Punatsangchhu-I (PHPA-I) will remove some part of the hill above the dam site that slipped in July 2013 delaying the project by more than a year. Excavation work on the hill is expected to begin soon.
Engineers say that removing the overburden at the crown of the weak zone could remove weight or pressure from the sliding area.
In July 2013, at the dam site, the hillside moved 5metres (m) vertically and horizontally stopping all work. The slide, engineers believe, was triggered when digging the dam pit.
The suggestion to remove the overburden came from the Norwegian GeoTechnical Institute (NGI) that the project employed to seek a “third opinion” on the strengthening measures of the slide area.
The Central Water Commission of India, WAPCOS and IIT New Delhi recommended three major remedial measures after the slide occurred.
PHPA-I’s managing director, RN Khazanchi, said that the project’s consultants and NGI are working on how much to remove and where it could be removed from. “We’re expecting the decision within two weeks,” he said.
The weak or shear zone, made of a mixture of loose mass and boulders, lies between two planes of rocks.
The project’s joint managing director, Dorji Pavo Phuntshok, said that project officials are working on applying for clearance from the government.
“There is no private land or government infrastructure within the identified area so it should not take long to begin work,” he said. The project’s drinking water supply and power lines fall within the area likely to be excavated.
The excavation work on the hillside will not cause any delays or add cost, the project officials said.
On the foot of the hill, workers are still stabilising the area. They have drilled 229 holes, each measuring 2m diameters into the ground until the solid rock. The holes are filled with concrete and steel rods.
“These pillars will partly support the dam, transferring the load onto the firm rock beneath and also avert the sliding of the hillside,” said PHPA-I’s chief engineer for the dam site, JS Bajwa.
So far, the project has inserted 419 cable anchors up to 90m long and will insert 500 more in the area. Each cable can bear a load of 100 tonnes.
The project injected more than six hundred thousand bags of cement, each bag weighing 50kg, into the weak band underground. There are numerous other measures.
Of the Nu 3.7 billion for the remedial measures, the project has spent about Nu 3 billion to date.
The project will begin monitoring-based excavation from the first week of October to dig up the last 50m on the right bank to the base of the dam, officials said.
On the left side, 10m remains to be dug until the rock.
“We’ll dig up 3m at a time and monitor if there is any movement of the land mass,” JS Bajwa said. “The stabilisation measures will also continue simultaneously.”
RN Khazanchi said the movement of the mass is being monitored every day and reports are shared with experts in India.
The PHPA has placed inclinometers, equipment to monitor the movement of the hill mass. The equipment is placed permanently along two sections of the hill face at 30m gap and extends until 10m below the shear zone into the “intact” rock.
Critics blamed the project management for shifting the damsite upstream and said the situation would not have been this worse if the project considered the previously identified site.
RN Khazanchi clarified that geology surprises are unpredictable especially in the young mountains of the Himalayas.
The advantage of shifting the dam site brought an increase of 105MW.
Moreover, the experts and consultants, including CWC, preferred the new site as it had a better working width, the managing director said.
“The new location, designers said, was as good or as bad as the previous site,” said RN Khazanchi.
The investigation work before identifying the site were more than what was done for Punatsangchhu-II or Tala hydroelectric projects.
“We’ve deferred the completion of other components to invest a little less than what we could be doing and save on interest during the construction,” he said.
Its estimated capacity is 5,700 million units of electricity in an average year. Construction of Punatsangchhu-I HEP commenced in November 2008 at a project cost of Nu 35.148B.
In July 2015, the Union Cabinet of India approved the revised cost estimate of Nu 93.75 billion for the project at 2013 price level excluding annual escalation. As of July 31, the project spent Nu 47.88B. The completion cost of the project as of today stands at Nu 96 billion.
Cyclone Aila in 2009 had already delayed the project by one year, and the right bank landslide added another year to the delay.
The initial completion date of the 1,200-megawatt project was November 2015. However, the sliding of the right bank above the dam site delayed it.
The new committed project completion deadline is December 2021.