step.The higher the horizon is,the larger the key stratum collapse step is,the later the key stratum breaks,the smaller the damage to the key stratum and overlying strata and even the surface when the key stratum is destroyed,and the smaller the subsidence of the key stratum and overlying strata and even the surface.And under repeated mining,the key strata have little effect on the movement of overlying strata and fracture evolution.4.The numerical simulation shows that the higher the key strata position is,the larger the key strata collapse step distance is,and the smaller the overburden subsidence is;the less the cracks at the key strata,the smaller the cracks on the overlying strata and the surface of the key strata,the larger the collapse step of the key strata,and the smaller the subsidence of the overlying strata during the collapse.The thicker the key stratum is,the larger the key stratum collapse step is,and the smaller the subsidence of overburden rock is.And in the case of repeated mining,the key stratum position and thickness will not have a great influence on the movement of overlying strata and the evolution of cracks.