Microseismic monitoring and stability evaluation for the large scale underground caverns at the Houziyan hydropower station in Southwest China

The stability of underground caverns and the excavatability of rock masses are important for geotechnical engineering practices during the design and construction stages. The risks associated with underground caverns at the Houziyan hydropower station in Southwest China are growing due to excavation-induced unloading. To assess the instability of underground caverns and resolve the complex subsurface conditions of the highly fractured rock mass, a high-resolution microseismic monitoring system was established in deep underground caverns. This system was used to determine the relationship between the measured microseismic activities and the excavation damage zones of the surrounding rock mass. The excavation damage zones and potential risk regions in the underground caverns were identified by analyzing the tempo-spatial distribution of microseismic activities. In addition, the correlation between microseismic activities and pre-existing geological structures was determined, and traditional monitoring results were analyzed. To validate the correlation between seismicity and the excavation damaged zones of the underground caverns, a numerical model was employed to further evaluate the deformation and stability of the surrounding rock mass. The monitoring results demonstrate that microseismic events mainly occurred in high-stress-concentration regions, corresponding with the results obtained from the numerical analysis. Therefore, this comprehensive method, which incorporates microseismic monitoring, numerical analysis, traditional monitoring and field observations, is promising for predicting the deformation and instability of surrounding rock masses in the underground caverns subjected to excavation.