Research On The Mechanism Of Mine Accidents Induced By The Instability Of Overburden Structure In Shallow Close-distance Coal Seams And Particle Flow Code Simulation

The coal stratigraphy beneath the Shendong mining area is a representative example of shallow and close distance coal seams in China.Roof stepwise subsidence and crushed supports occur during the weighting periods of long-wall mining of the upper coal seam,furthermore the abrupt release of the overburden pressure negatively affects the safety of the mining operation.These accidents seem to be unavoidable as they still occur even if supports with 1.52 MPa resistances are used.The existing roof management theories and techniques cannot account for the atypical nature of the roof movement of the Shendong coal mines,therefore an investigation was conducted on the stability of the structure of the overlying strata.Due to the depletion of the coal in the upper coal seam because of the adoption of high production and high efficiency mining techniques,the majority Shendong coal mines have moved their main mining operations down into the lower coal seam.As the panel of the lower coal seam advances to the area(±5m)directly below the edge of the barrier pillar in the upper coal seam,the roof becomes susceptible to violent overburden pressure induced subsidence.Furthermore,the subsidence may cause the interruption of mining operations because the height of the caved roof becomes smaller than the required height for the coal cutter to operate.A method to control and relieve the extreme overburden pressure is urgently needed to ensure the future safety of mining operations in similar conditions.Rock mass is the discrete medium of the continuum medium,which needs to interact with each other through discontinuous characteristics.This is consistent with the basic characteristics of the Particl Flow Code(PFC).PFC has a unique advantage in simulating and analyzing the movement of overlying strata-it can reproduce the whole process of deflection,separation,fracture,collapse,accumulation and compaction experienced by the overlying strata.And the process of mining fracture propagation can be traced explicitly.Based on the above,the mechanism of overburden structure instability caused by repeated mining in shallow coal seams is studied by using field observation,theoretical analysis,lab scale simulation experiments,and numerical simulation in order to realize the purpose of safe and efficient production.All numerical simulations in this thesis were conducted using PFC.The goal of both the calibration of the microparameters and the preparation of the similar material is to replicate the mechanical behaviors of any material.Furthermore,the commonly used trial and error method of microparameter calibration is both cumbersome and has a low-efficiency.Therefore,this thesis proposes a novel method to calibrate microparameters by using the theory of similarity.The comprehensive understanding of the mechanical behavior of jointed coal mass under uniaxial compression is of vital import in the study of the characteristics and evolution of overlying strata structure in shallow close-distance coal seams.Therefore,an investigation was conducted on the influence of cleat properties on the stress-strain behavior and failure modes of jointed coal masses,and the relationships between Uniaxial Compressive Stress(UCS)and cleat density,and stress-strain behaviors and failure modes.The factors contributing to the various stress-strain behaviors were found,and an analysis of the UCS scale effect was carried out.In the process of mining the upper coal seam,the most commonly observed accidents involve roof stepwise subsidence and support crushing.A model of roof stepwise subsidence was created and theoretical analyses of the model resulted in an explanation for the causes of roof stepwise subsidence.Moreover,numerical simulation,lab scale simulation experiments,field observations were conducted to analyze the behavior and mechanisms behind roof stepwise subsidence.Finally,a reasonable range for support resistances was determined.In the process of mining the lower coal seam,roof stepwise subsidence and the ensuing support crushing will occur once the panel reaches a position that is directly below the edge of the barrier pillar in the upper coal seam.Depending on position of the upper key strata and the horizontal distance between the cutoff(in the lower coal seam)and the edge of the barrier pillar(upper coal seam),the structure of the overlaying strata,above the upper coal seam,can be separated into Voussoir-beams and Step-beams.Furthermore,Voussoir-beams and Step-beams can further be subdivided by cantilever breaking and period breaking of key strata.Mechanical analyses were conducted for each of the possible structures of the overlying strata(based on the above categorizations),moreover mechanisms of roof stepwise subsidence were determined for each scenario.Numerical simulation,lab scale simulation experiments,field observations were conducted to verify the mechanisms for each scenario.To ensure the safety of long wall mining operations in shallow close-distance coal seams,a bottom-to-top analysis was conducted to determine to factors that influence support crushing.A contingency plan to control and relieve overburden pressure was proposed.The contingency plan consists of two possible methods of overburden pressure relief.The first method involves the refilling the mined cavities in the upper coal seam,while the second method involves the release of overburden pressure via blasting.