Microseismic Activity Characteristics And Surrounding Rock Stability Analysis Of Underground Caverns During Construction Period

The problems of surrounding rock deformation,rib spalling,falling blocks and rockburst caused by excavation unloading of underground caverns of hydropower stations have brought huge safety hazards to the project construction.Microseismic monitoring technology can capture the precursor information of rockburst and other disasters-microseismic activity.In recent years,it has been widely used in rockburst early warning and surrounding rock stability analysis in underground engineering.Based on the underground caverns of Huangou Pumped Storage Power Station,this paper studies the characteristics of microseismic activity and the stability of surrounding rock during the construction of underground caverns by means of microseismic monitoring and numerical simulation,aiming at the phenomenon of local surrounding rock falling and slight rockburst during excavation.The main research results of the thesis are as follows:?1?The multivariate linear regression method is used to analyze the initial geostress field of the project area.The initial stress field distribution characteristics of the underground caverns are determined,and the rockburst tendency is determined based on the critical depth and strength stress ratio.The results show that the distribution of stress field in engineering area is greatly influenced by geological structure,and the stress differentiation is obvious at fault.The maximum principal stress of underground powerhouse caverns ranges from 15 MPa to 20 MPa,which belongs to medium stress level,and there is the possibility of slight rock burst during excavation.?2?According to the temporal and spatial distribution of microseismic activity obtained during the period of microseismic monitoring for nearly four months,the severe disturbance zone of stress field in excavated surrounding rock is delineated,and the relationship between microseismic activity and construction dynamic response is discussed.At the same time,the evolution laws of source parameters such as energy index,Schmidt number and accumulated apparent volume are studied.The results of microseismic monitoring show that the microseismic activity mainly distributes in the upper side arch shoulder,side wall and the intersection of tailrace branch tunnel and main engine,and mainly distributes along the strike of fault F34 in strip shape.Microseismic activity is closely related to the dynamic response of construction,and influenced by geological structure,the main type of microseismicity is instantaneous microseismicity.Evolution rules of source parameters,such as energy index,schmidt number dropping sharply and cumulative apparent volume increasing,indicate that the stability of rock mass tends to deteriorate.?3?FLAC^3D is used to simulate the excavation process of underground caverns in stages.Distribution of disturbance stress field,displacement field and plastic zone during excavation is analyzed.According to the principle of energy dissipation,the damage model of rock mass element is established by using the relationship between microseismic energy and isotropic releasable elastic strain energy of rock mass considering damage,and the stability analysis of surrounding rock considering microseismic damage effect is realized.By comparing and analyzing the results of microseismic monitoring and numerical simulation,the causes of microseismic activity are revealed.The results show that the stress concentration area,plastic area and deformation area of surrounding rock obtained by numerical simulation are in good agreement with the spatial distribution law of micro-fracture of rock mass obtained by microseismic monitoring.The local stress concentration of surrounding rock under excavation disturbance and geological structure together lead to the occurrence of microseismic activity.When considering the effect of microseismic damage in surrounding rock stability analysis,the analysis results are more in line with the actual engineering situation than those without considering damage,because the information of construction disturbance,faults and fissure zones from microseismic monitoring are fully taken into account.