With a large number of infrastructure projects being implemented across sectors such as railways, roads and highways, water supply and sewerage, and urban mass transit, there has been a surge in tunnelling activity in the country. Recently, the segment has received greater attention due to increased congestion in urban areas that has left little scope for on-ground construction. As a result, tunnelling techniques that are now evolving offer greater flexibility to contractors. These mechanised techniques are gradually replacing traditional or non-mechanised methods of tunnel construction. While they usually involve higher equipment and transportation costs as compared to the conventional tunnelling techniques, they offer speedy project completion, thereby enabling faster inflow of revenues.
Key technologies
In recent years, the New Austrian Tunnelling Method (NATM) has gained importance, especially in the railway and metro rail sectors. It is a flexible and cost-efficient technique and is fast emerging as an alternative to traditional methods in congested urban areas. The NATM is a preferred method for tunnels in mountains with uncertain geological and hydrogeological conditions. Besides, it is appropriate for construction of approach roads of adequate geometry for caverns and mines. The NATM, which is based on deformation monitoring data to evaluate quantity and the quality of the support system, has also proved to be an appropriate tunnelling technique in the Himalayas. Further, with the pipeline of metro projects planned for implementation, the use of the NATM is expected to increase further in the times to come. Key projects implemented using the technique include various stretches under Phases I and II of the Delhi metro rail project. In addition, the technology has also been used in the Lucknow metro rail project. The NATM offers various advantages over other techniques of tunnelling such as a flexible support system, scope for additional support and flexibility in the choice of cross-section. Going forward, the scope of work for the NATM technology partner should include smooth blasting, quick support, rather short rounds at the face, proper geotechnical displacement monitoring, quality assurance and quality control on the primary support. This would make the NATM a flexible and cost-effective tunnelling technique.
Another advanced method gaining traction in tunnel construction is microtunnelling, also known as trenchless or pipe jacking technology. It is especially used for projects that require tunnels under roads with high traffic volumes, railways, rivers, etc. It is used for laying large diameter gravity sewers in cities where open cut installation is difficult, for the installation of product pipelines in areas where the soil condition does not allow for horizontal directional drilling (HDD) and for long individual crossings across rivers. This method is particularly used for constructing tunnels with diameters ranging from 600 mm to 3,000 mm. Key foreign players dominating the microtunnelling equipment market are Herrenknecht, Teratec, Robbins, Caterpillar and Akkerman. Meanwhile, the main equipment used in microtunnelling is the remote-controlled steerable boring machine, which is operated from a computer operator station using drive motor controls. Mumbai, the first city in the country to experiment with microtunnelling, used the technology in the World Bank-funded Mumbai sewage disposal project. Other cities such as Delhi and Kolkata have also used this technology. Microtunnelling is expected to gain prominence for laying deep water supply and sewer lines in areas where open cut tunnelling is not feasible due to existing surface utilities. Despite being expensive, microtunnelling is preferred over HDD due to its accuracy and reliability and lower maintenance cost of pipelines. Unlike the drill-and-blast-method (DBM) that has a higher environmental impact, greater noise and vibration levels and disturbance to wildlife, the microtunnelling method involves minimal interface with the local environment, making it suitable for constructing tunnels, especially in congested areas.
Tunnel boring machines (TBMs) have emerged as the preferred tunnelling technique in congested urban areas. They have been deployed primarily in the metro rail sector. With a number of tunnelling projects in the water supply and sewage sector also being undertaken in metro cities such as Mumbai, the use of TBMs, also known as moles, has gained traction. These machines are being custom-made to deal with site-specific requirements of projects. TBMs are primarily supplied by international players such as Herrenknecht, the Hitachi-Zosen joint venture, Okumura, Robbins, Shanghai Tunnel Engineering Company Limited and Seli-Kawasaki. There is a significant difference in the cost of imported TBMs versus that of domestically produced/assembled ones. In fact, imported TBMs are cheaper. The key reason behind this is that an imported TBM is 100 per cent duty-free, while a domestically produced TBM attracts a number of duties and taxes. Herrenknecht is the only company to have established a state-of-the-art TBM assembly and cutting tools manufacturing facility in the country (in Chennai). The TBM market has witnessed a recent trend of contractors entering into buyback arrangements with suppliers. Further, depending on the pipeline of projects, geology and cost-effectiveness of refurbishing existing machinery, contractors have been renting equipment, as many projects involve the use of different machines for short periods of time. The increasing tunnelling activity in urban areas is expected to result in the greater deployment of TBMs. Going forward, the TBM technology is expected to dominate the irrigation tunnelling segment.
Despite the mechanised techniques on offer, the DBM continues to be the most used method of tunnelling across all sectors. The hydropower, railways and metro rail sectors have deployed the technique for considerable tunnel lengths. One of the key reasons for the DBM being the preferred method is that it is flexible and is more capable of handling multiple challenges as against TBMs that require greater uniformity in operations. Unlike TBMs, the DBM is more suitable for use in tough terrains such as the Himalayas and the Western Ghats that pose challenging geological and hydrological conditions. Moreover, the mobilisation and demobilisation of TBMs is difficult in such terrains, making the DBM the preferred technology. In the past few years, the demand for computerised jumbos for tunnelling has increased. Besides ensuring high productivity, these jumbos are reliable and versatile. Further, the equipment is capable of minimising losses by eliminating the chance of over-excavating tunnels, which then require costly materials to fill in the space. The DBM is therefore expected to remain the dominant technique for tunnelling, given its advantage of being more flexible and having the ability to handle challenges vis-à-vis other advanced mechanised techniques.
The way forward
Indian industry is just beginning to use modern technology for tunnel construction. Almost all the upcoming tunnel projects are of much higher value than had been the case in the past. There is also greater room for the adoption of international standards in tunnel design and construction. However, the dependence on other countries for advanced tunnelling machinery remains very high. Key countries exporting tunnelling machinery to India are China, Finland and Germany. While China’s share in the total imports of tunnelling machinery declined from 58.47 per cent in 2017-18 to 47.27 per cent in 2018-19, it continues to dominate the segment. Therefore, India needs to move away from imports of tunnelling machinery towards greater self-reliance and encourage domestic production. A large amount of tunnelling activity in difficult terrains such as the Himalayas and the Western Ghats is still carried out using the conventional DBM technology. There is a pressing need to switch to mechanised ways in order to ensure safety of workers. Greater use of new high-end technology in tunnelling will go a long way in improving efficiency in operations for all the stakeholders involved.