TBM rescue for Tapovan hydro challenge 15 Dec 2016

Seli Overseas has been hired to recover the large TBM that has been stuck for the last few years on the delayed 520MW Tapovan Vishnugad hydropower project in northern India. Following rescue, the team will then use the refurbished machine to help complete the difficult tunnelling challenge in the mostly-finished Himalayan headrace tunnel.

Last month, Seli Overseas was contracted by Hindustan Construction Co (HCC) to undertake the recovery-and-completion challenge for the TBM-related works on the project in Uttarakhand State in the Himalayan foothills. The TBM is to be freed and refurbished insitu and more than 6km into the headrace, explained "Area Manager, Michele Sposetti", for Seli Overseas.

The combination of the rescue task and then using the double-shield TBM on the remaining approximately 2.8km length of drive are expected to take much of the next two years to successfully complete.

HCC was hired to complete the remaining headrace tunnel works by the project developer, National Thermal Power Company (NTPC), in early 2016. Seli Overseas will focus on completing the TBM part of the headrace and HCC will perform the remaining drill+blast works, said Sposetti.

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, on the Vishnugad Pipalkoti project which is located down the valley from the Tapovan Vishnugad for which tunnelling is about to get underway.

The Himalayas present significant tunnelling challenges, especially for TBMs, due to the difficult geology, weak zones, overburden stress and groundwater. 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 Selini.

For the Kishanganga project, the approach to overcome the tough tunnelling challenge for TBMs was to employ a combination package of TBM technology, using a double-shield machine plus a seasoned tunnelling crew capable of working in and out of the machine underground. The miners were able to exit through special hatches to excavate and open up small bypass tunnels and release the shield when required and to ensure continued progress of the TBM.

The same full service approach is being taken on to Tapovan Vishnugad, TunnelTalk was told.

The Tapovan Vishnugad hydropower project is located on the River Dhauliganga, in Chamoli District, in Uttarakhand. The run-of-river scheme involves significant underground infrastructure, the largest of which is the headrace tunnel. Principal consultant on the project has been the Central Water Commission of India.

Project owner, NTPC, began early construction of Tapovan Vishnugad almost 10 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, the power company was anticipating commissioning of the hydro plant by 2012. However, with tunnelling underway, geological and construction difficulties in the mountains were encountered along with instances of rock wedge movements and high volume water inflows. These difficulties were encountered primarily in 2009 and 2012, notes Geoconsult, with the TBM becoming trapped and excavation activities effectively halted from that point. By then, the JV had made much progress and despite the challenges had built the majority of the segmentally-lined headrace tunnel.

NTPC terminated the contract in early 2014, and has noted that the JV contractor had by then completed 7.65km of the overall headrace tunnel.

In March 2016, NTPC awarded HCC the contract to complete the balance of works on the headrace tunnel 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.

NTPC, in its annual report for FY 2015-16, said the project was approaching three-quarters overall completion. Other activities onsite include construction of the diversion barrage, the switchyard and installation of both the electro-mechanical and hydro-mechanical equipment.

TBM recovery and completion

The length of the headrace from the upstream intake to the surge shaft is approximately 11km, said Sposetti. The split between lengths of TBM bore and the drill and blast excavation along this section is approximately 9.1km and about 2km, respectively and with drill+blast progressing from the upstream end.

The previous contractor had bored just more than 6.3km of the tunnel, leaving a balance of work for the TBM of slightly more than 2.8km, Sposetti told TunnelTalk. “The TBM is stuck at chainage 5,859m,” he said, “and the geology is complex with gneiss and potential for further weak zones and associated geothermal inflows.”

For the recovery of the 6.6m o.d. double-shield, Seli Overseas plans to hand-excavate an access and open up the rock around the sides and over the top of the machines to create an open cavern in which the TBM will be refurbished. It then plans to excavate a 12m-15m long chamber in the heading to re-launch the shield with double-shield rams thrusting off the segmental tunnel lining.

The bypass tunnels beside and over the shield’s present position will be backfilled to consolidate the ground and enable the TBM to place new segmental concrete rings. Once the TBM completes its restarted bore, the plan is to complete excavation to the intermediate Charmi adit for disassembly and removal to the surface.