Experimental Study On Stabilities Of Roof-Coal Pillar Structural Body

In order to ensure the safe and efficient exploitation of coal resources,a large number of coal pillars have been left around the mining areas.The stability of composite system consisting of these coal pillars and their overlying strata determines the safety of the whole mining area,overlying strata and even the ground surface.Once the composite system occurs instability and failure,it will induce many disastrous consequences.Meanwhile,many engineering practices show that the instability and failure of the composite system under the influence of mining is the inducement of rock burst,coal and gas outburst and other disasters.With the transformation and rapid development of infrastructure construction of mining cities,the construction land of related mining cities is becoming increasingly tense,which has become a bottleneck restricting the development of mining cities.In order to alleviate the shortage of urban construction land supply,many mining cities in China begin to explore the engineering construction of coal mine subsidence area,and the surface above the composite system will also be converted into construction land,especially the surface above the composite system consisting of strip coal pillars and their overlying strata.Therefore,it is of great significance to study the stability of the composite system consisting of coal pillar and its overlying strata for understanding,preventing and controlling its instability catastrophe,and realizing the construction and utilization of surface engineering above the composite system.In this paper,the stability of roof-coal pillar(R-C)structural body with closely bonded is taken as the research object,using the methods of theoretical analysis,basic test and numerical simulation,etc.,the strength and macro-failure initiation(MFI)characteristics of the structural body,mechanical behavior of the structural body under different influencing factors,creep characteristics and creep damage evolution of the structural body and strengthening mechanism of filling walls on coal pillar were studied and analyzed.The main conclusions are as follows:(1)Axial compression mechanical models for R-C composites samples with large and small rigidity roof rocks were established.Combined with the deformation incompatibility theory between the roof rock and coal near the interface,the strength conditions of the composite samples were obtained.Based on the interaction theory between roof rock and coal,the strength characteristics of composite sample were discussed.The effects of original macro-defect and single joint in coal on the strength characteristics of composite sample were studied by laboratory test and numerical test,respectively.The results show that the whole strength of composite sample is a comprehensive reflection of the strengths of rock and coal far away from the interface and near the interface,which is affected by the interaction between rock and coal and natural defects in main bearing body of coal sample.(2)According to the macro-failure process of R-C composite sample under uniaxial loading,the macro-failure initiation(MFI)was proposed for the composite sample.MFI indicates the beginning of macro-failure of composite sample.MFI causes the occurrence of turning points on stress curve while the AE energy index reaches a peak value.MFI mode is mainly affected by the natural defect in coal and interaction between the roof rock and coal.MFI modes mainly include the development and evolution of micro-cracks and cracks in coal samples,and the tensile fracture of the roof rock or coal near the interface.At the same time,the elastic energy stored in the solid bearing structure around the original defect in coal can be released quickly through weak surfaces such as macro fracture surfaces,which causes the spalling or ejection failure for coal.Thus the MFI model is the macro-crack accompanying with different degrees of partial ejection or patch in coal.(3)Uniaxial compression tests of R-C composite samples under different rock-coal ratios,loading rates and lithological roof rocks were carried out.The mechanical behaviors of composite samples under different influencing factors were studied by combining acoustic emission system(AE)and digital video camera(DVC)system.Effects of height ratio of roof rock to coal,loading rate and roof rock lithology on whole strength,MFI characteristics and failure characteristics of composite sample were analyzed.Scanning electron microscopy(SEM)system was used to study the fracture morphology of composite sample under different loading rates.(4)Creep tests with different diffeerent loading time of stress level were conducted for R-C composite samples.The creep strength,deformation and failure characteristics of composite sample were studied by combining AE and DVC system.The relationship on creep strength between the composite sample and single coal sample was analyzed.And effects of interaction between roof rock and coal on creep strength of composite sample were discussed.Combined with the AE monitoring data,the creep damage coefficient S is defined and the creep damage evolution law of composite sample was analyzed.(5)A method of enhancing the stability of coal pillar with small-size filling walls was proposed,and the mechanism and feasibility of strengthening coal pillar with filling walls were analyzed by theoretical analysis and numerical simulation tests.The reinforcement effects are mainly reflected in the lateral confinement on coal pillar and sharing coal pillar bearing loads.The simplified models for calculating the supporting pressure and size of filling wall were established,respectively.The reasonable strength and size of filling wall for efTectively strengthening the stability of coal pillar were deduced.Finally,a construction method of filling wall to enhance the stability of coal pillar was put forward.