Significant progress in technology as well as increasing focus on cost efficiency and safety have necessitated vast changes in the methods and techniques employed in tunnelling. About three decades ago, India relied largely on conventional methods, which resulted in cost and time overruns. However, over the past five-six years, advanced mechanised methods have become more dominant, though traditional methods still continue to be used. Different infrastructure sectors deploy different methods of tunnelling depending on the geology of the location, soil conditions, contracting terms and conditions, length of the tunnel, etc.
Assessment of groundwater conditions, rock quality and stress state are the three pillars of effectively executing a tunnel project. In addition, other geological factors such as regional tectonics, palaeostress history, etc. are also taken into consideration to decide the tunnelling technique. Geotechnical investigations provide the best input in identifying the method of tunnel excavation. It is pertinent to note that an inappropriate choice of tunnelling method could result in considerable time and cost overruns. Thus, adequate attention needs to be paid during the geotechnical investigation and design stage.
Currently, drill-and-blast is the most widely used method of excavation in the tunnelling industry across sectors – hydropower, road and highway and railway. For these sectors, where most tunnels are constructed in the hilly terrain of the Himalayas and in the Western Ghats in peninsular India, drill-and-blast is the most common method of tunnelling.
Recently, mechanised techniques such as the use of tunnel boring machines (TBMs) and the New Austrian Tunnelling Method (NATM) have been gaining prominence with increased tunnelling activity in congested urban spaces. In the railway and road sectors, NATM is also gaining prominence for construction in the Himalayan region. One of the biggest railway tunnel projects under construction in Jammu & Kashmir — the Katra-Qazigund railway line, with 36 tunnels spanning 109.54 km — is being constructed using the NATM method.
Likewise, in the irrigation, water supply, sewerage and metro rail sectors, TBMs are also being deployed for tunnel construction in congested urban areas. Excavation using TBMs has been rather successful in projects such as the Delhi Metro and the Srisailam Left Bank Canal tunnel scheme.
However, the high cost of TBMs and the difficulty in mobilising machines to the job sites, especially in the Himalayan region, remain big deterrents. Flooding and mucking have sometimes resulted in TBMs getting buried, leading to time and cost escalations. Hydropower projects in Himachal Pradesh (Parbati II) and Jammu & Kashmir (Dul Hasti) are cases in point.
Further, special techniques are used in situations where rock conditions are poor and water ingress is high. These include the DRESS (drainage, reinforcement, excavation, support and solution) methodology (used in the Nathpa Jhakri hydropower project, Himachal Pradesh); the P5 system (plug, probing, pressure relief, protection of roof, and pre-grouting and support) (Allain-Duhangan hydropower project, Himachal Pradesh); ground freezing; and pre-grouting. These systems are now increasingly being used in tunnelling projects.
Various research studies and expert opinions reveal that costs of key project components such as material, equipment, transport and personnel vary significantly according to the technique adopted for tunnel construction. Mechanised tunnelling processes involve higher equipment and transportation costs in comparison to conventional tunnelling. However, these are faster and save on construction time; hence revenue flow from the project starts earlier.
Of late, micro-tunnelling is emerging as a new trend in India, and is being used to lay water supply pipelines and sewers in congested areas. Also known as trenchless or pipe-jacking technology, micro-tunnelling is especially used for projects that require tunnels under roads with high traffic volumes. This method is particularly used for constructing tunnels with diameters ranging between 600 mm and 3,000 mm. Micro-tunnelling is also used for laying large diameter gravity sewers in cities where open-cut installation is difficult; for installation of product pipelines in areas where the soil condition does not allow for horizontal directional drilling; and for long individual crossings across rivers.
The use of trenchless technology in India gathered momentum in the late 1990s. Mumbai was the first city to experiment with micro-tunnelling for 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). Other cities such as Delhi and Kolkata have also used this technology. Some projects in Chennai, Bengaluru and Ahmedabad could also use it in the near future.
Going forward, the drill-and-blast method will continue to remain the most commonly used tunnelling technique, particularly in the hydropower and railway sectors, where tunnels are mostly built in the Himalayan region and the Western Ghats. On the other hand, advanced NATM is fast emerging as a more cost-effective alternative for road and railway tunnels. That said, tunnelling activity in the Himalayas faces diverse geological problems such as difficult terrain conditions, thrust zones, shear zones, in-situ stresses, rock cover, ingress of water or gases, geothermal gradient, high level of seismicity, etc. Proper strategies counteracting these challenges coupled with modern tunnelling technologies are required to reduce time and cost overruns. Moreover, mechanised tunnelling solutions such as TBM and NATM should become a norm for metro rail, water supply and sewerage projects which are undertaken in densely populated urban areas.
