A network of tunnels has been carved into the hillside for this phase of the project
Punatsangchu I: Once a grazing pasture for the villages of Jala and Ula in Wangdue district, the corn-shaped ridge on the left bank of Punatsangchu river is today overrun by sophisticated machines, hundreds of trucks and thousands of people in helmets and safety boots.
Standing almost 500m high, with its base spread more than 3km, the inner wall of the solid rock mass of the ridge is being dug to house the powerhouse of 1,200MW Punatsangchu I hydroelectric project.
Excavation work for around 55 tunnels, of which many are access to the powerhouse’s main components, surge shaft, power house cavern, transformer hall cavern, surge gallery, head race tunnel and tail race tunnel are close to completion, and machines and equipment are being fitted in.
“More than 95 percent of the excavation work of the entire powerhouse is completed,” chief engineer of Punatshagchu I, Yogesh Kumar Dhir said. “The completion of excavation signifies the crossing of the risk portion of the work.”
Since it is still growing, the Himalayan region is considered to be the most difficult for tunneling, as it encounters loose fall because of weak rock mass, experts said.
The tunnels that are big enough for a huge truck to roll through are built with the Swedish made boomer, a drilling machine designed to hole the rock mass. There are seven boomers, each costing between Nu 40-45M, to dig the tunnels ranging from 40 to 1,557m in length.
As of now, around 200 trucks, with carrying capacity of 25 to 30 metric tonnes (MT) each, have dumped 0.30MMT of muck from the construction sites.
Like branches of a tree, a labyrinth of tunnels are being excavated in the 3km area at the base of the ridge. Visitors would get lost in the dark tunnels without project staff guiding them.
At the top of the ridge is the 117m long surge shaft or standpipe storage, which can accommodate 50M litres of water. “The surge shaft will control the velocity of water when there is rise and fall of the water pressure from an increase and decrease in the loads,” Yogesh Kumar Dhir explained. “The surge shaft monitors pressure variations due to rapid changes in velocity of water.”
About 25m in diameter, a 39-storey building can easily fit into the surge shaft.
The inner wall of the surge shaft was excavated with the help of a crane like machine that positions equipment and workers. From upstream, the surge shaft will be connected to a 10km long headrace tunnel, which will channel in the river from the dam.
Two pressure shafts with a vertical height of 300m will connect to the surge shaft to send water at a high speed to rotate six turbines in the powerhouse. The water will hit the turbines giving it continuous momentum and converting energy into electricity.
The pressure shafts can carry water flowing at the speed of 10m/s, said the chief engineer. It’s the tallest of all tunnels in the powerhouse.
The excavation of the pressure shafts has been completed and 32-40mm steel liners, 6m in diameter, are being placed in. Unlike the surge shaft, where concrete mixture are put in the earth surface to prevent crack and damage, the pressure shafts have steel rings imported from Japan.
The powerhouse, which has the six turbines, is the biggest hall of all underground tunnels. Measuring 237m in length, 23m wide and 53m in height, the powerhouse is almost half the length of an archery field and can easily fit in 17-storied buildings. With excavation work almost completed, electro-mechanical work has begun at the powerhouse.
Hindustan Construction company is building the powerhouse at a cost of Nu 6.88B, of which Nu3.75B has been spent so far.
The company’s project manager, V K Rajora, said, having a good set of experts and machines are assets for the progress of work.
Of more than 1,500 people working at the site, 80 are engineers. “Given our experience in powerhouse construction for more than three decades, we’re hoping to give the best,” V K Rajora said.
By Tenzin Namgyel, Wangdue