Experimental Study on Fracture and Instability Law of Lower Double Hard Roof in Extra-Thick Coal Seam
Aiming at the problems of overburden rock impact fracture in top coal caving mining with double hard roof in extra-thick coal seam,as well as large deformation of wide coal pillar gob-side entry,difficulties in roadway protection and repeated brushing and expansion,taking 14202 working face of Majialiang Coal Mine as the engineering background,through physical similarity simulation experiment,the dynamic load effect of lower double hard roof structure fracture in extra-thick coal seam was studied and analyzed.The results show that the lower hard roof has accumulated a total advance of 36 m from the initiation of the detachment layer to the complete caving of the working face,and the first fracture and instability occurs when the advance is 60 m.The caving thickness is 4.2 m,the fracture angle is 60°,the roof fracture line is 5.8 m from the coal wall,and the periodic caving step is about 23-25 m.The cumulative advance of the upper hard roof from the lower layer to the fully caving of the working face,advancing a total of 23 m,the fracture step is about 55-65 m,and the fracture angle is consistent with the lower hard roof.The displacement monitoring results show that the motion law of the lower hard roof and the middle-lower part of the weak interlayer,as well as the upper hard roof and the upper layer of the weak interlayer and the overlying weak rock layer are consistent.The fracture and instability of the double hard roof forms a certain range of stress rise zone on the side of the goaf.The peak stress is 23 MPa,the stress concentration factor is 1.57,and the influence range of the peak stress is 20 m.The lower hard roof fractures form a"short cantilever beam"structure,which forms a small periodic weighting with the dynamic forward movement of the working face.The upper hard roof fractures form a"long masonry beam"structure,and the dynamic load of fracture and instability acts on the lower"short cantilever beam",so that it breaks in advance and acts together on the working face support to form a large periodic weighting.The research results can provide a theoretical basis for surrounding rock control under the condition of multi-layer hard roof in extra-thick coal seam mining.
Extra-thick coal seamDouble hard roofSimilar simulationFracture and instability