Work on all components besides the dam construction were ahead of schedule by more than 18 months when disaster struck the 1020-megawatt Punatsangchhu-II hydroelectric project authority (PHPA II) on March 2 last year.
Six men were buried alive inside the downstream surge gallery (DSSG) of PHPA-II powerhouse when loose soil and muck fell on them. The incident occurred around 1am when the labourers were working on their night shifts.
The project’s superintending engineer, DD Thakur, said work on the gallery, which is one of the three major components of the powerhouse complex, stopped after the incident.
“We haven’t touched anything in the chamber since then,” DD Thakur said.
Managing Director of PHPA II, R N Khazanchi, said the project hired an international organisation from Italy to assess the size of the cavity. A cross-hole tomography showed that the cavity was 91 meters tall, 82m long and 30m wide above the chamber.
The downstream surge chamber is 314m long. Loose-soil and boulders fell between 118 and 200m. The muck will be reinforced with concrete and converted into a pillar, project officials said.
Only about 13m was left to excavate when the incident occurred.
DD Thakur said that because of movement of the rocks, nearby areas were distressed. “So we decided to reinforce the other two underground chambers next to the downstream surge chamber. We didn’t want to any take chance with the quality of our works.”
The strengthening measures have halted works on installing hydromechanical equipment and other components in the other two chambers.
“We’d already completed the foundation for the machines in the power house, but then for the new measures we covered it with sand and muck along with the machine we installed,” a senior hydromechanical engineer of the power house complex said.
To carry out the reinforcement work, the project had to refill the chambers up to 20m high. Excavation works were completed in both powerhouse and the transformer hall when the incident happened.
The chambers were reinforced with bolts and steel plates but more rock bolts are used to further stablise the excavated area.
About 7,600 more bolts will be inserted into the crown/roof and walls of the powerhouse. Ten percent of the reinforcement in rock bolts and grouting has been completed. The strengthening measures in the hall are expected to complete by July 2018.
The reinforcement measures, including 1,700 rock bolts ranging between 12m and 24m long, are completed in the crown or roof of the transformer hall which is 216m long, 14.7 wide and 26.5m high.
“As we excavate the muck from the chamber we’ll strengthen the walls, which would be fixed with about 4,000 more rock bolts,” the superintending engineer said. “Hopefully, strengthening works will be complete by February 2018,” he said.
Meanwhile, the downstream surge chamber will be completed in December 2019. There are machinery to be installed in the chamber, so the excavations can continue till the end of the project period, officials said.
The dam site is about 94km from Thimphu. The project was sanctioned at a cost of Nu 37.78 billion excluding IDC on March 2009 price level with installed capacity of 990MW, which was later revised to 1020MW.
The project is funded by the GoI on 30 percent grant and 70 percent loan at the rate of 10 percent interest per annum repayable in 30 equated semi-annual installments beginning one year after the date of commercial operation.
The project, a run-of-the river scheme, began implementation on December 17, 2010 with a completion schedule of seven years including two years of infrastructure development.
The Project is located on the right bank of Punatsangchhu along the Wangdue-Tsirang highway between 22km and 35km downstream of Wangdue bridge.