The troubled right bank after the January 2019 slide

NHPC’s third-party review says current P-1 dam site has to be abandoned due to right bank weakness

P-1 Civil Works Consultant CWC does not agree and feels right bank can be strengthened

The 1,200 MW Punatsangchu I project is not just faced with a geological challenge on its right bank, but now two expert Indian agencies, the Central Water Commission (CWC), which is the project’s civil engineering consultant, and the National Hydro Power Corporation (NHPC), brought in to review CWC’s proposal as an external expert third party do not agree on the ‘factor of safety’ of the right bank to build a dam at the current location.

A senior official said that the dam site would have to have a minimum factor of safety of 1 or above as per international standards while the European factor of safety is 1.5 to 1.8.

The CWC in its report and drawings give a factor of safety of 1.2 to 1.4, however, the NHPC hired to review this CWC report as a third party said the factors of safety is below 1.

Simply put, the NHPC is saying that the CWC’s strengthening measures and drawings for the right bank presented to the Technical Coordination Committee in October and December 2019 is not secure enough for a dam to be built right next to it.

The NHPC’s review effectively means that the current dam site on which quite a bit of work has already been done needs to be abandoned.

A senior official said, “NHPC feels a dam is not possible at the current site as the right bank is still moving and in the future once a dam is built there could be water leakages and slides.”

Another official pointed out that once the dam is constructed there will be 12 million cubic meters of water stored behind the dam. He said the dam and the surrounding structure should be able to deal with earthquakes and heavy rainfall in the future not just for a few years or even 20 to 30 years but it should be able to last for a 100 years or more.

The NHPC solution is to build a smaller structure in the form of a barrage 820 meters upstream of the current site.

A barrage would effectively mean no ‘pondage’ or storage capacity like a dam and it would be a ‘pure run-of-the-water project’ in the words of an official.

The NHPC feels that the current Head Race Tunnel can be elongated to reach this barrage, but the CWC does not feel it is possible.

The NHPC’s criticism is just not restricted to the right bank but it also feels that the ‘Stilling Basin’ or a depression to reduce the velocity and flow of the water coming out of the Dam’s sluice gates is not adequate.

Normally a stilling basin has hard rock under it but given its absence the proposal was to build a 7-meter concrete raft to take the impact.

The problem all started in 2008 when the then PHPA MD R.N Khazanchi proposed to change the original dam site to generate more power. The then cabinet agreed in June 2008 based on Khazanchi’s presentation.

A Joint Audit Report of the Royal Audit Authority (RAA) of Bhutan and the Comptroller General Audit of India (CAG) of 2012-13 said that the PHPA, its main consultant WAPCOS and CWC knew that there were geological weaknesses at the right bank area but still went ahead with the tendering of the dam on the same site in 2009.

A geo technical appraisal report prepared by the Geological Survey of India (GSI) identified some weak geological features and said that additional investigations were required for the exact delineation of the weak geological features.

However, instead of conducting further investigations PHPA’s consultant WAPCOS, on behalf of GSI, issued a clearance for the project dam bids to be opened on 16th February 2009.

The problem made itself known with a major slide on the right bank in July 2013 which at the time the overall consultant WAPCOS and the civil consultant CWC said could be fixed with Nu 3.5 bn of mitigation measures. However, despite the measures another major slide happened in August 2016 and again despite additional mitigation measures the same area slid on 22nd January 2019.

After this, the government in the 21st PHPA Authority meeting in January 2019 insisted to the Indian consultants to come up with a comprehensive solution.

CWC was tasked to come up with a holistic solution to strengthen the Right Bank without which construction of the Dam on the right bank side could not proceed.  The report was submitted by CWC on 1st October 2019 and detailed drawings later in December of the same year.

The CWC’s main solution rested on a German Company called Bauer which has proposed to drive in 26 two-meter diameter cement pylons or pillars of 100 to 110 meters’ length at the highway level to stabilize the right bank and prevent additional slides from above while excavation and strengthening works go on below. The idea is that the cement pillars will anchor the right bank to a hard rock surface.

Below there would be some excavation to remove the slide. Along with the cement pylons there would be 150 metric tons of cable anchoring used to strengthen the right bank and slide area. There would also be additional grid works to strengthen the area.

Apart from this strengthening works there has also been been a change in design to reduce excavation work in the right bank lower down from the dam site to ensure a shallower still basin (which NHPC has criticized).

PHPA officials at the time said they were confident enough with the report to start concreting of the dam from the stable left bank side. The total dam width is 241 meters the the starting work area in the left side was 51 meters.

One suggestion of the CWC was that the dam on the right bank side be constructed over 288 concrete piles driven into the ground to avoid excavating the shear zone below the right bank side

The Bhutanese side was not very confident of that plan since there are no similar examples of works done like this elsewhere.

The big question for the P I project was on whether the remaining 15 to 20 meters’ depth to reach the rock foundation needs to be excavated for the dam location on right bank side or other measures need to be taken.

The Bhutanese side not very convinced with the idea of concrete piles under the dam for the long term and feeling the need for an overall review of CWC’s proposal called for a review in the Technical Coordination Committee meeting

The National Hydro Power Corporation (NHPC) was called upon to do a third party review of the WAPCOS and CWC proposal. The NHPC in its preliminary study had already felt that driving piles under the right bank dam location will not be effective.

The NHPC’s review report not only pointing out holes in the CWC report on the right bank strengthening measures, but also effectively calling for abandoning the current dam site and building a barrage upstream is causing understandable confusion among Bhutanese officials.

The TCC has asked CWC and NHPC to sit together and go through their findings and come up with a reconciled report.

A project official said that CWC is asking IIT Delhi to also do a study on the safety aspects. The same official pointed out that NHPC had spent only two days at the site and did not meet the senior management there.

An official said that the two sides have said they will present a report within this month since the lockdown is being relaxed in India.

Though not official yet, the Bhutanese side is also thinking of seeking an independent international expert view on the divergent views of the CWC and NHPC.

The main concern of the Bhutanese side is that whatever final decision is taken on the right bank or the dam, it should be guided by full research and facts and it should be a safe structure in the end prepared to deal with all kinds of future situations from floods to earthquakes and last for around 100 years.

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