Tunnel construction activity in India has seen significant growth in the past few years owing to the development across key infrastructure sectors, including urban rail, roads and highways, hydropower, and water and sewerage. The introduction of initiatives such as the Pradhan Mantri Krishi Sinchayee Yojana, the Interlinking of Rivers Programme and the Jawaharlal Nehru National Urban Renewal Initiative for the exploitation of water resources have given a fillip to tunnel construction for water supply and sewerage. Similarly, increasing development in the metro rail sector has spurred tunnelling activity. Ongoing tunnel projects include tunnels under the Jammu-Udhampur-Katra-Qazigund-Baramulla Railway Line Project, the Sela tunnel, and tunnels under the Delhi Metro Rail Project and the Mumbai Sewerage Disposal Project.
Various methods are being used for the construction of tunnels. These include conventional techniques such as the drill-and-blast method (DBM), tunnel boring machines (TBMs), the New Austrian Tunnelling Method (NATM) and the micro-tunnelling method. Recently, contractors have also been exploring modern techniques such as sequential excavation and ground freezing. Moreover, innovative materials such as fibre bolts, lining stress controllers, geosynthetics and steel anchors are being used to enhance the execution of tunnelling projects.
DBM
DBM has emerged as the preferred technique for tunnelling across various sectors, especially in the Himalayan region, owing to its flexibility compared to other methods such as TBM. Furthermore, in difficult terrain with challenging geological conditions, mobilising and demobilising TBM machinery can be challenging, making DBM the preferred technology.
Keeping workers safe while drilling and blasting is one of the biggest problems contractors face today. DBM tunnelling presents a greater risk to construction workers than other methods. In addition, the Himalayan region is well known for its complex geology, which can create challenges such as cavities, loose falls, high water infiltration and difficult ground conditions. Contractors frequently have to alter previously approved tunnel designs, causing delays and cost overruns.
A few new technologies have been implemented to improve DBM’s effectiveness. These include advanced blast fragmentation modelling programmes, such as two-dimensional bench, which analyses and models the detonation of a proposed design. Furthermore, air overpressure measurement and blast vibration assessment are now possible with remote blast monitoring services. In addition, the use of technologically sophisticated and reasonably priced DBM equipment, such as computerised jumbos that can perform tunnelling with increased accuracy, has grown.
Among the projects tracked by India Infrastructure Research, more than 250 tunnels spanning a length of over 700 km have been constructed using DBM (including projects that have used other methods such as TBM and NATM along with DBM). DBM has predominantly been deployed in the hydro sector, accounting for around 470 km of tunnel construction, followed by the irrigation sector at 22 km. As of July 2023, 10 tunnel packages, spanning a length of about 55 km, are being constructed; and 10 tunnels, spanning a length of about 60 km, are in the pipeline. With regard to the number of tunnels, the hydropower sector accounts for the maximum share of upcoming projects (in the under-construction, planned and bidding stages) with (11 projects), followed by railways (6 projects). Some of the key completed tunnel projects constructed through DBM are the Contour Canal project, the Gundavali to Bhandup water complex and the Upper Vaitarna Scheme. The key upcoming tunnel projects using DBM include the Chandragala tunnel, the Jhelam Tamak hydroelectric project, the Devsari hydroelectric project and the Narihalla pumped storage project.
TBM
TBM offers advantages by limiting disturbance to the surrounding ground and creating smooth tunnel walls. Due to this, TBM has seen significant adoption in congested urban areas. It reduces a tunnel’s lining costs considerably, making it appropriate for urban settings.
The actual conditions of the soil are still unpredictable, causing geotechnical and hydrogeological uncertainties. This is one of the biggest challenges faced by TBM contractors. There is not enough skilled labour available to handle the increasing number of TBMs scheduled for deployment in upcoming projects. The mobilisation and demobilisation of TBM machinery and equipment presents challenges for TBM contractors as well, particularly when relocating them to project locations across the nation.
In the Indian TBM market, the soil composition and topography of project sites determine the use of open-mode TBMs, earth pressure balance machines and mixed shield or slurry TBMs. The Automatic Tailskin Clearance Measurement System and the TUnIS Ring Sequencing System are two new technologies being used for TBMs. In addition, the incorporation of data management systems in TBMs guarantees effective management and makes the administration of additional operational procedures in tunnelling projects easier.
Among the projects tracked by India Infrastructure Research, 63 projects, with a combined length of over 600 km, have been completed using TBM (including projects that have used other methods such as DBM and NATM along with TBM). The highest utilisation of this method has been witnessed in the urban metro sector (about 35 projects), followed by the water supply and sewerage sector (16 projects). Many projects have been undertaken using TBM machines. For instance, the biggest TBM ever used in the nation was 12.2 metres wide, and was employed to dig tunnels for the coastal road project in Mumbai.
Moreover, 51 upcoming projects (both ongoing and planned), spanning a length of over 90 km, are deploying TBM. With regard to the number of tunnels, the metro sector accounts for the largest share in upcoming projects (23 projects), followed by the water supply and sewerage sector (eight projects). Many upcoming projects will use this method. For instance, as part of the Mumbai-Ahmedabad bullet train project, India’s first underwater rail tunnel will be dug by a TBM with a 13.1 metre wide cutter head.
NATM
In circumstances in which geography and hydrogeology are uncertain, NATM has emerged as the method of choice for tunnelling. It is a cost-effective method, as it uses deformation monitoring data to determine the quantity and quality of the required support system.
This method is appropriate for the construction of approach roads of adequate geometry, caverns and mines. Among the projects tracked by India Infrastructure Research, 51 tunnels spanning a length of about 300 km have been completed using NATM (including projects that have used other methods such as TBM and DBM along with NATM). The highest deployment of the NATM technique has been witnessed in the hydro sector (23 projects), followed by the metro rail sector (14 projects). Moreover, 42 projects spanning a total length of about 280 km are in the pipeline.
Micro tunnelling
Pipelines can be installed beneath roads, railroads, runways, harbours, rivers and environmentally sensitive areas using a trenchless construction technique called micro-tunnelling. Large-diameter gravity sewer installation projects in cities where open-cut installation is challenging can benefit from this method. In locations where the soil conditions prevent horizontal directional drilling and lengthy individual river crossings, it is also used to install product pipelines.
The Mumbai sewage disposal project was the first project to deploy micro tunnelling. In recent times, this technique has also been used in other cities, such as Delhi and Kolkata. However, micro tunnelling also faces many issues and challenges. Because it depends so heavily on soil conditions, it is subject to hydrogeological and geotechnical uncertainties. Furthermore, because of the jacking procedures required, this technique is inappropriate for pipes with low strength. The cutting process may be complicated by obstructions in the soil profile, which would impede the tunnelling process.
In sum
Going forward, the overall outlook for tunnel development in the country remains promising. According to India Infrastructure Research, the tunnelling sector offers a strong pipeline of tunnel projects, spanning a length of over 3,600 km across various sectors such as roads, railways, hydro, irrigation, water supply, sewerage and metro rail. The sector holds immense promise for contractors, consultants, and technology and equipment providers. According to industry experts, going forward, DBM is expected to remain the dominant technique for tunnelling. This is because, compared to other advanced mechanised techniques that demand more uniformity during project implementation, it is more flexible and can withstand greater challenges.
Naina Gulati