Characteristics of Microseismicity during Breakthrough in Deep Tunnels: Case Study of Jinping-II Hydropower Station in China

Rockbursts are a common form of disaster that occur during the construction of deep tunnels in hard rock. This is especially the case in the breakthrough stage of excavation, when even more attention should be paid to the risk of rockburst in order to ensure construction safety. This work studied the characteristics of the microseismicity associated with 10 breakthrough cases in the deep tunnels (maximal depth 2,525 m) of the Jinping-II Hydropower Station in China. The results showed that the microseismicity was relatively more active in the breakthrough period (compared with that in adjacent sections) due to the effect of working two faces in tandem and was concentrated in the breakthrough section. Furthermore, the characteristic c-value associated with the seismic energy-potency relationship was larger, indicating that the apparent stress was greater in the breakthrough section. Spatiotemporal changes in microseismicity that are associated with rockburst development were found in the breakthrough section which can be used to qualitatively warn of the risk of rockburst. Rockburst risk can be quantitatively assessed during tunnel breakthrough based on the monitored microseismicity and a quantitative method of rockburst warning. Based on results thus obtained, the rockburst risk in the breakthrough section was found to increase continuously as the distance between the two working faces decreased. When both working faces are excavated in the breakthrough section, the quantitative risk of rockburst increases significantly. However, when only one working face is in action, the quantitative risk of rockburst increases only slightly. The results of this work will be helpful in warning of impending rockbursts, and thus improving the safety of the construction process, when breakthrough is carried out in deep tunnels excavated in hard rock. (C) 2019 American Society of Civil Engineers.