Fractal characteristics of micro-seismic volume for different types of immediate rock-bursts in deep tunnels

A fractal method is put forward to study the self-similarity of the volume distribution of micro-seismic events during the development of different types of immediate rock-bursts. The proposed method is used to study the fractal behaviours of the volume distribution of micro-seismic events during the development of immediate rock-bursts that occur in four deep headrace tunnels and one drainage tunnel at the Jinping II Hydropower Station. The results indicate that the volume distribution of micro-seismic events during the evolution of immediate rock-bursts displays fractal properties. The fractal dimension of volume can be used as the basis for estimating rock-burst type, that is, the fractal dimensions of immediate strain rock-bursts are >0.7 and <1, but those of immediate strain-structure slip rock-bursts are >0.2 and <0.6. For the immediate strain rock-bursts and the immediate strain-structure slip rock-bursts, if the intensity is lower, the fractal dimensions of volume will be smaller. For the immediate strain-structure slip rock-bursts, the more the number of structure planes, the smaller the fractal dimensions of volume. These conclusions can be used as the guideline to develop a warning system and to reduce the risk of rock-bursts during construction of deep, hard-rock tunnels.