Dynamic Stability Evaluation of Underground Powerhouse Cavern Using Microseismic Monitoring

Abstract

Construction of any excavation adds to the re-distribution of stresses around abutment and crown. This is stabilised by providing extra reinforcement. When a large underground cavern is excavated, stress re-distribution around abutment is obvious. Underground powerhouse cavern of Tala Hydropower Plant with adverse geological setting is one such cavern which encountered failure of a number of rock-bolts and series of other types of rockmass failure during its construction. It puts a question mark on the long-term stability of this powerhouse. To evaluate the dynamic stability of the rockmass (against induced stresses) around this powerhouse, microseismic monitoring network was installed. Microseismic data has been analysed in terms of static and dynamic parameters to assess the overall cavern stability. Spatio-temporal analysis of microseismic events could identify the potential risk zones which has significant cluster of events. To assess the overall stability due to these induced events, the ratio of transverse wave and longitudinal wave energy of these events was used as an index. It is found that cluster of events had shear failure. These events are basically of small non-damaging type and aid in releasing the built-up induced stresses. The slope of log–log plot of energy versus moment has a very high value. This indicates a stiffer rockmass which adds to its capability to absorb the induced stresses and aid to the overall stability of the region. Analysis reveals that no major seismic events are expected around this rockmass cavern and there is no dynamically unstable region against induced stresses.