Tunnel construction is critically dependent on geotechnical investigations, as these offer the best inputs for identifying the most efficient technique to be deployed for tunnel excavation. Another factor that determines the choice between mechanised and non-mechanised tunnelling is cost. Mechanised tunnelling usually involves higher equipment and transportation costs as compared to the conventional tunnelling technique. However, mechanised methods offer speedy project completion, thereby allowing faster inflow of revenues. As a result, mechanised techniques are gradually replacing traditional or non-mechanised methods of tunnel construction. Advanced, technology-driven, efficient and cost-effective methods of tunnelling are being deployed in almost all infrastructure sectors in the country.
As of January 2019, tunnels spanning a length of around 783 km have either been constructed, awarded or are under construction using the drill and blast method (DBM) of tunnelling. Overall, DBM remains the dominant method of tunnelling. Around 47 per cent of the total tunnel length, across all sectors (completed, ongoing and under-construction projects) have been constructed using this technique. DBM is mostly prevalent in the hydropower sector. Other sectors such as railways, metro and water supply also account for significant tunnel length being constructed/already constructed using this method. This technology is mostly deployed in areas that are less congested and has also been preferred in hilly areas (Himalayas and Western Ghats) where it is difficult to transport equipment like tunnel boring machines (TBMs).
In recent years, the New Austrian Tunnelling Method (NATM) has been gaining prominence, especially in the railway and metro rail sectors. While the hydropower sector accounts for the largest share of 49 per cent (78.3 km) of the total tunnel length constructed (completed projects) using this technology, the railway sector accounts for the largest share of 75 per cent (219.92 km) of the total tunnel length under construction.
With regard to the overall tunnel length completed, ongoing and awarded, the railway sector accounts for the largest share at 50 per cent (240.82 km), followed by the metro sector at 21 per cent (100.32 km) and the hydropower sector at 16 per cent (78.3 km). With the large pipeline of metro projects planned for implementation, NATM is expected to pick up further going forward.
As of January 2019, tunnels spanning a length of around 247 km have either been awarded, are under construction or have been constructed using conventional tunnelling techniques. Sector-wise, conventional techniques are mostly prevalent in the hydropower sector. One of the reasons behind this is the inability to use TBMs in hilly areas where hydro projects are developed. Tunnels spanning a length of more than 172 km have been constructed, are under construction or have been awarded using conventional technology.
DBM is the dominant method of tunnelling in India. Going forward, the trend is expected to continue across all sectors due to its benefit of being more flexible vis-à-vis TBMs and since it has the capability of handling multiple challenges unlike TBMs that require greater uniformity in operations. Unlike TBMs, the DBM technique is also more suitable for use in tough terrain that poses challenging geological conditions. Moreover, the mobilisation and demobilisation of TBM machinery is difficult in such terrains, which makes DBM a preferred technology.
Recently, GHH India Mining and Tunnelling Equipment Private Limited, incorporated in October 2016 as a subsidiary company of GHH Fahrzeuge (Germany), has emerged as a new entrant in the DBM market. Given the pace of infrastructural development, a greater number of foreign players are expected to enter the tunnelling equipment market going forward.
Cost-effective and technologically advanced DBM equipment such as computerised jumbos, designed to undertake tunnelling with greater precision, have been gaining prominence in recent years. This equipment is capable of minimising losses by eliminating the chance of over-excavating tunnels, which then requires costly materials to fill.
The NATM has existed in India for a long time now. However, this method requires a full package for implementation – smooth blasting, quick support at the face, proper geotechnical displacement monitoring, quality assurance and control on the primary support, and contract set-up in line with the tunnelling approach. NATM is a flexible and cost-efficient technique.
DBM, on the other hand, is very flexible and adaptable with comparatively lower costs. That said, there is a need for careful planning for the optimum selection of tunnelling alternatives, because a wrong choice can lead to costly and time-consuming consequences.