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Headrace Tunnel is Completed at Tala Hydro Project

Headrace Tunnel is Completed at Tala Hydro ProjectDespite fearing yet another delay, the 23 km headrace tunnel of the Tala hydro power project in Bhutan was completed on 8th November, 2004. The tunnel is the second longest in the Himalayas. The breakthrough ceremony between Mirchingchu and Kalikhola was attended by the Indian Ambassador to Bhutan Mr. K.S Jasrotia.The excavation on the tunnel has been delayed by more than seven months because of adverse geological conditions. In March 2003, workers at the Kalikhola upstream tunnel encountered a soft spot and six months later workers at the Mirchingchu downstream tunnel came across the same compact cake-like moist rock. This tricky geological condition was encountered at the 21st kilometre of the 23 km-long headrace tunnel which starts at Wangkha dam and ends at the surge shaft in Kalikhola. Water made the rock moist and the whole ground became a flowing mass of soil very difficult to stabilise. This is possible because the Himalayas are young mountains. With special equipment and methods needed to tackle such terrain, the problem considerably slowed tunnelling.The core of the Tala project is the 23 km headrace tunnel, divided in four contracts. Contract C1 includes a dam located near Honka, about 90 km from Phuentsholing, and the first 6.4 km headrace tunnel section. Contract C2 includes the construction of a 5.13 km long 6.8 m finished diameter horse shoe-shaped headrace tunnel and the 260 m-long 7 m-diameter D-shaped Padechu adit tunnel. Contract C3 is the next headrace tunnel section (4.4 km) and the 960 m-long Geduchu adit tunnel. Contract C4 is the last section of the headrace tunnel (7.2 km) and two adits. Contract C5 includes the construction of an underground surge shaft, two 4 m-diameter pressure shafts totalling 2,153 m, a 3,108 m-long 7.75 m-diameter horse shoe-shaped tailrace tunnel, a 206.4 m-long x 21.4 m-wide x 44.5 m-high machine hall underground cavern, a 191 m-long x 16 m-wide x 24.5 m-high transformer hall underground cavern, 609 m of 3 m x 4 m D-shaped galleries around the caverns, and a 376 m-long 7.5 m x 8 m D-shaped main access tunnel to the machine hall cavern. The contactors are Hindustan Construction Company Ltd (C1 and C4), Larsen & Toubro (C3) and Jaiprakash Industries (C2 and C5).Drilling, bolting and transport equipment was supplied by Atlas Copco. They include five Rocket Boomer 352 drilling rigs, five Boltec 435H rigs and sixteen MT-420 20t dump trucks. Atlas Copco's Rocket Boomer 352 is a large two-boom face drilling rig. The Boltec 435H has special features including a pressurised cement injection system for easy use and maintenance, and includes a single feed concept where only one feed is used for both drilling and bolt installation. Visit www.boomer-rig.com or www.atlascopco.com/ureThe Tala tunnel will be ready for delivering water to the six 170 MW generators by October 2005. The project, which is estimated to have cost about Nu43 billion, will generate 1020 MW of electricity on its completion in June 2006. The Tala hydroelectric project is a joint venture between the governments of Bhutan and India. Subscribe to E-News Weekly 44/2004, 17/2004, 23/2003 & 25/2002. Click bt/11. More in www.tunnelbuilder.com/facedrilling/edition2pdf/page63.pdf 47/04. Atlas Copco Rocket Boomer 352 drill rig at Tala's Kalikhola downstream tunnel.


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Beakaert Maccaferri: bm-underground.com