Warning Index Associated with Rock Burst in Deeply Buried Tunnels

Abstract

Rock bursts are disastrous when occurring during the construction of tunnels in a deep underground rock mass, and they occur under two key conditions: energy release from surrounding rocks; and fracture propagation. Based on the construction process of the excavated segment of the marble around a deeply buried tunnel at the Jinping-II hydropower station (Sichuan Province, China), an analysis was conducted on the whole initiation and propagation processes of 40 rock bursts of different intensities. The analysis was carried out by combining numerical calculation with field microseismic (MS) monitoring. During the analysis, the local energy release rate (LERR) and fractal theory-based calculations were introduced. Furthermore, the dynamic warning index for rock burst risk of deeply buried tunnels in hard rock was established. When the peak LERR reached 2.51 × 105 J/m3 and the energy fractal dimension of MS events was greater than 0.32, there was a high rock burst risk. When the logarithm of the LERR was between 1.25 × 105 and 3.16 × 105 J/m3 and the logarithm of the energy released during MS was increased to 0.25–0.32, a slight rock burst risk arose. When the LERR was less than 105 J/m3, no rock burst risk arose. By applying the aforementioned warning indices to engineering practice, the probability of occurrence and potential risk of a rock burst during subsequent construction operations were decreased. The aforementioned results provide an important theoretical basis for smart and green excavation and safety life cycle solutions under high geostress conditions.