Tunnelling activities across infrastructure sectors have seen a steady rise over the years. The construction of tunnels has aided in enhancing connectivity to remote locations and areas of unpredictable terrain such as the Himalayan regions. Due to these complexities, tunnelling techniques have also evolved over the years. Conventional techniques have been replaced by newer methods, and the deployment of advanced machinery and technology has aided in the overall enhancement of tunnelling activities. Various techniques of tunnelling are being used, including the drill and blast method (DBM), the New Austrian Tunnelling Method (NATM) and by using tunnel boring machines (TBMs).
DBM
DBM is a conventional tunnelling technique used across infrastructure sectors, which involves drilling holes into the rock surfaces. Blasting takes place through explosives that are filled into these holes, causing the rocks to fragment, paving the way for the subsequent stages of constructing tunnels. In comparison to other techniques, DBM has lower requirements of initial investment. It is cost-effective, especially for the construction of shorter tunnels in comparison to using TBMs. Further, its ability to adapt to varied geological conditions and hard rock formations adds to its use cases in different tunnelling projects. DBM is also helpful for the construction of non-circular tunnels.
As per projects tracked by India Infrastructure Research, as of May 2024, more than 110 tunnels with a length of over 260 km (in terms of package length and the number of tunnels in packages) are being constructed using DBM. These tunnels also comprise those that are being constructed using DBM in combination with other methods such as NATM, TBM and cut and cover. Sector-wise, DBM is majorly being used in hydro tunnelling, with more than 90 tunnels in various packages spanning a length of over 230 km being currently constructed. Some of the key under-construction projects using DBM in the hydro segment include the Dibang Multi-Purpose Project with 21 tunnels spanning more than 8 km, and the Luhri Hydroelectric Project Stage I comprising two tunnels with a length of more than 38 km.
Processes under DBM have come a long way since its initial use cases. For instance, preliminary survey works for the construction of the Atal Tunnel utilised technologies such as GPS. Various technological advancements have emerged on the drilling side, emphasising its importance in the overall process. Precise drilling in the required formation ensures that blasting can be carried out with precision so that the desired and best results are obtained. New equipment such as drilling jumbos and road headers have aided in the process. Explosives have also witnessed advancements over the years, with the shift from gunpowder to emulsion-based explosives. Other explosives like liquid oxygen explosives and nitroglycerin-based explosives are also being used. DBM has also witnessed the utilisation of remote and electronic blasting technologies. Furthermore, there is scope for using technologies that simulate the outcomes of a blast, as well as surveying technologies such as light detection and ranging and interferometric synthetic aperture radar.
TBM
TBMs are gaining widespread recognition due to their advantages, leading to their increased use in various sectors. In comparison to DBM, they are faster and less disruptive. TBMs have high excavation rates and can work without delays. They also cause comparatively lesser ground disturbances than other techniques. As per projects tracked by India Infrastructure Research, as of May 2024, there are more than 80 tunnels spanning a length of over 360 km (under various packages) currently under construction that use TBMs or combinations of TBMs with other methods. Among these, the metro rail sector has the maximum utilisation of TBM, with more than 30 tunnels under different packages.
Various lines, as part of the Mumbai Metro project, have used TBMs for tunnelling. The availability of various types of TBMs, such as The Robbins Company’s Crossover XRE TBMs and machines by Terratec, has aided in the deployment of the appropriate machinery based on geological conditions. TBMs have also been used in other metro projects. For example, in March 2024, the TBM Narmada, a refurbished Terratec earth pressure balance machine, achieved a breakthrough at the central warehouse station under Line 1 of Surat Metro Phase I. The machine was also used in other sections of the project. In another notable development, the 394 metre additionally driven intermediate tunnel (ADIT) of the Mumbai-Ahmedabad bullet train project recently witnessed successful excavation. This is likely to facilitate the construction of the 21-km long tunnel between the Bandra Kurla Complex and Shilphata. Notably, this tunnel plans to deploy a TBM with cutter heads having a diameter of 13.6 metres, in comparison to cutter heads spanning 5-6 metres that are usually used for urban tunnels.
Various technologies and innovations have been used for enhancing tunnelling works using TBMs. For instance, Amberg Engineering India provides solutions such as tunnel seismic prediction (TSP). The incorporation of these TSPs in TBMs offers advantages including being lightweight and flexible. They also help in overcoming challenging conditions that affect TBMs by continuously acquiring seismic data and predicting weak zones.
NATM
The NATM is also known as a sequential excavation method. The technique usually follows a “build as you go” approach, where ground conditions are continuously monitored. In this method, the surrounding ground strength or rock strength is used to the maximum extent to strengthen the tunnel. Materials, including rock bolts and sprayed concrete, are also used for the same. Further, NATM has wide applications in areas with varying geology and is particularly suitable for weak ground conditions. Various projects in the Himalayan region have made use of this method for tunnelling activity.
As per projects tracked by India Infrastructure Research, as of May 2024, there are more than 120 tunnels spanning over 350 km (under various packages) currently under construction in different sectors. These include tunnels that use NATM alone, and those that use NATM along with other techniques. Among these, the concentration of tunnels and length is the highest in the railway sector (with more than 70 tunnels spanning over 240 km), followed by roads (more than 30 tunnels with a length of over 60 km). For instance, the Rishikesh-Karnaprayag New Rail Project has a total of 17 tunnels under different packages spanning over 100 km. Of these, 16 tunnels are being constructed using NATM, and the other tunnels employ both TBM and NATM.
NMT
Another method witnessing uptake in India is the Norwegian method of tunnelling (NMT). This technique is primarily used for hard rock surfaces, with excavation commonly carried out using DBM. Rock supports are majorly provided in the form of bolts. Reinforced ribs of shotcrete (RRS) are also used in weak zones. The technique uses the Q-system for assessing the quality of rock masses and understanding support needs. For instance, the Rishikesh-Karnaprayag project has installed RRS at the ADIT-3 tunnel. Global use cases suggest that the use of NMT has substantial cost- time- and material-saving benefits.
Other advancements
Various tunnelling techniques like NATM and TBM are witnessing an increased use of monitoring instruments for tunnel excavations including surface settlement points, optical displacement sensors, and seismographs for peak particle velocity. India has also witnessed the use of the I-System tunnelling method. The construction of the Udhampur-Srinagar-Baramulla Rail Link Project encountered several challenges. Among them was the construction of tunnel T-1, which passed through a shear zone called the main boundary thrust. Though tunnelling was initially carried out using NATM, it was later replaced by the I-System tunnelling method. Another method commonly deployed for the construction of water supply and sewerage tunnels is micro-tunnelling. Other methods, including the box pushing method and double-decker underground structures, are upcoming techniques.
Conclusion
Various techniques have been used for excavation and construction in the tunnelling segment. These are selected based on the geographical location of the project, geological factors, cost aspects, environmental considerations, on-ground conditions and numerous other factors. Conventional techniques such as the DBM are witnessing advancements in processes in order to adapt to varying conditions and for better construction. With the emergence of new technologies, these tunnelling techniques can be further developed to meet challenging requirements.