TBM ‘Shakti’: The Tech Behind India’s Longest Railway Tunnel Between Rishikesh and Karnaprayag

India's longest railway tunnel is currently under construction in Uttarakhand. Dubbed Tunnel No. 8 or T-8, it will be a 14.57km tunnel between Rishikesh and Karnaprayag, and connect Devprayag and Janasu in Uttarakhand. While the length of the tunnel in itself is a major engineering achievement, the technology that went into the excavation and construction of the structure in such a challenging terrain also deserves recognition. To carve a hole in the mountains, engineers used a combination of a German-imported Tunnel Boring Machine (TBM) named 'Shakti' and the New Austrian Tunnelling Method (NATM).

Using TBM Shakti and NATM to Build India's Longest Railway Tunnel

Larsen & Toubro (L&T) announced that it had achieved the breakthrough in the tunnel excavation project in April. Tunnel breakthrough is the point of excavation when both ends of a tunnel finally connect for the first time. The tunnel, which is part of the Rishikesh-Karnaprayag Broad Gauge Rail Link Project of Rail Vikas Nigam Limited (RVNL), is expected to be operational by the end of 2026.

Once it is operational, T-8 will supersede the 12.75km long T-49 tunnel between Khari and Sumber stations on the Udhampur-Srinagar-Baramulla Rail Link in Jammu and Kashmir as the longest railway tunnel.

At the time, L&T said that 10.4km of the tunnel was completed using a Single-Shield TBM named Shakti. With a diameter of 9.11m, it is said to be the largest TBM to be deployed in the Himalayan region. It excavated at an average rate of 413 metres per month. The remaining 4.11km of the tunnel was completed using the NATM.

A TBM is a giant cylindrical machine that is capable of excavating tunnels through both soil and rock while causing minimal disturbance to the surrounding ground. It contains a rotating steel disk with disc cutters that cuts through rocks at high pressure. The cutterhead is designed to cut through quartzite, schist, and phyllite, which are common in the Himalayan rock deposits.

A segment erector system is placed behind the cutterhead, which adds precast concrete segments to add structural integrity to the freshly excavated tunnel. Additionally, the debris (also known as muck) is continually removed from the tunnel using a conveyor system.

While the TBM was used for the major portion of the tunnel, it cannot be used in tricky geological areas coinciding with fault zones, water ingress, and more. Using a large machine to excavate these portions could negatively impact the structural integrity of the entire tunnel.

L&T says it used NATM, which uses drill-and-blast or mechanical excavation technique with constant monitoring of ground condition. Usually, small sections are excavated at a time using this technology. Once rock and muck is removed, shotcrete (sprayed concrete), rock bolts, and steel ribs are added to the exposed area to prevent deformation and collapse.

Throughout this process, crewmates monitor tunnel stress and movement using instruments such as extensometers and load cells. NATM is critical to tunnel excavation in challenging terrains, as they offer reduced dependence on large machinery and flexibility in the construction process.