Tender calls have been issued to build a battery of major tunnels for the Dibang multipurpose project in Arunchal Pradesh state in India.

The project, being developed by National Hydroelectric Power Corp (NHPC), calls for the construction of five 12m wide diversion tunnels totalling 6,260m in length, six 9m wide headrace tunnels with combined length of 2,700m and a similar scale of tailrace construction. An underground powerhouse will include three caverns.

Dibang’s underground works are included in two bid packages: the diversion tunnels in Lot 3, which also includes construction of a dam and intakes structure; and, the powerhouse and both headrace and tailrace tunnels in Lot 4. Deadlines for bids for Lot 4 and Lot 3 are 31 October and 3 November, respectively. NHPC expects Lot 4 will take 78 months to complete and Lot 3 would last 90 months.

The five horseshoe-shaped diversion tunnels will have lengths varying from 1,175m to 1,325m. The six horseshoe-shaped headrace tunnels will vary in length from 300m to 600m and leading to 7.5m diameter circular pressure shafts of 184.8m length.

Of the three caverns, the powerhouse is the largest: 24.5m wide by 54.8m high by 382.8m long. The transformer cavern will be 19m wide by 31.5m high by 325.8m long. The smallest cavern is the MIV: 17m wide by 26.1m high by 277.8m long.