• Terratec

Vishnugad Pipalkoti TBM finally set to start 10 Oct 2019

Patrick Reynolds for TunnelTalk

Launch is finally set for a TBM to excavate the majority of the 13.4km long headrace tunnel for the Vishnugad Pipalkoti hydro project in the Indian Himalayas after main contractor delays severely held up the start of tunnel boring.

TBM set for launch at Vishnugad Pipalkoti
TBM set for launch at Vishnugad Pipalkoti

The original plan was to launch the new 9.86m diameter double shield almost three years ago with main contractor Hindustan Construction Co (HCC) having awarded the TBM tunnelling subcontract to Seli Overseas. While manufacturer Terratec and Seli Overseas had the TBM ready to despatch to site from mid-2016, the project developer, Tehri Hydro Development Corporation India (THDC), has reported slow progress by HCC over the last few years on much of the early tunnelling works, including preparing the TBM launch area. Shipment of the TBM was therefore delayed.

HCC is working on an engineering, procurement and construction (EPC) contract to build the 444MW Vishnugad Pipalkoti project. The contract was awarded five years ago, shortly after which the tunnelling subcontract was signed. Early underground works that have been gradually progressed by HCC are drill+blast excavations, including access adits and the diversion tunnel as well as the powerhouse complex that includes:

TBM to follow previous successes in Himalayas
TBM to follow previous successes in Himalayas
  • Machine hall: 146m x 20.3m x 48m
  • Transformer hall: 140m x 15m x 25.5m
  • Three desilting chambers, each 390m x 16m x 21.25m
  • Upstream surge shaft: 154m deep x 15m and 22m diameter
  • Downstream surge shaft: 150m x 13m x 25m
  • Tailrace tunnel: 3.1km long

With work gradually advancing on multiple fronts, the major tunnelling task yet to start is boring the majority of the 13.4km long headrace tunnel which will eventually convey water to the power plant on the Alaknanda River in Uttarakhand Province. The original plan was to see launch of the TBM in October 2016. The balance of work on the headrace has been done by drill+blast.

Gianluca Ciocca, India Projects Director for Seli Overseas, told TunnelTalk that the TBM should be launched in November to start boring the main reach of the headrace tunnel. The machine specification was developed by Terratec with Seli Overseas, which will be supplying the operating crew. The machine will be capable of switching quickly from double shied to single shield EPB mode.

Ciocca told TunnelTalk the TBM bore is expected to take 36 months to excavate the 12.3km long headrace. He further explained that the geology for about the first 200m of the drive is “quite critical,” consisting of mainly sandy silt layers. During this stage Seli Overseas will use face consolidation, chemical grouting and hand mining to open a top heading.

Beyond the initial section, competent rock is anticipated, consisting of low grade quartzitic sandstone, dolomitic limestone, and slates with metabasic sills and dykes.

Powerhouse crown slashing (Jan 2017)
Powerhouse crown slashing (Jan 2017)
Lining in the diversion tunnel (Nov 2016)
Lining in the diversion tunnel (Nov 2016)

Seli Overseas worked with HCC previously on a TBM hydro project, demonstrating the excavation technique can be used successfully in the complex geology of the Himalayas but also the importance of an experienced crew in achieving progress. Their success at the Kishanganga project in Kashmir, took a 6.18m diameter double shield through the 14.8km long drive despite several challenges. In the past, TBMs have suffered problems when trying to bore hydro tunnels in the Himalayas under conditions of high overburden and variable geology, resulting in most tunnelling projects staying with the slower drill+blast method.

References

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