Characteristics of the microseismicity resulting from the construction of a deeply-buried shaft

The China Jinping Underground Laboratory phase II (CJPL-II) is currently the world's deepest laboratory with a maximum overburden over 2.4 km. For this project, a deeply-buried shaft was constructed on the floor of the existing large-span laboratories under conditions involving high geo-stress. To explore the failure characteristics encountered and optimize the excavation and support scheme of the shaft, an in situ microseismic (MS) monitoring experiment was carried out. Firstly, an appropriate monitoring scheme was developed for this special project (i.e. the details of the sensor array required, velocity model, and system protection). Secondly, the location accuracy of the monitoring system was analyzed. On this basis, the effect of excavation and support on the fracturing processes induced in the surrounding rock was analyzed by interpreting the MS information. Thirdly, the spatial relationship between the potential damage zones and direction of the maximum principal stress was validated by analyzing the spatiotemporal evolution of the MS activity with the help of numerical simulations. In addition, some suggestions for excavation and support schemes are provided based on the monitoring results. Hence, the MS monitoring scheme and results described in this paper constitute a reference for construction design and MS monitoring of similar projects.