Disaster characteristics and mechanisms of the time-delayed rockburst development process in tunnels at 2500 m depth

Abstract

Based on the headrace tunnels of Jinping II hydropower station with a buried depth of up to 2500 m, a database containing 74 cases of time-delayed rockbursts with various intensities was established through investigations and monitoring of a cumulative tunnel section of 30 km. According to the evolution characteristics of microseismic events during the development process of rockburst, the time-delayed rockburst was first classified into two types, including rockburst with and without a “sleep period”. The spatiotemporal distribution and development characteristics of rockburst were statistically analyzed, and the mechanism of rock fracture in the development process of rockburst was revealed. The results indicate that geological structures, structural planes, rock mass integrity and support measures all have an impact on the spatial distribution and intensity of time-delayed rockburst. There are differences in the delay time, delay distance and microseismic evolution characteristics between the two types of rockbursts. During the development process of time-delayed rockburst, the type of rock fractures is primarily tensile, with occasional shear and mix type, and their proportions are influenced by the presence of rigid structural planes. Furthermore, some time-delayed rockbursts are often accompanied by at least one immediate rockburst occurrence at the same location or adjacent area, indicating that time-delayed rockbursts tend to occur in regions where energy accumulation is relatively concentrated. This study can provide important guidance value for the mechanism interpretation and early warning of time-delayed rockburst.