Evolving Methods: Tunnel construction techniques in use

Tunnel construction techniques in use

Mechanised techniques are gradually replacing traditional or non-mechanised methods of tunnel construction in the country. Advanced, technology-driven, efficient and cost-effective methods of tunnelling are being deployed in almost all infrastructure sectors. However, despite the advances in technology, non-mechanised methods remain a dominant technique of tunnel construction. In fact, various sectors have been deploying different methods of tunnelling depending on factors such as geology of the location, soil conditions, contracting terms and conditions, tunnel length, etc.

Factors determining tunnelling techniques

Tunnelling depends heavily on geotechnical investigations which offer the best input in identifying the most efficient technique for tunnel excavation. The surveys assess the three most crucial factors for ensuring the smooth execution of tunnel construction – groundwater conditions, rock quality, and stress state of the land. Further, the geotechnical investigation assesses other factors such as regional tectonics, palaeo stress history, etc. However, it is pertinent to note that given the geological complexities, tunnelling is a risky undertaking, as any deviation from the right choice of technique could lead to serious cost and time overruns. Therefore, serious attention needs to be paid during this stage of investigation.

Besides geotechnical investigations, another factor that greatly influences the choice between mechanised and non-mechanised tunnelling is cost. Several studies reveal that project components such as material, equipment, transport and personnel tend to vary significantly in terms of cost across various techniques of tunnel construction. Mechanised tunnelling usually involves higher equipment and transportation costs in comparison to the conventional tunnelling technique. However, the former offers speedy completion of projects, thereby allowing faster inflow of revenues.

Tunnelling methods and techniques

The most common methods of tunnelling are conventional, drill-and-blast, tunnel boring machines (TBMs) and the new Austrian tunnelling method (NATM). Of these, drill-and-blast is the most commonly deployed technique across all infrastructure sectors, accounting for nearly 55 per cent of the total length of tunnels completed, ongoing or currently in the pipeline for development. Sector-wise, drill-and-blast is the most common technique in the hydropower, road and railway sectors, accounting for a share of 68 per cent, 50 per cent and 64 per cent respectively, while TBM is the most common technique in the metro rail and irrigation and water supply sectors accounting for an 85 per cent and 56 per cent share respectively.

While the non-mechanised technique, drill-and-blast, is mostly undertaken in hilly terrains like the Himalayas and the Western Ghats, mechanised techniques like TBMs and NATM have gained prominence in congested urban areas. Sector-wise, NATM has been gaining traction in the road and railway sectors, with a share of nearly 45 per cent and 34 per cent respectively. Further, besides congested urban areas, the use of NATM is also becoming prominent in the Himalayan region. The recently inaugurated Chenani-Nashri tunnel in Jammu & Kashmir, said to be the country’s longest tunnel at 10.89 km, was constructed using the NATM technique.

The use of TBMs is the most prominent in the irrigation and water supply, sewerage and metro rail sectors for tunnel construction in congested urban areas. This technique has been successful in projects such as the Delhi metro and the Srisailam Left Bank Canal tunnel scheme. However, a major deterrent in employing this technique has been the high cost associated with it and the complexities involved in mobilising machines to job sites, especially in hilly terrains. Further, there are considerable chances of cost and time overruns in the case of flooding or mucking, which could cause TBMs to get buried. Two examples of projects which faced this issue are the Parbati II and Dul Hasti hydropower projects in Himachal Pradesh and Jammu & Kashmir respectively.

Besides these techniques, other methods such as the DRESS (drainage, reinforcement, excavation, support and solution) methodology, used in the Nathpa Jhakri hydro project in Himachal Pradesh; the P5 system (plug, probing, pressure relief, protection of roof, and pregrouting and support) used in the Allain-Duhangan hydropower project in Himachal Pradesh; the ground freezing technique; and the pregrouting technique are being used in cases where the terrain is rocky and water ingress is high.

In addition, micro-tunnelling, also known as trenchless or pipejacking technology, has gained importance in recent years, with the technique being employed in laying water supply pipelines and sewers in congested areas where tunnels need to be constructed under roads with high traffic volumes. This method is especially used for constructing tunnels with diameters ranging from 600 mm to 3,000 mm. Micro-tunnelling is also 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 horizontal directional drilling; and for long individual crossings across rivers. The use of this technology has gained momentum in the past two decades. Mumbai was the first city to test this technique, in the World Bank-funded Mumbai Sewage Disposal Project. Under this project, micro-tunnelling contracts worth Rs 340 million were awarded to TTI Consulting Engineers (Sydney). Since then, cities such as Delhi and Kolkata have also deployed this technology. Going forward, congested cities like Chennai, Bengaluru and Ahmedabad are also likely to use this technology.

The way forward

Going forward, developing tunnelling technologies domestically, especially in the case of carving out tunnels under water and in difficult hilly terrains like the Himalayas and the Western Ghats, will go a long way in saving huge foreign exchange currently being spent on such technologies. Tunnelling activities in the Himalayas face diverse geological problems like difficult terrain conditions, thrust zones, shear zones, in-situ stresses, ingress of water or gases, geothermal gradient, high level of seismicity, etc. Developing effective technologies to tackle these problems will help in reducing time and cost overruns.

Regarding tunnelling methods, drill-and-blast is likely to remain the most dominant technique of tunnel construction in the country, especially in sectors like hydropower and railways, where most of the tunnels are being built in hilly terrains.

Meanwhile, mechanised techniques like NATM are also rapidly emerging as a cost-effective alternative to non-mechanised techniques in the road and railway sectors. In the times ahead, the use of such techniques should become the norm, especially across metro rail, water supply and sewerage projects which are undertaken in densely populated areas.