India headrace TBMs stuck and needing rescue 15 Apr 2021

TBMs engaged to excavate and complete headraces for the Vishnugad Pipalkoti and Tapovan Vishnugad hydro projects in northern India are both stalled once again in difficult unfavourable ground.

TBM ready for launch at Vishnugad Pipalkoti, more than three years later than planned

On the Vishnugad Pipalkoti project on the Alaknanda River, in district Chamoli in the State of Uttarakhand, Tehri Hydro Development Corporation India (THDC), a wholly owned subsidiary of NTPC, formerly the National Thermal Power Corporation, which is a public sector electricity generator owned by the Government of India Ministry of Power, is developing the 444MW project to harness a gross head of 237m.

Hindustan Construction Company (HCC) was awarded an EPC, engineering, procurement and construction contract to build the project that comprises a 65m high dam across the Alaknanda; three intakes to three underground desilting chambers; a 13.4km long headrace; two pressure shafts bifurcating into four penstocks to the underground powerhouse, comprising a machine hall and a transformer hall cavern; and a 3.1km long tailrace (Fig 1).

Civil engineering works of the Vishnugad Pipalkoti hydroscheme

HCC began drill+blast excavation of the underground elements in 2014 and engaged Seli Overseas on a subcontract to complete 12.3km of the 13.4km long headrace by TBM. Seli Overseas procured a 9.86m diameter Terratec double shield TBM capable of converting to single shield EPB mode to excavate the drive and erect a precast concrete segmental lining.

The original plan was to launch the TBM in 2016 but, slow progress, including to resolve land acquisition issues and advance early works by HCC to prepare the TBM launch area, delayed shipment and assembly for launch of the TBM until October 2019.

For the 520MW Tapovan Vishnugad hydropower project on the River Dhauliganga, also in Chamoli District in the State of Uttarakhand and further up the Himalayan valley from the Vishnugad Pipalkoti project, Seli Overseas is subcontracted again by HCC to recover a Herrenknecht double-shield TBM that was stuck in difficult geology at about 6.3km from the downstream portal of an 11km long x 6.6m o.d. diameter headrace drive some 9km of which were planned to be excavated by TBM with drill+blast advancing from the upstream portal for the remainder.

Project owner, NTPC (also involved on the Vishnugad Pipalkoti project through its wholly owned THDC subsidiary) began construction of the Tapovan Vishnugad project in late 2006 with award of the civil works contract to a JV of Larsen & Toubro of India and Alpine Mayreder Bau of Austria, with Geoconsult of Austria advising the owner on the TBM works. Principal consultant on the project is the Central Water Commission of India.

Hand-excavation work created the recovery chamber of the trapped TBM on the Tapovan Vishnugad project

Once launched the TBM encountered difficult geological conditions, eventually becoming trapped at chainage 7.65km, halting further excavation without renegotiation of the contract in light of the conditions. In early 2014 NTPC terminated the contract and in March 2016, awarded a contract to HCC to complete the works, with the 34-month contract based on an item rate basis. In November 2016, HCC awarded the subcontract to Seli Overseas to rescue the TBM and complete its remaining 2.8km portion of the headrace over an estimated three year contract with about two years estimated to complete the drive.

After recent investigations to know the status of the two TBM drives, following the devastating glacial flood of the River Dhauliganga valley in the State of Uttarakhand in February 2021, the news was that both TBMs are stuck again in poor geological conditions and with discussions on-going as to how to progress and pay for the works needed to solve the situations.

Vishnugad Pipalkoti

At Vishnugad Pipalkoti, Seli reported to TunnelTalk in October 2019 that the geology for about the first 200m of the drive would be “quite critical”, consisting of sandy silt layers in which face consolidation using chemical grouting and hand mining to open a top heading would be required. The reality is that large boulders were also encountered and these had the TBM at a standstill within meters of launch.

Large boulders were noted by a visiting engineer in March 2016, when the World Bank, as a funding contributor, made a visit to the project. It was noted that the TBM portal was sited within a stiff deposit of river boulders rather than bedrock and that the oversight was due to misinterpretation of geophysics as the only geological investigation, and no borehole investigations.

Large boulders evident at the launch site being prepared for the Vishnugad Pipalkoti project TBM

Vishnugad Pipalkoti is the first project in India to prepare and apply a GBR (geotechnical baseline report) and beyond the initial section, competent rock is anticipated, consisting of low grade quartzitic sandstone, dolomitic limestone, and slates with metabasic sills and dykes. A review of how well the GBR relates to actual conditions encountered and how the mechanisms are applied will be of interest to the industry.

Seli reported that, while knowing that the first 200m of the drive would be critical, the main contractor and the owner decided to try to use the TBM through this reach, rather than use drill+blast to excavate a TBM starter tunnel. The gamble has not paid off and bypass headings to rescue the TBM are now needed. It reported that the large boulders were unforeseen, that hard rock was expected after the initial 200m, and that advance is stopped to prepare and agree the consolidation methods and works needed to progress.

Tapovan Vishnugad

For the Tapovan Vishnugad project, Seli took about a year to rescue the trapped TBM and refurbish it in the 19m long excavated works cavern at more than 6km into the drive from the portal. In 2019 it relaunched the renewed machine and advanced only 55m before it became trapped once again in highly squeezing conditions.

In those 55m, two bypass headings were needed to free the shield or clear the cutterhead. A new cavern is now needed around the TBM to free it and plan the way forward. Less than 2km of the troubled and planned 9km long 6.6m o.d. segmentally lined tunnel is left to excavate.

Stockpiles of segments await continuation of the Tapovan Vishnugad TBM drive

Experience is a root factor

HCC and Seli Overseas have experience of working together on a previous hydropower project in India, on the Kishanganga project in Jammu Kashmir, also with NHPC and in a joint venture with UK-based consulting engineer Halcrow Group (before eventually becoming part today of Jacobs Engineering) on a 98-2% share to design and build the 330MW scheme, where conditions were challenging but TBM operation for the 6.18m o.d. double shield on the 14.8km headrace was successful. This was despite the need for several bypass headings around the shield to clear the cutterhead.

Seli Overseas has a well-experienced crew of TBM tunnellers who know how to excavate out and around a TBM shield. The company has designed in portals in the TBM shields to provide access for these purposes and the Seli teams have earned a reputation around the world for succeeding with TBMs where others may founder. But as Lok Home said in his presentation to the British Tunnelling Society in January 2021, there should be no need for bypass headings to free trapped machines, no need for sending crews outside the safety of the shield into what is evidently unstable, weak, friable and squeezing rock, with or without significant inflows of groundwater. He maintains that TBMs can be designed and manufactured with the capabilities required to cope with a wider and widest range of conditions, if the money is made available to include these facilities in the TBM design and supply.

On a wider perspective there is the issue of corporate experience within the main contractors and the owners of these high mountain hydro projects that are well known as presenting difficult tunnelling for waterways and huge caverns for power stations, be it by drill+blast or TBM. HCC is more experienced in India as an urban contractor with its portfolio mainly listing metro projects and road and rail infrastructure. The portfolio of stations operated by NTPC, as the largest power generator in India and owned by the Government of India, are mostly fossil fuel fired stations mainly coal. Hydropower generation is a relatively new sector of procurement and operation for NTPC as an owner, when the Government persuaded the company to develop hydro schemes in its urgent need to meet rapidly rising demand in India for electricity. There is also lack of corporate experience in the consulting and design engineers engaged on these difficult projects. CWC, the Central Water Commission, is engaged by NTPC as its main consultant and designer for hydro projects but has little knowledge and experience in implementing underground works.

Several experienced professionals have been invited to visit the projects and advise on the way forward, but the inexperience of the owner, main consultant and main contractor is exposed in that there are no, or incomplete, systematic records of construction and of the series of failures in the tunnels and caverns, and no detailed documentation of geological and hydrological data as the excavations have proceeded. Professional advisers who visit the project sites to study the issues can find insufficient data for any analysis. It seems there is little understanding on site of the reasons for extreme geological conditions and high water flows and how to handle them, and this is compounded by the major complication of cost of treatment, which is difficult for project owners and their consulting engineers to compensate.

The discussion and experience continues.