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Long term stability of an underground cavern is significant for construction engineers. For structures constructed in the geologically active regions of the Himalayas, continuous monitoring is required to ward off the sustained threats due to seismic activities and associated local geological hazards. Underground powerhouse of Tapovan Vishnugad Hydropower Power Project (TVHPP), Chamoli, Uttarakhand is one such powerhouse that encountered various types of rockmass failures both during and post construction. The project area is located about 2.0 km north of the Main Central Thrust (MCT) and the rockmass cavern has numerous joints, shear and seam zones and fractures. This pose threat to the long term stability of this underground powerhouse cavern. To analyse the rockmass stability of the powerhouse, real time microseismic monitoring system was installed in this underground cavern. Spatio-temporal variation of the recorded seismic events has been analysed for microseismic events distribution in terms of seismic energy, displacement, cumulative apparent volume and various other contours led to the identification of potentially hazardous zones in the underground rockmass structure. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The study of micro-cracks, shear zone and redistribution of stress after excavation in the rockmass is required for a stability analysis of the underground structure. The powerhouse of the Tapovan Vishnugad Hydropower Project (TVHPP) of NTPC Ltd. is located in the seismic zone V of India and is positioned at the junction of the middle and higher Himalayan region under challenging geological conditions. The powerhouse cavern constructed by drill and blast method encountered a number of rockbursts of various intensities during and after construction. So, a real-time remote microseismic monitoring network was used to evaluate the stability of the powerhouse cavern rockmass. Strata condition, rockburst occurrence during construction and drilling feasibility for sensor installation were studied for an array of designs of the microseismic monitoring network. The primary monitoring period demarcated the micro-crack locations and probable failure zones in the powerhouse rockmass by processing and analysis of various temporal and spatial variations of microseismic parameters to evaluate the underground powerhouse structural stability. Although a large number of microseismic events occurred in the powerhouse which resulted in high displacement in a few areas, due to the low amount of energy released from the events occurring inside the powerhouse, the structure appears to be stable. [ABSTRACT FROM AUTHOR]