Study On Rockburst Nucleation Process And Early Warning Method Of Deep-buried Tunnels

Under the rapid development of national social economic environment in our country, the requirement of resources becomes increasingly expanded as well as the resources exploitation scale. Exploitation and construction tend to deeper within the rock mass in the field of water resources and hydropower, mining, national defense industry and transportation. The geological disaster problems faced in deep rock engineering are becoming increasingly challenging due to the high in-situ stresses for deep rocks. Rockburst, as one of the most common and prominent one in the underground engineering, which performs as the phenomenon that the surrounding rock catapults and casts to the excavation space abruptly and fiercely. Such sudden hazard always leads to the failure of support facilities, large deformation of the excavation outline and surface subsidence, which will further result in the huge losses of economy and casualties. Moreover, some rockbursts with high magnitude may lead to other disasters such as local earthquake and ground surface subsidence. So far, the rockburst nucleation mechanism and prediction have been a key scientific problem and technical difficulty in the field of rock mechanics. Therefore, it is of great engineering interests and significance to study the rockburst nucleation mechanism, prediction and early warning in the condition of high in-situ stresses for deep-buried tunnels.Previous studies demonstrated that there are close relationships between rock failure (rockburst) and its internal microseismic activities. Abnormal performance of microseismic activities always indicates the premonition of rockburst. In this study, the Yalongjiang Jinping Ⅱhydropower station is used as the engineering background and the deep-buried tunnels which are in the condition of high in-situ stresses are selected as the monitoring object. A microseismic monitoring system which has the characteristics of independence, easy installation and flexible moving is established in the deep-buried tunnel to investigate the behaviors of rockburst. The spatial-temporal distribution characteristics of microseismic activities and the research achievements of rockburst mechanism were combined, real-time monitoring,analysis and early warning of rockburst are conducted in the diversion tunnels and drainage tunnel in Jinping Ⅱ hydropower station. The main achievements are list as follows:(1) The microseismic monitoring system is successfully established in the deep-buried hard rock tunnels for Jinping Ⅱ hydropower station. Real-time monitoring, analysis and early warning of the potential rockburst zone in the diversion tunnels and drainage tunnel are preliminarily realized. Considering the complex geological structure condition, geomorphic features, special construction methods and construction sequence of Jinping hydropower station, a proper monitoring program including the construction of monitoring system, sensor installation and improved method, the layout of sensor array and optimization, the classification and establishment of the noise database is formulated. Continuous real-time monitoring of tunnel rockburst is realized by collecting the micro-fracture data. The digital information of seismic activities in the process of rockburst induced by progressive failure of rock mass can be obtained after the integration of data.(2) Based on large numbers of recorded rockburst cases and microseismic monitoring data in the tunnels at Jinping II project, the development process and occurrence regularity of strainburst and tectonic rockburst are carried out study. According to microseismic activity related parameters, including microseismic accumulated counts, mircoseismic consumed energy and those density ect., microseismic activity regularity and dynamic migration characteristics of different types of rockburst are summaried and extracted during the rock failure process. The internal relationship between rockburst and microseismicity was revealed during the surrounding rock gradual failure progress.(3) According to the spatial-temporal distribution characteristics of microseismic activities, the microseismic monitoring technique and numerical simulation method are conducted to study the damage progress of underground excavation from the view of dynamic crack propagation, e.g. micro-fracture initiation, extension, development and interaction and failure. Moreover, the rockburst nucleation and development process can be divided into three stages and three failure modes by considering crack expansion progress and microseismic activities behaviors. And detailed germination mechanism and failure characteristics in each stage are analyzed. In addition, from the view of seismology theory, the precursor of micro-fracture in the rockburst nucleation process is analyzed by considering the evolution regular pattern and variation characteristics of microseismic energy, microseismic magnitude, cumulative apparent volume, energy index and b value.(4) The multivariate early warning method of rockburst is propoesd for deep-buried tunnel in condition of high in-suit stresses in term of the investigation of evolution regular of mircoseisms parameters, including microseismic count density, microseismic energy density, stress buildup, stress shadow and stress transference (called 3S theory), apparent volume energy index and b value under various rockburst nucleation stages.