If GoI approves changes in concreting methodology of dam construction by the end of this month
PHPA I: If the government of India approves the slight change in methodology of the dam construction by the end of this month, the 1,200MW Punatshangchhu I will be completed only in April 2019, a further delay of six months.
The project is proposing to substitute the conventional vibrated concrete (CVC) of constructing the dam with roller compacted concrete (RCC) method.
The managing director of the authority, RN Khazanchi, said the authority expected the Indian government’s approval in September or early October last year. “But it’s still pending and we’ve already lost about six months.”
The CVC methodology involves concreting the blocks cement, aggregate, earth, sand and other common additives and it is time consuming because the cement takes more time to dry up.
The RCC method uses fly ash and less water, thus consuming less time to dry up.
Using RCC concreting for half of the dam would save 13 months, and enhance the power earning in the country by Nu 18B, besides lowering the interests on loan and saving extra escalation in cost.
“But the entire dam can’t be constructed with RCC, because, at the opening of the dam, gates needs to be constructed which can’t be done using RCC,” the director general of department of hydropower and power systems, Dasho Yeshi Wangdi said.
The RCC method, Dasho Yeshi Wangdi said, was cheaper because fly ash, one of the main ingredients was a byproduct from thermal power plants available in India. “Although it (fly ash) isn’t very cheap when including the transportation cost, it’s still cheaper than CVC,” he said.
The approach, RN Khazanchi said, was top down. The first stage of the remedial measure was conducted from more than 1,300m above sea level till the dam top.
“Right now, we’ve gone slow on the excavation and reoriented our focus from the top down,” he said. “Initially, the plan was they would implement the remedial measures from top down, and also start construction of the dam from below.”
After the approval of the decision on the RCC, it would take time to mobilise the equipment and install them. “As we’ve already lost about six months, we’ll be re-planning the whole thing, and the RCC will be aimed after this monsoon.”
Had the decision been given then, RN Khazanchi said, in six months time, the project could have started the building foundation.
“We’re supposed to complete the excavation of the dam foundation and come up with construction of the dam foundation, so that before monsoon we have a support across the two banks and that was to be done with RCC.”
“On the recent Gujarat visit, the prime minister and economic affairs minister, the chairman of PHPA, also raised the RCC proposal clearance issue with the highest level of the government of India and the decision is expected soon,” RN Khazanchi said.
As for the exaction of the dam foundation, the managing director said that the last 15m was pending, which means 70m below the river level had been excavated.
But since the unstable hill is part of the dam foundation, it was cut and treated simultaneously.
Although small cracks were noticed on January 22 along the remedial measures taken, the managing director said that it was expected since last monsoon, which did not materialise. As the hill stabilises, he said, this was expected.
“But we’re closely monitoring it and sending the reports to Delhi for review,” he said.
The project already lost time due to a geological surprise at the right bank, resulting in the movement of the hill, on which the dam foundation was supposedly to be laid, by five metres vertically and horizontally.
Following this, investigations has been conducted by geologival survey of India, central water commission and IIT India, which conducted several tests and considered the treatment that was in full swing since April last year.
“About 600,000 bags of cement have been used in injecting the cement into the hill to solidify,” he said.
Besides, concrete piling up to 100m depth at 573 different locations has been completed, and the remaining 163 is expected to complete before monsoon this year.
Insertion of cable anchors of 100 tonnes capacity up to 100m is also expected to complete by May this year. As of January, 177 cables were already laid.
About 15m deep drainage hole to tap the sub-surface water has also been initiated to stabilise the hill, and all the water is being guided through a proper drainage.
Meanwhile, the intake tunnel, four desilting chambers and the 8.9km head race tunnel is expected to complete by end of the year. The surge shaft at the end of the head race tunnel, two vertical pressure shafts and related proponents are expected to complete by November next year, although the testing could extend till March 2017.
By Tshering Dorji