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 S.p.A., which has been engaged by the main contractor – the Hindustan Construction Company (HCC) – to complete mechanised tunnel excavation.
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.
The project includes construction of dams, waterway tunnels, vertical shafts and a powerhouse that will generate a capacity of 444 MW of electricity. Seli Overseas will execute by TBM 12.3km of the low-pressure headrace tunnel. The remaining 700m will be excavated 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 800–1500m, with squeezing rock conditions, spalling and rock bursts, as well as potential challenges from groundwater inflows, anticipated.
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, but retains 12 large openings to prevent clogging during excavation through fractured ground. The 4,200 kW electric VFD main drive will allow the cutterhead to excavate through the hardest rock zones at the maximum specified rotation of 6rpm, but will also also be able to deliver torque of more than 22,000 kNm to cope with the fractured geology of some sections of the alignment. 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’ ability to supply a tunnelling crew to execute the works was a key factor in winning the contract, according to the company’s Area Manager Michele Sposetti. He said the crew comprises tunnellers with strong mining and hand-digging bypass experience. Seli Overseas previously completed TBM excavation on the Kishanganga hydro scheme, also in India, which demonstrated that TBMs operated by experienced crews could deal with problem geology and open a new chapter for shield drives in the Himalayas.
The machine will now to be dismantled and shipped to Mundra Port in India, from where it will be transported by road to the jobsite ahead of scheduled assembly in July 2016.