Research On Disaster-pregnant Characteristics And Evolutionary Disaster-causing Mechanisms Of Rock Burst Under Large-scale Stope Structure

Deep mining of coal deposits is an unavoidable trend.Mining conditions and stope structures get increasingly complicated as mining depth and intensity increase,and regional-local dynamic and static stress fields are stacked.Specifically,the activation of large-scale fault structures,the movement and fracture of the high-level thick and hard roof,and the long-distance disturbance,all combined with the high concentration of stress within the large-scale stope structure.As a result of these factors,there is an increasing risk of serious and complex rock burst in coal mines;rock burst accidents still occur on a regular basis;the disaster-causing mechanism of the coupled evolution of large and complex rock burst is unknown;the characteristics of precursors are unknown;and there is a need to improve the pertinence and accuracy of prevention and control.The primary scientific question addressed in this dissertation is how Rock burst under Large-scale Stope Structure（RLSS）evolves into a disaster and what factors lead to it.To this end,a variety of research methods,including survey statistics,physical simulation,theoretical analysis,numerical simulation,and field study,are used to carry out in-depth analysis of the disaster-pregnant characteristics of RLSS,to study the damage characteristics of the tunnel surrounding rocks with the multi-parameter spatio-temporal response rules under varied impact-level,in addition to simulating the disaster evolution process caused by the RLSS,revealing the disaster-causing mechanism of RLSS,and providing recommendations for the precise prevention and control of RLSS in remote areas.The main research work is as follows:The disaster-pregnant characteristics of RLSS were defined and clarified,and the key factors of RLSS were investigated,based on survey and statistical analysis.The study found that the far-field large-scale stope structure of the overlaying area is abnormally altered by the collective action of numerous goafs in the stope.Due to the high static load stress environment in the near-field and the high-level dynamic loads released by these fluctuations,the roadway sustains significant damage.These are the typical manifestation traits of RLSS rupture.Three categories comprise the key factors:fundamental characteristics and required conditions（impact tendency）;static load sources and breeding conditions（coal pillars,structures,etc.）;and dynamic load sources and starting conditions（large fault structures,high-level,extremely thick hard roofs,etc.abnormal changes）.Under various impact levels,a large-scale physical simulation test of meter-scale"true three-axis static load+dynamic load"impact instability was conducted to investigate the features of impact instability and multi-parameter spatio-temporal response rules of the three-dimensional tunnel surrounding rock.This test revealed the far-field secondary damage evolution occurs as a result of a high-level dynamic load on the tunnel’s surrounding rock structure.The results show that under the action of high energy level impact,the surrounding rock of the tunnel shows significant nonlinear progressive secondary damage characteristics,two quick non-linear rises of"rising-steady-sharp rise-falling"were seen in the stress of the surrounding rock of the roadway,and at the time of the second increase,the stress state also shifted from compressive strain to tensile strain.After several large-scale fissures,the rock finally gave way,obstructing more than two thirds of the tunnel’s length with broken coal.During the secondary damage of the roadway,there are a large number of signals,the acoustic emission signal from the surrounding rock is extremely active in the tunnel,with significant increases in count and energy per unit time,longer signal duration,and decreased attenuation rate;The electromagnetic radiation signal shows a changing trend of"rising-steady-sharp rise-decline",which is consistent with stress changes;The waveform of the micro-seismic response exhibits directionality.In the temporal domain,there is less energy attenuation and a notable increase in the vibration speed and instantaneous peak energy in the Z direction relative to the X and Y directions;In the frequency domain,the main frequencies in the X and Z directions decrease but the response frequency range increases,and the upward change in Y is not obvious;Regarding the distribution of energy within the frequency bands,the Z-direction is focused in the high-frequency band,while the X-and Y-direction energy is concentrated in the middle and low frequency bands.With the development of a large-scale stope structure model at the mine size and a large-scale model block modeling approach,the spatial relationship between the stope structure and subsurface stratigraphic structure was mostly restored;Based on the continuous-discrete(FLAC^3D-PFC^3D)coupling numerical method,the simulation reproduces the disaster evolution process caused by RLSS,and analyzes the effects of the far-field stope structure and the near-field tunnel surrounding rock structure on the dynamic load stress propagation and control of disaster-causing processes.The findings demonstrate that several goafs in the stope structure work together to create a pressure relief zone,which causes the high-level thick and hard roof of the overlying space structure to flex and shatter extensively;Dynamic load stress causes tunnel damage in the range of 0～50 m,60～90 m and 120～140 m in front of the working face,and the deformation exceeds 0.9 m;The closer the wave impedance ratio of adjacent formations is to 1,the smaller the energy dissipation and the higher the transmission efficiency;The higher the strength of the surrounding rock of the tunnel,the greater the energy absorption capacity of the structure and the stronger its ability to resist deformation.A theoretical fracture mechanics model was established for high-level thick and hard roof by employing numerical solution to determine their fracture energy.The dynamic impact effect caused by the conversion and release of dynamic load energy was studied,the coupling process of dynamic load stress and surrounding rock structure was analyzed,the energy criterion of rock burst catastrophic energy was established,the dynamic damage index of rock burst was proposed,and the disaster-causing mechanism of RLSS is revealed.The findings demonstrate that variables including roof thickness,lithology,and the structural thickness-to-span ratio are related to the breaking energy of a high-level thick and hard roof;The produced dynamic load stress is combined with the static load of the tunnel surrounding rock,resulting in changes to the coal’s and rock’s physical characteristics,an expansion of the plastic failure zone’s area,and a shift in the vertical stress peak’s location;There are two phases to the tunnel surrounding rock structure’s reaction to dynamic load stress:asymptotic deterioration and plastic dissipation.The rock burst dynamic damage index is defined as the ratio of plastic energy dissipation to impact kinetic energy.Based on this,the rock burst is divided into three categories:high static load dominant type（1～2）,dynamic and static coupling type（2～100）and dynamic load dominant type（≥100）.The type of RLSS is the dynamic load dominated type.The reaction law of precursors to rock burst under big stope structure settings was investigated based on stress,electromagnetic radiation,and micro-seismic monitoring data collected during on-site mining.Taking a certain mine as an example,it further explained the evolution and disaster-causing process of rock burst under large stope structure conditions,and then proposed an idea for precise prevention and control of rock burst in remote areas.The results show that the shallow stress decreases before the occurrence of rock burst,while the deep stress increases.The electromagnetic radiation intensity shows a trend of"rising stable sharp rising falling".The micro-seismic results show that after the breaking of the high-energy event,multiple secondary high-energy events appear near the roadway in a short period of time;The disaster-causing mechanism of RLSS provides a good explanation for the occurrence and evolution process of rock burst in a certain mine;By using a kilometer directional drilling rig to construct directional long boreholes and branch boreholes in the area of dynamic load sources or dynamic load stress wave propagation paths,energy sources can be weakened at a larger spatial scale,propagation paths can be cut off,and high stress in surrounding rock can be unloaded,thereby achieving precise remote prevention and control of RLSS.The research findings presented in this dissertation have effectively illuminated the evolutionary disaster-causing mechanism and disaster-pregnant traits of RLSS.It has practical value for guaranteeing the safe mining of deep coal resources and energy in China,as well as significant theoretical significance for advancing and enhancing the theory and prevention technology of rock burst in coal mines.The dissertation contains 108 figures,12 tables,and 198 references.