The first two Pelton wheel turbines each of 84 Hi were commissioned late in 1986 for the 336 MY (4x84 MW) run-of-the-river Chukha H.E. Project (Bhutan) located in the geologically young, unpredictable rocks of the Himalayas. Work on the predominantly underground scheme began way back in 1973 when the construction of buildings, roads and other infrastructures in three villages namely Chimakothi, Tsimalakha and Chukha located on the banks of Wangchu river was taken up. Work on the diversion dam and the major underground structures viz., power house cavern, power tunnel, de-silting chambers, valve house caverns and inclined pressure tunnels began in 1978–79. Due to their tectonic behaviour, geologically young age and unpredictable nature, there were rock falls in the power house cavern and power tunnel, land slides near the diversion dam and large scale water gush in the power tunnel and this affected the general pace of the work. The geotechnical problems encountered and the solutions provided thereof have been outlined in the paper. Recommendations regarding the precautions required to be taken for the projects located in similar geological set ups have also been made.
Chukha Hydro-electric project is a 336 MY run-of-the river hydro-electric scheme located in the Thimphu district of Bhutan - a north-easternly neighbour of India. The project features consist of a 40 metre high diversion dam constructed across Wangchu river for diverting a maximum of 120 cubic metre per second discharge into two.semicircular vertical intakes situated on the left bank of the river. Two desilting chambers trap the silt particles down to 0.2 mm size and flush the same back to Wangchu river downstream of the dam via a free flowing steel＿lined silt flushing tunnel consuming 24 cumecs of water in the process. Thus 96 cumecs of silt-free water is diverted into a 6.5 km long 4.9 m × 4.9 m D-shaped power tunnel which terminates into a 1i.2 m dia-meter surge shaft. Two 3 m internal diameter steel-lined pressure shafts take-off from the surge shaft. An underground butterfly valve chamber is provided which houses two butterfly Valves ＿ one for each pressure shaft. Two inclined pressure shafts have been provided immediately downstream of the butterfly valve chamber. Each of the two pressure shafts bifurcates into two separate horizontal pressure shafts of 2.25 m internal diameter to feed four Pelton wheel turbines - each of 84 MV to generate 336 MW of peaking power under a head of 468 metres. The tail water is l＿Cl back to Wangchu river via a 900 m long D-shaped free flowing tunnel. The schematic layout is presented in Fig. 1
GEOLOGY Being primarily an underground project the geological and geophysical tests assumed great importance for evolving safe and economical designs for the underground works. The preliminary geological investigations of the project were carried out in April, 1963. As the works progressed, this information was supplemented with detailed geotechnical explorations providing useful data to the designers.