Underground caverns stability and excavatability of rock masses are of significance in geotechnical engineering, in both design and construction stages. Risks associated with underground group caverns of Houziyan hydropower station in southwestern China are growing as a result of continuous excavation-induced unloading. To improve our understanding of the underground caverns instability and resolve the complex subsurface conditions of the highly fractured rock mass, a high resolution microseismic monitoring system was established in the deep underground caverns, pursuing the relationship between the measured data about microseismic activity and excavation damage zones of surrounding rock mass. Through analyzing the tempo-spatial distribution of microseismic activity, excavation damage zones and potential risk regions in underground caverns were identified. Furthermore, the correlation between microseismic activity and pre-existing geological structures, as well as traditional monitoring was analyzed. In order to validate the correlation between seismicity and potential slip surface of underground caverns, a model was implemented into numerical code to further evaluate the deformation and stability of surrounding rock mass. The monitoring results demonstrate that microseismic events mainly occur at high stress concentration regions, which coincides with the results obtained from numerical analysis. Therefore, the comprehensive method incorporating microseismic monitoring and numerical analysis, as well as traditional monitoring and filed observation has been proven to be very promising in deformation and even instability prediction of surrounding rock mass in underground caverns subject to delineation of potential slip surfaces and excavation damage zones.
Myriads of large scale deep underground powerhouses for huge hydropower stations are under construction, at the design or planning stage in the southwest of China; the stability of the surrounding rock mass in underground caverns plays a remarkable role on the engineering safety. The effective prediction and control of the stability of underground caverns are crucial to ensure engineering safety of these projects.
Numerous studies have been conducted to investigate the behaviour of surrounding rock mass and the stability of underground caverns subjected to excavation activities. In past decades, computer methods have been increasingly popular in investigating underground cavern stability. For instances, Feng et al. (2012) proposed an intelligent and dynamic design method for stability analysis of large cavern group, focusing on characteristics of strong excavation unloading of high sidewalls of multi-cavern group. Cai et al. (2007) investigated the AE activities in large-scale underground excavations using FLAC/PFC coupled numerical method. Dhawan et al. (2004) analyzed surrounding rock mass stability on specific hydroelectric projects using finite element method and discrete element method.