Study On Overlying Strata Movement In Close Distance Seam Mining And The Activation Law Of Step Fault

In this paper,Chaili Coal Mine actual geological conditions as the starting point,by means of theoretical analysis,similar research means of experiment and numerical simulation,and considering the confined water effect,we discussed the movement of overburden,the stress change of roof and floor and the activation law of step fault during the process of downlink mining of associated step fault in close range coal seam.The main conclusions or conclusions obtained are as follows:(1)Through the elastoplastic mechanics analysis,the mechanical models of overlying rock collapse,water inrush from coal seam floor and fault activation are constructed,and the criteria of water inrush and fault activation are obtained,and the ultimate breaking water pressure and bottom failure depth of the upper and lower coal seams are obtained.The results show that the bottom coal seam is disturbed by the mining of the upper coal seam,and the bottom failure depth of the coal seam is further increased,and the shear and tensile ultimate water pressure are less than the upper coal seam.(2)Through the simulation test of similar materials,it is found that the overburden structure can be divided into three types:"cantilever beam,masonry beam,transition structure",and its collapse form also corresponds to three kinds.During the mining of close coal seam,the collapse of the overlying strata in the lower coal is characterized by"collapse with the excavation","no obvious periodic pressure step distance" and "hard to form a beam type structure",which is distinct from the overlying strata collapse of the upper coal.(3)The study of fault found that because of the "barrier" and"adsorption" of the fault,the stress of the roof is difficult to cross the fault,so the closer to the fault,the more obvious the degree of stress concentration is.Compared with the excavation of the upper coal seam,the collapse step of the lower seam is smaller and the support pressure is far less than the upper coal.(4)The numerical simulation shows that the support pressure is directly proportional to the water pressure of the aquifer,and the stress of the two step fault is obviously reduced by the stress barrier of the first step fault,and the plastic failure zone is gradually transferred from the"round arch" to the " saddle shape "with the increase of the step distance,and eventually becomes the "central saddle,the whole round arch".It is found that the lower coal mining will result in different effects on the two ends of the goaf and the working face after the upper coal mining.The pressure relief of the goaf is aggravated,the stress is reduced,the stress concentration at both ends of the working face increases and the stress becomes larger.