Conventional tunnelling describes the cyclic construction of underground tunnels of any shape where there is access to the excavation face. Earlier, conventional methods of tunnel construction excluded tunnel boring machines (TBMs); however, in recent years the use of TBMs has become more common and is, therefore, it is being increasingly included within the ambit of conventional tunnelling techniques.
Due to access to the excavation face, conventional tunnelling is considered a very flexible process and is preferred in situations where there is a variation in ground conditions or tunnel shape. This flexibility includes the ability to vary the support, the blasting technique and size, ring closure time and the excavation technique. The most common methods of tunnelling are conventional, drill-and-blast (DBM) method and TBMs. Of these, DBM is the most commonly deployed technique across all infrastructure sectors.
Recent trends and developments
TBMs have predominantly been deployed for the construction of tunnels in the metro rail sector (41 per cent). Metro is followed by the irrigation, hydropower and water supply sectors at 22 per cent, 15 per cent and 15 per cent respectively. With respect to the irrigation sector, TBMs have started replacing DBM, which was used earlier. The same holds true for the water supply sector as well. So far, tunnels spanning a length of around 144 km and 100 km are either being constructed or are under construction using TBMs. The plausible reasons for the use of TBMs in the water supply sector in urban areas are heavy traffic movement, congestion and space constraints.
The key tunnelling projects in the metro rail sector completed by TBMs include the Delhi Metro Rail Project, Phase III, Janakpuri West-Kalindi Kunj-Botanical Garden (Magenta Line) stretch; Delhi Metro Rail Project, Phase III, Majilis Park-Shiv Vihar (Pink Line) stretch; Delhi Metro Rail Project, Phase II, Delhi Airport Express Line (IGI Airport-Dwarka Sector 21); Delhi Metro Rail Project, Phase II, Central Secretariat-HUDA City Centre (Line 2 extension); and Delhi Metro Rail Project, Phase I, Vishwa Vidyalaya-Central Secretariat (Line 2). Further, there are various ongoing projects in the metro rail segment that are using TBMs for tunnel excavation.
One of the major challenges faced by TBM contractors is with regard to geotechnical and hydrogeological uncertainties. Despite proper soil investigations and other preliminary investigations, the actual soil conditions are unpredictable. This is particularly true for the Himalayan region. The lack of infrastructure and efficient logistics in the country is another challenge. The timely availability of raw material and spare parts at a reasonable cost is one of the prerequisites for ensuring the timely completion of projects. As infrastructure is not fully developed and proper logistics facilities are not available, companies are forced to maintain an inventory of spare parts and components, adding to the costs of manufacturing. Further, TBM contractors face problems in mobilising/ demobilising TBM equipment and machinery, especially while transferring the same equipment to project sites located in different parts of the country.
DBM has predominantly been deployed for the construction of tunnels in the hydropower sector. As many as 323 tunnels spanning a length of over 680 km have been constructed (awarded, ongoing and completed tunnels) using DBM. Hydropower is distantly followed by railways – around 178 km. Overall, DBM is mostly deployed in less congested areas. It is also the preferred method for the construction of tunnels in the Himalayan region and the Western Ghats.
Some of the key hydropower tunnels excavated through DBM are the Rangit tunnel project, Nathpa Jhakri tunnel project, Teesta 5 tunnel project, Chamera 3 tunnel project and Rampur tunnel project. Further, various other tunnels are being excavated through DBM in the hydropower and irrigation sectors in the country.
One of the main challenges faced during tunnelling using DBM pertains to geotechnical and hydrogeological uncertainties. Geological complexities (faults/shear zones) causing cavity, loose fall, high ingress of water and squeezing ground conditions are common phenomena. These conditions often result in contractors altering the previously approved tunnel designs, leading to time and cost overruns. Another challenge faced by contractors is ensuring the safety of workers during drilling and blasting. Compared with other techniques of tunnelling, DBM involves a higher risk to construction workers. The shortage of trained workforce for undertaking drilling and blasting activities is another major issue faced by contractors.
The way forward
Going forward, DBM is likely to remain the most dominant technique of tunnel construction in the country, especially in sectors such as hydropower and railways, where most of the tunnels are being built in hilly terrain. Meanwhile, mechanised techniques such as the new Austrian tunnelling method are rapidly emerging as cost-effective alternatives to conventional techniques in the road and railway sectors.