Difficult Track: Experience in rail tunnel construction in tough terrain

Experience in rail tunnel construction in tough terrain

Indian Railways has been steadily working to expand its network. The Himalayan region, owing to its difficult topography and access constraints, poses various challenges in the construction of tunnels. Despite this, several projects are being undertaken in the region, in­cluding the Udhampur-Srinagar-Baramulla rail link and the Bhanupali-Bilaspur-Beri new railway line project. A look at the experience of Rail Vikas Nigam Limited (RVNL) and Konkan Rail­way in tunnel construction in India…

RVNL perspective

Established in 2003, RVNL has a wide spectrum of projects. The Bhanupali-Bilaspur-Beri new rail line project is one of the key projects be­ing executed by RVNL, at an estimated in­vestment of Rs 67.53 billion. The total length of the project stands at 62.6 km, of which 26.13 km (42 per cent) is tunnels. The project is ex­pected to be completed by March 2025.

Of the total tunnel length of 26.13 km, one 3.81 km-long is a major one, while nine tunnels of a total length of 17.67 km are medium tunnels (with lengths ranging between 1 km and 3 km). Further, there are 10 minor tunnels of  4.65 km length. Of the four packages for the tu­n­nel, Package 1 is at advanced stages of cons­truction, where breakthrough has been achiev­ed in all seven tunnels. Construction un­der Pa­ck­ages 2 and 3 commenced recently. Un­der Package 4, tenders have been invited.

The key factors essential for deciding a tunnel’s cross-section include the kinematic envelope as per the Indian Railway Schedule of Dime­nsions, ensuring walkways for evacuation (a width of 1.2 metre has been kept on either side), and ventilation and overhead electrification space requirements. Geological considerations play a major part in deciding the cross-section of the tunnel. Given the poor rock mass conditions, a robust support system and a greater excavated profile are required.

Under Package 1, various investigations were performed to obtain the physical properties of the rock in the project area, before finalising the alignment and design of the proposed structures. For determining the final alignment, it was ensured that there would be minimum enc­oun­ter with geologically weak thrust zones and that the majority of the alignment was easily accessible for construction. Many other investigations, including in-situ permeability, pull-out test, pro­be-hole investigation, petrographic studies, etc., were carried out in the construction phase.

The project faced a major challenge due to the geological conditions. The rock mass was of very poor quality. Other challenges included low overburden, soft ground conditions, difficulties in pipe-roof drilling and water seepage during tunnelling (under Tunnel T4).

Konkan Railway perspective

Konkan Railway was the real game changer in railway tunnelling. With 91 tunnels aggregating 84.5 km un­der Konkan Railway, it was proven to administrators that tunnelling is possible on a large scale and in difficult terrain too.

The ongoing Katra-Banihal section under the USBRL project has a total of 28 tunnels, with a length of 94 km. This section is one of the most difficult projects to be undertaken in the country, with more than 85 per cent of the project length comprising tunnels. The longest tunnel has a length of 12.3 km. All the tunnels are at advanced stages of construction.

While designing and implementing tunn­els, the important components to be kept in mind include the choice of cross-section, layout of lo­ng tunnels, correct design philosophy and meth­ods, and advanced and safe excavation metho­ds. Correctly predicting the maximum moving dimension will help plan the tunnel in a more efficient manner. Clearances required for a track as mandated; structural behaviour during excavation, which can be gauged through geotechnical investigations; allowance for maintenance re­serves; walkways; drainage and ventilation sy­s­tems; power and communication cables; and track systems are other factors influencing the cross-section of a rail tunnel.

Some of the key issues faced during the implementation of rail tunnels include inadequate investigations, complexities in the Hima­layan region, complexities in shallow-depth tunnels, safety risks and contractual challenges. Over the years, improvements have been made in fire competency, drainage and safety in tunnels. Further, adoption of geospatial tools for de­termination of alignments, geophysical investigations, airborne electromagnetic surveys and in situ horizontal drilling, that is, tunnel seismic prediction, are key technologies that have help­ed in better execution of rail tunnel projects.

With the passage of time and increasing adoption of advanced technologies, more complex tunnels with bigger diameters are being taken up. Going forward, appropriate measures need to be taken by project developers, tunnel contractors and relevant government de­part­ments to mitigate the challenges associated with tunnel construction. Addressing these risks will provide the much-expected boost to the tunnelling sector. w

Based on presentations by Anmol Nagpal, Joint General Manager, Project, RVNL; and G.B. Nagendra, General Manager, Konkan Railway, at a recent Indian Infrastructure conference