New award follows Seli Kishanganga success 20 May 2014
Patrick Reynolds for TunnelTalk
- Seli strikes a double success in Indian Himalayan hydro projects by signing the TBM tunnelling subcontract for the Vishnugad Pipalkoti headrace works while simultaneously preparing to complete final breakthrough on its landmark TBM drive at Kishanganga.
Pipalkoti dam location in Indian Himalayas
- The Italian firm will supply a 9.85m diameter double shield TBM, capable of switching to EPB mode, for the 12.3km headrace drive. Seli is due to launch the machine mid-2015, using its own crew. It will also supply the carousel for casting the segmental lining for the 8.93m i.d. headrace tunnel.
- Vishnugad Pipalkoti is located in the Indian state of Uttarakhand, and the 444MW hydro scheme is being developed by Tehri Hydro Development Co Ltd. The main civils work contractor is major Indian firm Hindustan Construction (HCC) – the company that is also executing the principal civil works on the Kishanganga project in Jammu & Kashmir.
- The already-proven double shield TBM technology will be used once again for the latest Himalayan hydro tunnel challenge at Vishnugad Pipalkoti, said Michele Sposetti, Commercial Manager for Seli for the Middle East & India. The key to success in the Himalayas, where difficult geographic and geological conditions predominate, is using a highly experienced crew, Sposetti told TunnelTalk.
- The TBM for the new headrace tunnel at Vishnugad Pipalkot will be notably larger than the 6.18m diameter machine used for the Kishanganga drive. Additional features have been added to the double shield specification to cope with the different ground conditions expected.
- “The main difference to highlight is that the TBM for Vishnugad will also be EPB, due to the geology, with an extra screw conveyor to keep the pressure,” explained Sposetti. “We can switch from open mode to EPB mode in two hours without unscrewing a single bolt.”
- The use of a TBM for the Vishnugad Pipalkoti project has been planned for several years and from before the Kishanganga TBM tunnel was launched.
- In addition to TBM works, both the Vishnugad Pipalkoti and Kishanganga projects feature major drill+blast excavations for varied tunnels and caverns. While World Bank funding for Vishnugad Pipalkoti was approved in 2011, it is only recently that the civil works contract was awarded.
- For Kishanganga, Seli supplied a 6.18m diameter double shield with 19in discs, and its own crew, for excavation of a 14.8km long, 5.2m i.d. section of headrace tunnel. The 330MW project is being developed by NHPC Ltd (formerly National Hydro Power Corporation). Consultant Halcrow of the UK, now a part of the CH2M Hill engineering corporation of the USA, is a joint venture partner with HCC in construction of the hydro project and completed detailed design of the civil works, co-ordinated with the electrical and mechanical designer/contractor, and provided site engineering and supervision support.
- The Kishanganga double shield achieved a best monthly advance of 816.2m in November 2012, “This is a record in TBM tunnelling with a segmental lining in the Himalayas,” according to Sposetti.
TBM assembly and marshalling area
- Average progress between launch in June 2012 and the end of the drive earlier this month (May 2014) is recorded at 403m/month, though final breakthrough has been purposefully delayed to allow other works to be completed.
- During the near two-year drive Seli crews recorded 13 months of 500m+/month progress, and maintained rates above 300m/month for most of the TBM boring programme except for a period when national security curfews in the disputed territory of Kashmir were introduced that affected all projects in the region. Average net boring time on the drive was 9.5hr/day, and the TBM finished 60 days ahead of the contractual schedule, said Sposetti.
- Geology along the alignment comprised basalt, andesite and intrusive granite/granodiorite, which it was noted might have some association with hydro-geothermal activity at faults and discontinuities.
- Historically, applications of TBMs in the geological conditions of the Himalayas have run into difficulty, such as such as on the Dulhasti and Parbati II hydro projects. As a result, the majority of tunnels are built using drill+blast. The decision to use TBM technology for Kishanganga was a major departure from the norm but Seli took a different approach in its choice of a shielded TBM, rather than an open gripper machine, and, perhaps crucially, performed the tunnelling using its own team.
6.18m diameter TBM for Kishanganga
- The key choices in Seli’s specification of the TBM at the outset – in addition to choosing a double shield type machine – were high installed power (approximately 90kN), and having a retractable telescopic shield, which enabled the crew to “switch from double shield to single shield operation in those areas where the geology was unstable and to enable excavation to progress in safer conditions,” according to Sposetti.
- The TBM design also included drill hole portals in the shield that provided the ability to continue TBM excavation while performing probe drilling and ground conditioning cycles; extra cutterhead motors; auxiliary thrust rams cylinders; and a high conicity of the shield to allow continuous over-excavation by 100mm to help the machine manoeuvre in squeezing ground.
- Sposetti said of the TBM: “It is a great machine though as always it is the site crew that makes the difference. Seli’s experienced crew was vital to the contractor’s ability to overcome the classic adverse ground conditions associated with long reaches through weak zones in mountainous regions under high overburden.”
Tunnel portal for 14km long Kishanganga drive
- After the Kishanganga TBM was launched in Spring 2011, the first 1.5km of the drive proved difficult. “There were adverse ground conditions, unforeseen in the GBR (Geotechnical Baseline Report), and the heading encountered Class V rock conditions,” said Sposetti. “The crew had to contend with a combination of collapsing ground, cutterhead blockage and the shield becoming jammed. The crew had to hand-mine bypass tunnels over the top of the TBM on three occasions – once in a shear zone where the overburden was 650m.”
- Having been prepared for dealing with the unexpected, Seli had the double shield fitted with top hatches near the rear of the machine to allow the seasoned crew to hand-dig towards the cutterhead, he explained. Other measures to mitigate geological challenges included ground consolidation with self-drilling bolts and foam and resin grout injections to fill cavities.
- “I cannot praise highly enough the coordination, bravery and experience of our guys in executing the TBM operation and the bypass tunnels,” said Spisotti.
- Martin Knights, Global Leader of the Tunnelling and Earth Engineering and Sciences division of Halcrow, and a Director for the joint venture partnemship with HCC for the Kishanganga design-build contract told TunnelTalk from Delhi, and en-route to Kashmir for the TBM breakthrough celebrations, that: “this is a great achievement by HCC and Seli and Halcrow India. The Seli TBM project teams are to be congratulated on this successful breakthrough and l hope it will signal to clients that TBMs can be used to manage the construction of long tunnels to overcome the geological risks presented by the Himalayas.”
Breaking excavation records in the Himalayas - TunnelTalk, December 2012
World Bank backs Vishnugad Pipalkoti hydro - TunnelTalk, July 2011
Special TBM needed for Himalayan drive - TunnelTalk, March 2010
Seli set for tough drive on Kishanganga - TunnelTalk, May 2009
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