Study On The Overburden Breakage Law Of The Coal Face Under The Goaf In The Close Distance Coal Seams

Mining goaf of close distance coal seam,coal seam mining influence each other,leading to the coal strata movement,stress unloading area and clusters superimposition effect,stope stress distribution and rock movement becomes more complex,may cause the lower coal seam mining process middling appeared abnormal pressure,threat of coal mine safety and efficient production.In order to obtain the rule of overburden fracture and its supporting strength in the lower working face of mined-out coal seam,theoretical analysis,numerical simulation,similarity simulation and on-site verification are adopted to obtain the change rule of stress field of overburden fracture in the near coal seam with different interlayer structure types.The main research conclusions are as follows:According to the interval of coal seam and the damage of the upper coal seam to the interbedded strata,the structure of the lower coal overburden is divided into the basic top without interbedded top,the basic top with complete interbedded top,the basic top with partial interbedded top and the basic top with complete interbedded top,and the classification criterion is obtained.Among them,the law of fracture of overburden with no basic roof and complete damage between layers is obviously different from that of single coal seam mining,and the law of fracture of overburden with partial damage between layers and complete damage between layers is close to that of single coal seam mining.Through numerical simulation,it can be found that when the upper single coal face is mined,the overburden fracture law is as follows:significant periodic pressure,constant increase of stress concentration coefficient K,rapid growth in the early stage and slow growth in the later stage,and the average periodic pressure step distance is 18.6m.Continue to mine the lower coal seam.Taking the basic roof structure with complete damage between layers as an example,K has a small drop in the early stage of mining,stable growth in the middle stage,and decrease in the later stage.The plastic failure area between the two coal seams has no obvious change,indicating that there is no obvious periodic pressure in the lower coal seam mining.The main factors that influence the breaking rules of various overburden structures are the failure depth of upper coal seam floor and the spacing between coal seams.Meiyukou below 3 coal mine 23307 working face 23307(bottom)and 3 coal working face get close distance coal seam as the object of analog simulation experiment,the upper part of a single coal seam 3 phase,coal mining has obvious periodic interval average periodic 19 m,while the lower coal seam 3 characteristics during coal mining under periodic is not obvious,increases with the increasing mining roof fall;After the lower coal seam is mined out,the overburden compacted degree of the upper coal seam goaf increases greatly,the caving zone develops to the upper level to reach 26.12m,and the fracture zone height is 63.97m.According to theoretical calculation based on field sampling measurement,the maximum damage depth of 23307 working face bottom plate is 11.9m,and the roof of 23307 working face(bottom)is judged to be the basic roof structure with complete inter-storey damage.On-site observation of mine pressure shows that the average breaking step of the basic roof of 23307(bottom)working face is small,which is 9.6m;the average working resistance of the support is 33.4MPa when breaking;on the whole,the periodic pressure is not obvious;the roof falls with mining.The research results are of reference significance to roof management in the mining process of near face and are beneficial to mine safety production.