In late May, TBM manufacturer Robbins signed contracts with Jaiprakash Associates for the supply of two TBMs to bore the 43.5km long Tunnel-1 of the Srisailam Left Bank Canal Tunnel Scheme (SLBCTS) in south central India.
When complete, it is claimed the tunnel will be the longest single tunnel without intermediate access in the world. Robbins will supply two 10m diameter hard rock double shield TBMs, one to bore from each portal, associated equipment such as conveyors, spares and cutters, as well as support personnel for the contract. Robbins USA will supply the TBMs, conveyor system, spares and cutters. Robbins India will provide local components such as the backup gantry structure, personnel and other services.
Robbins told T&TI that the expected geology consisted of 60% very hard and abrasive quartzite and 40% coarse grain granite. Some water is expected near the upstream portal, but mostly the tunnels are expected to be relatively dry. Talking to local press, Shyam Chengalath, managing director of Robbins India, said that a major challenge was ventilating the 22km long headings. He added that spoil removal was required to take place continuously at high volume rates as the TBMs are planned to bore in excess of 1,000m per month in total.
Probing of the ground will take place ahead of all excavations and T&TI was told that grouting would be carried out on an ‘as required’ basis. The entire tunnel will be segmentally lined and the TBMs will be equipped with vacuum pad erectors.
SLBCTS is part of the Alimineti Madhava Reddy (AMR) project that aims to transport water from the Srisailam Reservoir to the plains of Nalgonda in the Andhra Pradesh region by way of two tunnels totalling 50.75km. The region is affected not only by a scarcity of water, but also with an excess of fluoride in the groundwater resulting in an accumulated poisoning, characterised by bone diseases, stooped backs, crooked limbs, deformed teeth and blindness.
Tunnel-1 runs for 43.5km, at 9.2m diameter, across the Amrabad plateau with a maximum cover of roughly 500m into the Dindi valley. The 7.25km long, 9m diameter Tunnel-2 will run from Dindi, crossing a range of hills with 350m cover. From the exit portal of Tunnel-2, an open canal will run for 25km to join a node of the AMR main canal at 25km chainage. Jaiprakash has been awarded the design and construct contract for both tunnels. Tunnel-1 is felt to be the more sensitive of the two as it traverses under a wildlife sanctuary and tiger reserve.
Robbins is adopting the same approach as it has for the Niagara project, (p12), by assembling the machines onsite.
