Train to Sikkim: Railways succeeds in mining tunnel in sandy soil below NH-10, averting cave-in risk

New Delhi: The Sivok-Rangpo Rail Project, which will connect Sikkim to the railway network for the first time, has got a major boost with the completion of an 855-metre-long emergency exit tunnel.

A 20-metre section of the tunnel runs under National Highway-10, along the Teesta River, and according to officials, mining through its soft and loose soil was the most challenging part of the whole project as there was a high risk that it might cave in, snapping Sikkim's road connectivity with the rest of the country.

"NH-10 is the only road network that connects Sikkim with other parts of the country. It was a huge challenge for us to mine a tunnel 18 metres below the highway, because the soil is very soft and loose, just like the river sand bed (fine to coarse-grained), with alternate bands of pebble, cobble and boulders," an official from Ircon International, which is constructing the project, said.

"Mining for only that 20-metre distance, which is the width of NH-10 including the impact areas on both sides along the highway, has been the most challenging moment of the project so far. We completed it successfully, without causing a single millimetre of deformation to the highway," he added.

Railway officials said they used the secant-piling method, rarely used in tunnel mining. After getting it proof-checked by the Indian Institute of Technology (IIT), Ircon officials prepared a case study, explaining the ingenuity of the mining work.

The study says, "To provide fool-proof safety to NH-10, it was suggested to prepare a secant pile wall. Secant piles are designed by constructing reinforced concrete piles that interlock with each other and are reinforced with steel rebar. Typically, primary piles are cast in situ, leaving space in between them, and this is followed by secondary piles cut into the primary piles to form a continuous wall of piles." The Sivok-Rangpo Rail Project (SRRP), covering a total length of 44.96 km between Sivok in West Bengal and Ranpo in Sikkim), has 14 tunnels, 22 bridges (13 major, nine minor) and five railway stations -- Sivok, Riyang, Teesta Bazar, Melli and Rangpo.

A railway official told PTI that of the 14 tunnels, mining work has been completed in 10 and of these 10, the lining work has been finished in four. The longest tunnel of the project is 5.3 km and the shortest is 538 metres.

"Of the 13 major bridges, 12 are ready with the substructure. The maximum height of the pier (of Bridge-17) is 85 metres, constructing which was itself an uphill task. The most demanding aspect of the project is that 86 per cent of the total rail line passes through tunnels and 5 per cent over bridges," the official said, adding that only 9 per cent of the project is on the ground.

The Indian Railway has set a new deadline of August 2025 to finish the SRRP that will connect Sikkim with the rest of the country through the rail network for the first time.