Run of River Project in Bhutan : Tala Hydroelectric Project
The Tala Hydroelectric Project is the largest run – of – the – river scheme of the Royal Kingdom of Bhutan. It is located in Chukha Dzongkhag in the Western Bhutan Himalayas. It envisages construction of 92 m high concrete diversion dam across the river Wangchu intercepting a drainage area of about 4,028 sq km and carrying intake water through three egg shaped desilting chambers, each of size L = 250m, B = 13.925m and Headmistress = 18.5 m, to remove silt particles larger than 0.20 mm size, 22.884 km long 6.8 m diameter modified horse shoe shaped concrete lined Head Race Tunnel on the right bank of the river Wangchu with design discharge of 142.5 m3/sec. The Head Race Tunnel is provided with four intermediate D- shaped adits of 7 m x 7 m diameter for facilitating the construction of the tunnel. A 180 m high, 15m diameter under ground restricted orifice type surge shaft is provided at the down stream end of the Head Race Tunnel to alleviate any overpressure from 863 m of water head. It will be connected by two inclined pressure shafts, each 992 m long and 4 m diameter trifurcating near the power house (L=190m, B = 18m and H = 43 m) with an installed capacity of 1020 MW (6 x 170 MW). The water will be discharged back into the river Wangchu through a tail race tunnel 2.2 km long, 7.5 m diameter.
Commissioning of the plant was planned for June 2005, but because of geological problems this has been delayed until June 2006. Cost estimates have been revised, too. The project was originally estimated at BTN14 billion (the Bhutanese Ngultrum is equal to an Indian Rupee), equivalent to $0.3 billion, but this has been increased in stages to around BTN40 billion.
The Tala Hydroelectric Project area falls within the central crystalline belt of Thimphu Formation and meta-sediments of Paro Formation. The Thimphu Formation comprises a variety of granitoid rocks, such as migmatite, augen-gneiss, banded-gneiss, granite-gneiss, schistose rocks with subordinate quartizite and marble bands. The Paro Formation consists of high grade calcareous rock and meta-sedimentaries such as marble, calc-silicate rock, quartzite, quartz-garnet-staurolite-kyanite-silimanite schist, graphite schist etc. with subordinate felspathic schist and gneiss bands.
The desilting complex mostly lies in biotite-gneiss with intermediate schist bands. The rock is thinly to moderately foliated with several sets of intersecting joints. The orientation of the foliation joints is almost parallel to the cavern alignment.
HYDROELECTRIC PROJECT CONSTRUCTION CONTRACTS
The plant was supplied by Bharat Heavy Electrical Limited of India, with other Indian contractors including M/s Hindustan Construction Company, M/s Larsen and Toubro and M/S Jaiprakash Industries.
Five main contracts were awarded for construction: the dam itself and first 6.4km section of the headrace tunnel (C1), a 5.13km horseshoe-shaped headrace tunnel and a 260m D-shaped adit tunnel (C2), the next 4.4km headrace tunnel section and a 960m adit tunnel (C3), the last 7.2km section of the headrace tunnel and two adits (C4), and an underground surge shaft, two pressure shafts, a tailrace tunnel and underground caverns (C5). Hindustan Construction Company Ltd were awarded C1 and C4, Larsen & Toubro was awarded C3, and Jaiprakash Industries was awarded C2 and C5.
Atlas Copco supplied the drilling and transport equipment, including five Rocket Boomer 352 two-boom face drilling rigs, five Boltec 435H rigs with pressurized cement injection, and 16 MT-420 20t dump trucks.
Drilling of the upstream and downstream tunnels both hit soft spots (moist crumbly rock which is difficult to stabilize) in 2003. Around 80% of the strata were adjudged ‘poor’ or ‘very poor’, which caused considerable tunneling delays. Blockages in inclined and vertical pressure shafts were a big threat and could cause a delay in completion within time. Geologists had described the conditions as the worst for tunneling that had been met anywhere in the world. Despite all the hurdles the job was still taken up with a positive approach and was finally completed.
Excavation Methodology for Desilting Caverns
A common construction adit (Inv. El. = 1334.85 m) of size 7 m x 7 m and length 36.130 m was provided. This adit bifurcates into two construction adits (named here-in-after Top and Bottom) of size 6 m x 6 m, one reaching Inv. El. of 1338 m and the other 1328 m.
In order to avoid the disturbance of rockmass during cavern excavation and to have the better control for installing the supports, five stages of excavation were proposed.