Research On The Mining-Induced Seismicity Caused By Extra-Thick Overlying Stratum And The Hazards Triggering Mechanism

Mining-induced seismicity(MIS)caused by extra-thick overlying stratum is the MIS induced by the fracture and movement of the extra-thick overlying stratum above the coal seam and resulting in obvious seismic effects or dynamic failures.In recent years,with the increase of mining depth and mining intensity,MIS caused by extra-thick overlying stratum has occurred frequently in the major mining areas of China,which has attracted great concerns.In this dissertation,the main types and characteristics of MIS caused by extra-thick overlying stratum,the mechanism of MIS caused by extra-thick overlying stratum,the macro-and microscopic cracking process of the extra-thick overlying stratum and the influencing factors,the hazard-causing mechanism and risk assessment of MIS caused by extra-thick overlying stratum,the split-source seismic reduction-hazard prevention effects were studied by means of field investigation,theoretical analysis,discrete element numerical simulation,physical simulation test and engineering practice.It is shown that there is a key layer or a group of key layers in the roof that has a controlling effect on the occurrence and distribution of MIS caused by extra-thick overlying stratum,which is called the key stratum of mining-induced seismicity(KSMIS).The KSMIS must be a key layer,while the key layer is not always a KSMIS.The location of the main key layer,the mining height and whether it is in full mining can be combined to initially determine the location of the KSMIS.Based on the characteristics of the KSMIS,we can subdivide the MIS caused by extra-thick overlying stratum into three types: far-field rock layer type,near-field rock layer type and near-field rock group type.The low-frequency components are absolutely dominant in the far-field rock layer type and the near-field rock group type,and the high-frequency components are more abundant in the near-field rock layer type;the time-frequency diagram of the near-field rock layer type is "narrow and long",the time-frequency diagram of the near-field rock group type is "flat",and the time-frequency diagram of the far-field rock layer type is in between.The seismic duration is from large to small: far-field rock layer type,near-field rock group type,and near-field rock layer type,and the main seismic duration of the far-field rock layer type and near-field rock group type is significantly higher than that of the near-field rock layer type.The main vibration frequency of the far-field rock layer type is slightly larger than that of the near-field rock group type,and the main vibration frequency of the near-field rock group type is more discrete and has more frequency centers.The mechanism of KSMIS strength failure induced MIS was revealed.When the actual maximum stress exceeds the ultimate strength of the KSMIS or structural contact surface,the KSMIS breaks or the structure loses stability,releasing a large amount of elastic energy and potential energy and forming a MIS caused by extra-thick overlying stratum.The stress distribution characteristics and influencing factors before the initial breakage of the KSMIS were analyzed,and the form of instability of the hinged structure after the initial breakage of the KSMIS and the calculation method of seismic energy were discussed.The bending deformation of the KSMIS is “stratified” and is broken in a gradual manner,and the number of tensile seismicity is always greater than that of shear seismicity in the process of breakage.The influence of the thickness,strength,distance from the coal seam,number of stratification and number of working faces on the breakage of the KSMIS was obtained.The mechanisms of coal burst and ground damage induced by the MIS caused by extra-thick overlying stratum were explained,and the criteria and risk assessment method for identifying the coal burst and ground damage were proposed based on energy and vibration velocity,respectively.The ratio of the residual energy per unit area of vibration wave propagation to the ultimate energy per unit area of the support structure was defined as the risk factor,and then the radius of the strong,medium and weak risk zones of the MIS caused by extra-thick overlying stratum were determined respectively.The effects of MIS intensity,propagation distance,mining fractures and support strength on the hazard-causing effects of the MIS caused by extra-thick overlying stratum were analyzed.The seismic reduction effect of the weakened seismic source,the disaster prevention effect of the deteriorated propagation path and the disaster prevention effect of the reinforced support were studied for the MIS caused by extra-thick overlying stratum.The research results were applied and verified at the 63 Upper 06 working face in Dongtan Coal Mine,and its application effect is remarkable.There are 166 figures,17 tables,and 285 references in this dissertation.