Stability analysis of surrounding rock of a large hydropower cavern in the Himalayas

The stability of a large underground powerhouse in the Himalayas has been evaluated. This underground powerhouse cavern constitutes a major component of a Hydro Electric Project in Bhutan. It has experienced a number of instabilities during and after construction. Approximately 5 percent of the bolts in the powerhouse are reported to have failed and the walls of the cavern are continuing to converge, albeit at a slow rate since its completion in 2008. Plans are underway to stabilize this important underground structure. The cavern is located in the complex geological conditions of the Himalayas and close to a high stress thrust zone known as the Main Central Thrust (MCT). This paper evaluates the stability of the powerhouse and discusses measures to strengthen the cavern. Both finite and distinct element analysis are performed to better understand the behavior of the rock mass surrounding the cavern. The results from numerical simulations showing convergences in the walls of the cavern are in fair agreement to those observed at the site. An energy-absorbent support system is recommended for the rehabilitation of the side walls of the cavern.