Research On The Evolution Of Fracture And Disaster Incubation Mechanisms In Ununiform "Bow-shaped" Ultra-thick Key Stratum

The Gaojiapu Coal Mine in Binchang Mining Area,Shaanxi Province,is a typical deep mine with a depth of over 1 km.Multiple mining pressure impacts occurred during the first and second mining panels under fully mechanized mining conditions with an average mining thickness of about 10 m.The maximum surface subsidence coefficient was only 0.05.This is closely related to the overlying ultra-thick key stratum(UTKS),with an average thickness of about 260 m.The UTKS is thick and strong and can maintain a suspended state for long.Once damaged,it will lead to serious disasters.In addition,the thickness variation of the UTKS within the mining area leads to localized high-stress concentration in some areas,posing a higher impact risk.In order to effectively prevent disasters caused by the UTKS,it is necessary to clarify the failure forms and hazard mechanisms of the UTKS and understand the effect of thickness changes in the UTKS on mining pressure hazards in the mining area.This thesis uses theoretical analysis,laboratory tests,physical simulation,numerical simulation and field measurement methods to systematically study the evolution of rupture and hazard mechanisms of non-uniform UTKS.The main research results are as follows:(1)Based on the Reissner thick plate mechanics theory,the theoretical solution of stress in the UTKS is derived,and the Mises yield stress analytical equation is solved.A thick plate mechanics calculation program for UTKS is developed to analyze the influence of shear and compression deformation of strata bodies within UTKS on stress states and to explore the thickness and suspended span of UTKS on corresponding stresses.The deformation,stress distribution and yield characteristics of UTKS during the mining process are grasped.(2)The evolution rules and hazard mechanisms of rupture of UTKS are revealed.A similar simulation experiment method of microfracture monitoring based on carbon fibre modified material(CFSM)is proposed,and the corresponding relationship between CFSM resistance change rate and acoustic emission damage coefficient is calibrated.A large-scale similar simulation experiment model for multiple mining panels is constructed to clarify the evolution rules of microfractures in UTKS under a long-term suspended state and to grasp the bending and rupture characteristics of UTKS.Furthermore,a large-scale numerical model for multiple mining panels is constructed to analyze the rupture motion characteristics of UTKS,study the rules of elastic energy accumulation and release under different mining scales of UTKS,discuss the influence of thickness and coal seam spacing of UTKS on energy release characteristics,and analyze the vibration phenomena caused by concentrated energy release during rupture of UTKS to reveal the energy release characteristics and hazard mechanisms of UTKS.(3)The abnormal concentration rule of stress in the lower coal pillar under the "bow-shaped" thickness variation form of UTKS is revealed,and the influence rule of the "bow-shaped" form on the rupture characteristics of UTKS is clarified.A mechanical model of the "bow-shaped" reserved form of UTKS is established.The stress and elastic energy variation equations of coal pillars under the "bow-shaped" form are solved.The influence of the "bow-shaped" form on the distribution of stress elastic energy in the coal pillar area is analyzed clearly.The influence of the "bowshaped" form on the rupture characteristics of UTKS and the transmission path of the overlying load is explored.The "step-shaped" distribution characteristic of mining stress in large roadway coal pillars is discovered under an "bow-shaped" form.The reasons for frequent mining pressure impacts in large roadway coal pillars under "bowshaped" form are discussed.The working mechanism of the "bow-shaped" form on mining pressure impacts in one mining panel large roadway coal pillars is clarified.(4)A prediction method and control measures for disasters caused by non-uniform "bow-shaped" UTKS are proposed.Based on the influence of the "bow-shaped" form on rupture characteristics and stress distribution of UTKS,the high-risk mining areas of 205 working faces in Gaojiapu Coal Mine are successfully predicted.Based on the hazard mechanism of energy release caused by rupture of UTKS,control schemes of reducing the suspended span of UTKS and forming a supporting body system of composite support of isolated coal pillar-mined-out area backfilling are designed from two aspects.The risk relief effects and application conditions of these two schemes are simulated.It is determined that the width of large coal pillars between the No.2 and No.3 panels should reach more than 200 m.The research results guide safe production under non-uniform "bow-shaped" UTKS in Gaojiapu Coal Mine.In-situ measurement of rock mass migration within UTKS is carried out to verify the research results.The research results of the thesis guide the engineering practice of 205,301 and302 mining faces and provide theoretical references for safe mining under similar UTKS conditions in deep mines.It has important theoretical and engineering value.The thesis contains 137 figures,14 tables and 226 references.