Page 10 - TunnelTalk Annual Review 2016

Headrace challenge for double shield
Terratec News Release
Factory testing is complete for the double shield TBM by Terratec that will excavate the major part of a 13km-long headrace tunnel for the Vishnugad-Pipalkoti Hydropower Project in India. The machine will be delivered to tunnelling subcontractor Seli Overseas SpA, which has been engaged by the main contractor, Hindustan Construction Company (HCC), to complete mechanised tunnel excavation.
Factory acceptance ceremony
The 9.86m diameter hard rock machine will now be transported from the Terratec workshops in Wuhan, China, to the jobsite in Uttarakhand, approximately 500km northeast of Delhi. Seli Overseas will execute by TBM 12.3km of the low- pressure headrace tunnel. The remaining 700m will be excavated by HCC using drill+blast.
Geology is expected to comprise dolomitic limestone across approximately 33% of the alignment, slates across 64%, with the balance of 3% crossing 150m of critical thrust zones under shallow overburden, five shear zones and three fault zones. The majority of the tunnel will be excavated under an overburden of 800–1,500m, with squeezing conditions, spalling and rock bursts, as well as potential challenges from groundwater inflows.
The TBM is designed with particular attention to the critical zones. The cutterhead is equipped with heavy-duty 19in disc cutters for hard rock conditions, and retains 12 large openings to prevent clogging during excavation through fractured ground. The 4,200kW electric VFD main drive will allow the cutterhead to excavate through
the hardest rock zones at the maximum specified rotation of 6rev/min, and deliver a torque of more than 22,000kNm to cope with the fractured geology. The segmentally lined tunnel will feature precast concrete rings of 300mm thickness and 1.5m in length.
Other features of the TBM include a high-speed re-gripping system, single- shield advance mode, high-pressure emergency thrust, and flood control doors to mitigate against water inflows. The key parts were produced in Australia and Japan, with final assembly carried out at Terratec’s factory facilities in China.
Seli Overseas was selected to execute the works because its crew previously completed difficult TBM excavation on the Kishanganga hydro scheme, also in India. According to the company’s Area Manager Michele Sposetti, the project demonstrated that TBMs operated by experienced crews could deal with problem geology and open a new chapter for shield drives in the Himalayas. n
References
• New Award follows Seli Kishanganga success – TunnelTalk, May 2014
TBM rescue for Tapovan hydro challenge
Patrick Reynolds for TunnelTalk
Seli Overseas was hired to recover the TBM that has been stuck for several years on the delayed 520MW Tapovan Vishnugad hydro project in northern India. Following rescue, the team will use the refurbished machine to help complete the difficult Himalayan headrace tunnel.
Seli Overseas is contracted by Hindustan Construction Co (HCC) to undertake the recovery-and-completion challenge. “The TBM is to be freed and refurbished insitu and at more than 6km into the headrace,” explained Area Manager, Michele Sposetti, for Seli Overseas. Rescue of the double-shield TBM and operation to complete the remaining 2.8km of the drive is expected to take two years.
HCC was hired to complete the remaining headrace tunnel works by the project developer, National Thermal Power Company (NTPC), in early 2016.
HCC and Seli Overseas have experience of working together on two hydropower projects in India, the Kishanganga project, which is generally viewed as the first successful TBM hydro
tunnel in the Himalayas, and the Vishnugad Pipalkoti project which is located down the valley from the Tapovan Vishnugad for which tunnelling is about to get under way.
The Himalayas present significant tunnelling challenges for TBMs. Hydro projects that have experienced difficulties with TBM applications in the past include the Dulhasti and Parbati II projects, where gripper shields were employed. The experiences led to much hesitation in using TBMs in the Himalayas, until success of the TBM drive at Kishanganga by Seli and before the company split into Seli Technologies, the designer and manufacturer of the Seli TBM machines and Seli Overseas which continues the contracting and tunnel excavation activities with other parts of the original Seli company being sold and taken over by fellow Italian construction contractor Salini.
For the Kishanganga project, Seli used a double-shield machine, with special hatches in the shield through which crews could pass to excavate and open up small bypass tunnels and release the shield when required and ensure continued progress. The same full service approach is being taken on to Tapovan Vishnugad.
Construction of Tapovan Vishnugad began almost ten years ago. The 12km long headrace of the scheme was awarded to a joint venture of Larsen & Toubro and Alpine, in late 2006. The JV would use a Herrenknecht double-shield for the main portion of the headrace. Geoconsult has been advising the owner on the TBM works.
By 2009, NTPC was expecting the hydro plant to be operational by 2012. However, difficulties with instances of rock wedge movements and high water inflows were encountered and the TBM became trapped, halting excavation activities from that point.
NTPC terminated the contract in early 2014, noting that the JV had completed 7.65km of the segmentally lined tunnel.
In March 2016, NTPC awarded HCC the contract to complete the balance of works with the 34-month contract based on an item rate basis. In November, HCC awarded a subcontract to Seli Overseas for the TBM portion of the works. n
References
• Breaking excavation records in the Himalayas – TunnelTalk, December 2012
• Freeing stricken TBMs in tough Asian conditions – TunnelTalk, March 2012
Left and centre: Job site high in the Himalayas; right: Hand-excavation work to create a trapped TBM recovery chamber
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