India’s longest under-construction rail tunnel nearing completion

Punching through the mighty Peer Panjal ranges in the Lower Himalayas, in J&K, the railways are finishing the longest railway tunnel in India

Indian Railway’s longest tunnel that is under construction is now nearing completion. Named T49, the tunnel is the longest railway tunnel in the country and the Afcons constructed 7.32 KM (nearly 60%) of the total tunnel It is linking the country’s northernmost part, i.e., the Jammu and Kashmir (J&K), with the rest of India.

To do so, it is constructing a series of tunnels and bridges to navigate the rugged mountain terrain of the Lower Himalayas (or Lesser Himalayas). Punching through the mighty Peer Panjal ranges in the Lower Himalayas, in J&K, the railways are finishing the longest railway tunnel in India. The Udhampur-Srinagar-Baramulla Rail Link (USBRL) project undertakes this arduous mission to join J&K with the mainland.

The T49 tunnel is 12.75 KM long and connects the Sumber and Arpinchala Stations in the Ramban district of J&K. Afcons has constructed 7.32 KM of the total tunnel which is nearly sixty per cent of the entire tunnel length. Besides the main tunnel, Afcons has also constructed two adits, twenty cross passages, two bridges, and a station yard among others.

Defying Gravity

Tunnelling itself is a veritable challenge in the Himalayas. Imagine the levels of complexity when constructing a tunnel with a downward gradient. The gradient of a tunnel is a measure of how steep a slope or a line is. It plays a major role in overall engineering & construction. Afcons constructed 7.32 KM of the tunnel from the north portal (Arpinchala Village) which is at an altitude of 1,600M.

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As a result, one of the major challenges was the pouring of overt gantry concrete from an upward gradient to a downward gradient. “The pouring rate of overt gantry concrete should not be more than 5.8 cum/hr. If the pouring rate is increased then there are chances of bulging of the gantry which will lead to misalignment. To avoid this, the team effectively and meticulously ensured an accurate pouring rate, and the gantry mechanism was minutely surveyed,” said Chandra Shekhar Dixit, Afcons’ Project Manager.

Excavation Challenges

Besides the unique challenge of gravity, there were few other known challenges in the region, and, especially, in the context of constructing such a long tunnel. Arranging ventilation systems, dewatering and navigating adverse geological and climatic conditions were clear and present dangers. In one of the tunnel sections, a 450M-long stretch had a thick overburden of 800-850M. “The overburden was causing incidents of rock bursting, spalling, and popping during excavation. We drilled pressure relief holes to release the accumulated stress, and installed Swellex bolts to provide immediate support," Dixit said.

In another two-kilometre stretch of the main tunnel, there was a risk of high deformation due to squeezing ground conditions. “We were facing a daunting challenge of diameter reduction by almost 1-1.5M. This would have significantly impacted the project progress. To avoid this, we identified the stretch, and reprofiled it," the Project Manager explained.

There were other geological challenges like cavity formation due to the presence of a shear zone in the escape tunnel. The construction activities were also impacted by huge water ingress. “The dewatering system for more than 4 KM-long tunnel in a downward gradient was extremely challenging. The increase in the length of the tunnel meant more water inside. Dewatering arrangements for such a long tunnel having a head of 40M was daunting,” Dixit said.
Despite numerous challenges, the breakthrough of the tunnel was achieved in February this year, and thanks to the incredible efforts of Afconians and other engineers, T49 will very soon be a matter of pride for the nation and the Indian Railways.
 
T49 Highlights
•  T49 is the longest railway tunnel in the country
•  The tunnel is 12.75 KM long and surpasses the Peer Panjal Tunnel (11.2 KM) 
•  It is part of the USBRL project 
•  Breakthrough of the tunnel was achieved on Feb 15, 2022
•  Afcons has constructed 7.32KM (nearly 60%) of the total tunnel
•  Afcons has constructed the tunnel from the north portal which is at an altitude of 1,600M

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