The right bank

Ray of Hope for the Punatsangchu I project with new solutions

The 1,200 MW Punatsangchu I project, over the years, has become a collective national nightmare with its slides, delays and huge cost escalations but with no guarantee of a solution.

The project from its original completion date of November 2016 moved to 2024.

However, there is now some light at the end of the tunnel as there are some new solutions to first control the slide on the right bank through a different technology and then build the dam using a special construction technique.

Minister for Economic Affairs Lyonpo Loknath Sharma who is also the Chairman of the Punatsangchu Hydroelectric Project Authority (PHPA) said that there is some likely solution from an Austrian Company called SparGeo Infra to ensure the slope stability of the much troubled right bank.

He said that the other important development is that there is now a plan to use construction piles to construct the dam on the right bank side.

Both the above new developments will mean that the earlier plans to carry out massive excavations at the base of the slide and also above it may not need to happen.

Both the issues were discussed in the 24th Technical Coordination Committee between experts from both sides in New Delhi on 18th June.

Right Bank Stability

The main reason for the delay of P I is three slides in the right bank area that occurred first in July 2013, followed by another major one in August 2016 and then a third smaller slide earlier this year.

While the first slide showed the weakness of the right bank the second and third slides showed that the various solutions including getting in third-party experts was not paying off as expected.

Lyonpo said that the reason the SparGeo Infra proposal is interesting is that unlike earlier ideas it does not look at moving a lot of the overburden above and below but just strengthening the area.

He said the difference this time is a change in the earlier approach from just looking at the slide areas to now focusing on strengthening the entire right bank. He said this is something he himself insisted on from the time he took over as the Chairman of PHPA.

The minister said that sensors placed earlier showed that whenever there was excavation the movement worsened and it stopped when the work stopped.

SparGeo in the TCC meeting made a presentation on ensuring slope stability based on their study of the slope on the right bank. The company did a design analysis using Slide software at certain sections.

The company’s solution, according to the minister, is the provision of anchored cladding wall with cable anchors after removing the August 2016 slide along with the rock fall barriers and RCC walls.

Cladding in construction is used to build an outer wall or layer for protection of the inner surface. The use of cable anchors will hold it firmly in place.

The company also presented an overall scope of work that has to be done to the TCC.

Lyonpo said that it was decided that SparGeo Infra through the Center Water Commission will provide the detailed design of the cladding wall with cable anchors by 15th July initially for the area below the National Highway which is the August 2016 slide.

He said the company already has pre-monsoon data and it had to collected monsoon data on the slope for the design.

It was also decided that the award of the work for the anchored Cladding Wall shall be decided after techno-economic discussions and work will be carried out either through Larsen and Toubro or with engagement of SparGeo independently.

The PHPA-I will be implementing the most suitable of either arrangements to strengthen the right bank.

Lyonpo said that this is a more feasible solution as the earlier solutions would have entailed huge excavation and removal of overburden leading to the diversion of the highway and also removing a part of the project colony above it.

The minister pointed out that, earlier, when excavation was carried out from below, it resulted in slides and even when it was done from above it weakened the area.

Dam Construction at Right Bank

The main reason why all the experts are so worried about the right bank stability is because eventually the right side of the dam has to be constructed close to it.

This was made challenging from the fact that it took a lot of excavation to reach the bed rock, more than originally estimated.

Lyonpo said here too there is a solution forthcoming as the idea is now to place construction piles at the foundation of the right side of the dam to strengthen it. Construction piles is a commonly used technique in construction where cylindrical piles made of either metal or concrete are driven deep into the earth below a structure to provide additional support and stability to the structure above it.

The piles would importantly avoid excessive excavation beyond a point at the foundation of the right bank of the dam which is close to the right bank. The concern is that excessive excavation at the foundation of the right bank of the dam can disturb and weaken the right slope.

The CWC made a presentation on using the piles based in turn on the recommendations of the 9th Technical Group meeting held earlier in June itself.

The CWC said that as per its own research the option of placing piles under the dam on the right bank without further excavation is the best for the project.

The piles will be around 2 meters in diameter and a total of 265 piles will be used under three sections of the right bank of the dam.

CWC and the Indian consultant WAPCOS will submit the detailed designs for each of the study and analysis carried out so far for the right bank stability by the end of June.

To ensure that the piles can withstand seismic shocks and also a full dam 3 D physical model studies with dynamic loadings shall also be carried out by the consultants soon.

The details of instruments required to be provided in the piles area under the dam block foundations to monitor the piles will be worked out by the consultants.

CVC concreting better for Dam

The Punatsangchu Dam is supposed to be a mix between Conventionally Vibrated Concrete (CVC) and Roller Compacted Concrete (RCC). The difference is that the first uses a vibrating method to settle the concrete down and the second uses a roller machine to compact it.

However, with new CVC technology it was found that a full CVC dam is more appropriate for P I. Also, RCC can be implemented only after a certain height.

Lyonpo said that the CVC dam would also be stronger.

The placement rate of CVC is comparable to that of RCC, and thus, RCC method does not have a fast placement advantage over CVC.

It was felt that using CVC fully will take 30 months as opposed to the current CVC and RCC mix taking 40 months.

The TCC decided that CVC is the only technically feasible option for dam concreting under the prevailing conditions, and the concreting shall be carried out with a modern set of equipment giving equal or better placement rate than the cable cranes of RCC.

The Director Finance of PHPA-I and PHPA-II will come up with cost implications and present it to the Authority for a final decision.

Lyonpo Loknath said that his main hope and push had been for a breakthrough for the project by this year and he is happy that it is coming true.

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