Energy Characteristics of Micro-Seismic Events Associated with Immediate Rockbursts in Deep Tunnels

This study integrates micro-seismic data and data from hundreds of rockbursts of different intensities that occurred in four deep headrace tunnels and one water drainage tunnel at Jinping II hydropower station in Sichuan province, China. The overburden of the studied tunnels is between 1900 m and 2525 m, largely excavated in marble. The tunnels have a total length of 12.4 km. The micro-seismic energy produced during the development of immediate rockbursts excavated with the drill-and-blast method (DBM) and a tunnel boring machine (TBM) were studied. The results indicated that the daily maximum micro-seismic energy can be used as a basis for estimating the rockburst intensity: no rockburst, weak rockbursts, moderate rockbursts, and intense rockbursts, and that the common logarithms of the micro-seismic energy of each level was <4, >4 and <5, >5 and <6, >6, respectively. For the same intensity, rockbursts excavated by TBM and DBM have the same range but different distribution characteristics. The micro-seismic energy can be used as a guide to establish a dynamic warning system based on the evolution of micro-seismic events to reduce the risk of immediate rockbursts during excavation of deep, hard-rock tunnels.