Tunnel construction activity in India has picked up pace in recent years. A growing focus on improving road and rail connectivity, the emergence of several new urban mass rapid transit systems and the need to upgrade water supply and sewerage systems, hydropower projects, etc., are key factors driving the demand in the sector. According to projects tracked by India Infrastructure Research, as of May 2024, India has more than 1,470 completed and operational tunnels spanning over 3,400 km. A sector-wise analysis indicates that the hydropower sector has the highest share of completed tunnels with over 1,100 km, followed by metro rail (more than 840 km), irrigation (more than 550 km), railways (more than 500 km), water and sewerage (more than 260 km) and roads (more than 120 km).
Innovative techniques being explored by contractors
In India, several tunnelling techniques are being used, such as the new Austrian tunnelling method (NATM), the drill and blast method (DBM) and tunnel boring machines (TBMs). In recent years, the sector has seen an increasing shift from conventional tunnelling techniques to more mechanised techniques and cutting-edge equipment and technologies, improving the overall tunnelling operations.
As per projects tracked by India Infrastructure Research, as of May 2024, more than 110 tunnels spanning a length of over 260 km are being constructed using DBM (including tunnels that are being constructed using DBM in combination with other methods). Meanwhile, there are more than 80 tunnels spanning a length of over 360 km currently under construction that use TBMs or combinations of TBMs with other methods. Further, more than 120 tunnels spanning over 350 km in different sectors are currently under construction using the NATM method, or combinations of NATM with other methods.
TBMs are being extensively used in the implementation of metro tunnel projects due to advantages such as high excavation rates, faster construction, less disruption and lesser ground disturbances compared to other techniques. NATM is taking over conventional methods in the Himalayan region, as it better adjusts to the geographic conditions and also speeds up project implementation. In this method, the rock strength or surrounding ground strength is used to the maximum extent to strengthen the tunnel.
Growing technology and sustainability focus
Sustainability is emerging as a key focus area in the tunnelling sector. Practices such as the reutilisation of excavated muck, use of solar lighting, judicious water usage and restrictive use of fossil fuels are being adopted. Further, the use of cutting-edge technologies in the sector is improving operational efficiency for all the stakeholders involved. Sophisticated drilling techniques, computerised jumbos and navigation systems are being increasingly used to improve precision and monitoring. The application of strength monitoring using thermal imaging technology enables the real-time evaluation of the strength of the entire shotcrete lining, quality control and productivity benefits.
Moreover, innovative machines and equipment are being used for tunnel construction in the country. These are helping to revolutionise tunnel construction, allowing faster and more efficient excavations. For instance, variable density TBMs offer increased safety and flexibility in choosing tunnel face support and discharging muck. Another example is the use of a remote cutter monitoring system, which allows contractors to minimise TBM downtime and maximise efficiency and excavation potential.
The quality of construction materials used in tunnelling has also evolved over the years. New and innovative materials are being deployed during tunnel construction to ensure safety, minimise vibrations and resistance, and make tunnels last longer with fewer maintenance requirements. The growing complexity of tunnel construction, especially in the Himalayan and peninsular regions, has necessitated the use of new and advanced materials. For instance, contractors are using 
innovative materials such as fibre bolts, fibre-reinforced shotcrete, lining stress controllers, geosynthetics, steel anchors and self-drilling rock bolts. Geotextile membranes are being widely adopted for waterproofing tunnels. Materials such as steel fibre-reinforced shotcrete, safer emulsion-based explosives, geosynthetics such as 5D steel fibres (for increased tensile strength), mineral admixtures, steel anchors and self-drilling rock bolts are being deployed to improve the durability and strength of tunnels. There is also an increased use of materials for primary support, such as sprayed concrete, face bolting, forepoles and lattice girders. Furthermore, advanced materials are being deployed for the final lining, such as sheet or sprayed membrane for waterproofing systems and polypropylene microfibres for fire resistance.
Issues and challenges
While there have been several advancements, challenges such as ambiguity in contracting practices, lack of skilled manpower, safety and risk issues, geographical complexities and inadequate investigations remain, leading to significant cost and time overruns. Issues arising from geological complexities and inadequate investigations continue to hamper the pace of tunnel construction. Moreover, poor and insufficient investigation results in contractual delays and disputes.
Deficiencies in contract documents and contracting practices are yet another challenge. Contracts are being drafted on age-old norms, which leaves room for ambiguity between the implementing agency and contractors. Further, the shortage of skilled manpower for handling sophisticated tunnelling equipment presents considerable challenges in tunnel construction. Inadequate ventilation during the construction of long tunnels and insufficient safety measures (in the case of sudden ingress of water), also affects the construction progress.
Another key issue is the inadequate investigation of ground and soil conditions, which leads to time delays, cost overruns and occasionally, the failure of tunnelling projects. With the growing tunnelling activity in the country, environmental concerns are increasing. For instance, seismic activities in hilly areas can increase the risk of landslides. It may also lead to gradual settling or sudden sinking of the earth’s surface.
Outlook and opportunities
Based on projects tracked by India Infrastructure Research, as of May 2024, the tunnelling segment offers a lucrative pipeline of more than 1,140 tunnels, spanning a length of approximately 1,750 km, including planned and under-bidding projects. Of the total number of tunnels analysed, 89 per cent are under the planned category, and the remaining 11 per cent is under bidding. Meanwhile, in terms of length, around 10 per cent are under bidding and the remaining 90 per cent is under the planning stage. In terms of investment, according to India Infrastructure Research, the sector requires an investment of around Rs 3.5 trillion.