ISSN : 2277-1328 (Online) - ISRM

16 ISRM (India) Journal
ENGINEERING GEOLOGICAL AND GEOTECHNICAL
EVALUATION OF DAM SPILLWAY (II-C) OF BUNAKHA
HYDROELECTRIC PROJECT, BHUTAN HIMALAYA
A.K. Naithani, L.G. Singh and Devendra Singh Rawat
National Institute of Rock Mechanics, Kolar Gold Fields, Karnataka
P.C. Nawani
Former Director, NIRM, H.No. 273, Nirvana Country, Sector 49-50, Gurgaon, Haryana
ABSTRACT
The 193.3 m long and 64 m wide spillway of Bunakha Hydroelectric Project is proposed 89.50 m from the
right abutment to pass the flood discharge and lies mainly on the right abutment and partly in the river
section. The spillway area was investigated through detailed engineering geological mapping, exploratory
drilling and laboratory testing. The rock mass properties, i.e., joint sets, weathering grade, RQD, RMR,
permeability etc. of the rock masses to be encountered during the excavation of spillway have been
analyzed in detail. Core samples from the exploratory drill holes drilled at the spillway alignment were
tested for physico-mechanical properties of rocks in the laboratory. The mapping details indicated that the
major rock types which are exposed to the surface and cover the entire spillway area are foliated gneisses
with bands of banded gneisses. On the basis of detailed investigations and laboratory testing, inferences
and recommendations have been made which will be helpful during the construction of the project.
Keywords: Hydroelectric, Rock Mass Rating, Spillway, Thimphu Formation, Bhutan
INTRODUCTION
The proposed Bunakha Hydroelectric Project (BHEP) for
generation of 180 MW (3 x 60 MW) hydropower is located
near village Bunakha, in Chukha Dzong (District) in the
Western Bhutan, covering toposheet No. 78E/11 & 78E/
12 India and adjacent country series. BHEP is the upper
most major project in the development of the power
potential of the Wang Chhu river in Bhutan. Wang Chhu is
formed by the confluence of two major snow-fed rivers
viz. Paro Chhu and Thimphu Chhu, join at an elevation of
2068 m at Chhuzom, i.e., upstream from the proposed
project site. BHEP is a storage project utilizing the head
upstream of the existing Chukha HE Project (336 MW)
and the confluence of Paro Chhu and Thimphu Chhu. This
proposed storage scheme envisages important structures
such as 198 m high concrete dam, spillway, intake, steel
lined pressure shafts partly along the slope of the nonoverflow
section and partly through the underground
excavation, power house and a tail race channel. The
estimated gross storage of this project is about 237.52
MCM. In the spillway area, river Wang Chhu flows in 170 o
direction and takes a south-westerly turn after the
confluence with Sherjalum Chhu. The confluence of
Sherjalum Chhu with Wang Chhu is at an elevation of
1848 m. In this paper attempt has been made to bring out
the rock mass condition of the foundation of the spillway
of new dam axis (II-C) on the basis of detailed engineering
geological mapping, geological logging of drill holes, rock
mass permeability values, and laboratory test result. The
basic purpose of these investigations was to identify /
map different rocks and structures like joints, shear zones,
faults, fracture zones etc and to determine engineering
properties of rock by lab testing. Rock mass classification
using Rock Mass Rating (RMR) system [Bieniawski, 1979,
1989] was attempted. The rock mass classification can
be used for estimating the unsupported span, the standup
time and the support pressures of an underground
opening. It can be also be used for selecting a method of
excavation and permanent support system.
The spillway is proposed 89.50 m from the right abutment
of the dam to pass the flood discharge and lies mainly on
the right abutment and partly in the river section. The
length of spillway is 193.30 m (up to Flip Bucket) and the
overall width of the spillway is 64.00m. The crest of the
spillway is at EL 1917.50 m. The flood/MPF through the
spillway will be regulated by means of six gates each of
size 6.0 m (width) x 9.0 m (height). The waterway for the
spillway has been designed to cater to the Maximum
Probable Flood (MPF) with a peak discharge of 10028
Cumecs and the total flood volume of about 845 million
Volume 1 No. 2 July 2012