Abstract: Excavations of large-scale hard-rock underground caverns with high stress often result in rib spalling and collapse of surrounding rock, threatening the safety of on-site builders and equipment. From the perspective of reducing the risk of local failure of surrounding rock, framing excavation method was adopted in the fourth floor of the right transformer chamber of the Baihetan hydropower station. In order to study the microseismic (MS) characteristics and stability, MS monitoring was carried out in the right transformer chamber. The results show that: By the excavation of upstream rock, the number of MS events and energy release were greater than the downstream rock. In the process of working face advancing, the number of MS events and the energy increased, which was in accordance with the overlay abutment pressure. When the upstream rock was excavated, the excavation unloading induced seismic events and the cluster phenomenon. In contrast, the excavation of downstream rock only induced few MS events. The Schmidt number, the activity rate, the cumulative volume, and the energy index were combined to evaluate the stability of the study area. The study found that compared with the strain hardening phase, the rock mass in the stage of strain softening has a greater risk of unstable deformation and failure. The results can provide valuable reference to the optimization of construction scheme of the Baihetan hydropower station. At the same time, the results can also provide reference to other similar projects of construction.

Key words: high stress, framing excavation, local deformation, stability evaluation