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.


The construction of underground structures, such as tunnels and caverns in the seismically active region of the Himalayas have generated new thoughts in anticipating the assessing the instability problems posed in such structures. The relatively high tectonic stresses in the area can cause instability in underground structures especially if the rock support requirements do not take into consideration the existence of high stresses in the region.

By virtue of its geographical location on the Southern slope of the Eastern Himalayas, Bhutan is blessed by nature with abundant hydropower potential. Several large hydropower projects are currently under construction in Bhutan and Tala hydroelectric project is the biggest operating hydro power project in Bhutan. The 1020MW hydroelectric project is a joint project between India and Bhutan generating 4865 GWh/yr.

Due to the complex geological conditions in the area the commissioning of the Tala plant was delayed from 2005 to 2008. Druk Green Power Corporation (DGPC) assumed control of Tala in April 2009. The total cost of the project was about 1 billion USD.

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