There has been a significant increase in tunnelling activity in the country over the past few years, accompanied by advancements in tunnelling techniques. The choice of a tunnelling technique is heavily influenced by soil conditions. As different techniques are being implemented and tunnelling equipment continues to evolve, the process is becoming increasingly mechanised. The industry is adopting new technologies and techniques to enhance the execution of tunnelling projects.
Indian Infrastructure takes a look at the advancements in tunnelling techniques…
DBM
The drill and blast method (DBM) has emerged as the preferred technique for tunnelling across various sectors, especially in the Himalayan region, owing to its flexibility as compared to other methods like the tunnel boring machine (TBM). Moreover, the mobilisation and demobilisation of TBM machinery can be difficult in tough terrain with challenging geological conditions, establishing DBM as a preferred technology.
Based on the projects tracked by India Infrastructure Research, more than 200 tunnels spanning a length of over 250 km have been constructed using DBM. DBM has predominantly been deployed in the railway sector, accounting for 82 per cent of tunnel construction, followed by the irrigation sector at 7 per cent. Currently, 42 tunnels, spanning a length of about 31 km, are being constructed, and 23 tunnels, spanning a length of about 25 km, are in the pipeline. With regard to the number of tunnels, the hydropower sector accounts for the maximum share in the upcoming projects (in the under-construction, planned and bidding stages) at 61 per cent, followed by railways (38 per cent) and roads (1 per cent). 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 the DBM technique include the Chandragala tunnel, JelamTamak hydroelectric project, Devsari hydroelectric project and Narihalla pumped storage project.
One of the main challenges faced by contractors is ensuring the safety of workers during drilling and blasting. As compared to other tunnelling techniques, DBM poses a higher risk to construction workers. Besides, the Himalayan region is known for its geological complexities, which can lead to issues such as cavities, loose falls and high water ingress and challenging ground conditions. As a result, contractors often need to modify previously approved tunnel designs, leading to time and cost overruns.
Some new technologies have been deployed to enhance the effectiveness of DBM. These include advanced blast fragmentation modelling software such as two-dimensional bench, which is used to analyse and simulate the detonation of the proposed design. Moreover, remote blast monitoring services have been developed for blast vibration assessment and air overpressure measurement. Rail India Technical and Economic Services has deployed remote sensing and GPS to survey the Atal Tunnel project. Besides, there has been a rise in the popularity of cost-effective and technologically advanced DBM equipment such as computerised jumbos designed to undertake tunnelling with greater precision.
TBM
The TBM method has witnessed significant adoption in congested urban areas, primarily due to its advantages in limiting the disturbance to the surrounding ground and creating smooth tunnel walls. This significantly reduces the cost of lining the tunnel, making it suitable for urban areas.
Based on the projects tracked by India Infrastructure Research, 51 tunnels with a combined length of about 387 km have been completed using the TBM method. The highest utilisation of the New Austrian Tunnelling Method (NATM) has been witnessed in the urban metro sector at 53 per cent, followed by the water supply and sewerage sector at 31 per cent. Presently, more than 160 TBMs have been deployed or are planned to be deployed for the construction of underground metro tunnels across key metro projects in cities such as Delhi, Mumbai, Chennai, Kolkata, Jaipur, Pune, Ahmedabad, Patna and Lucknow (N-S corridor). Moreover, 64 tunnels spanning a length of about 257 km are being constructed and 30 tunnels spanning a length of about 88 km are in the pipeline. With regard to the number of tunnels, the road sector accounts for the largest share in upcoming projects at 56 per cent, followed by the urban transport sector at 26 per cent. Some of the other key completed projects using the TBM method are Sardar Sarover Narmada Unit-3 (powerhouse), Kishan Ganga head race tunnel (HRT) and the Dulhasti HRT. The key upcoming projects include the Hetawane Dam-Jite WTP raw water tunnel, the Chennai Metro Rail Project Phase II and the Dehradun-Tehri Tunnel Project.
One of the major challenges faced by TBM contractors is the presence of geotechnical and hydrogeological uncertainties as the actual conditions of soil remain unpredictable. As the number of TBMs being planned for deployment in the upcoming projects increases, there may be a shortage of skilled manpower. TBM contractors also face problems in mobilising or demobilising the TBM equipment and machinery, especially when transferring them to project sites located in different parts of the country.
In the Indian TBM market, the deployment of open-mode TBMs, earth pressure balance machines and mixed shield or slurry TBMs is based on the soil content and geographical terrain of project sites. Some new technologies being deployed for TBMs include the TUnIS Ring Sequencing System and the Automatic Tailskin Clearance Measurement System. Besides, the implementation of a data management system in TBM ensures efficient control of TBMs and facilitates the management of other operational processes within tunnelling projects.
NATM
NATM has become a preferred method for tunnelling in uncertain geological and hydrogeological conditions. It relies on deformation monitoring data to evaluate the quantity and quality of the support system, making it a cost-efficient method of tunnelling.
It has predominantly been deployed for the construction of tunnels in the railway sector. This method is appropriate for the construction of approach roads of adequate geometry, caverns and mines. Based on the projects tracked by India Infrastructure Research, 52 tunnels spanning a length of about 155 km have been completed using the NATM. The maximum deployment of the NATM technique has been witnessed in the railway sector (65 per cent), followed by the urban metro sector (15 per cent). Moreover, 112 tunnels spanning a length of about 287 km are in the pipeline.
In the past few years, this method has gained prominence in the metro sector, particularly in cities that face space constraints. This tunnelling method is commonly used on both sides of underground metro stations to create a secure opening for TBM launch. Delhi Metro Phase III Corridors I, II and III, Delhi Metro Phase II and Lucknow Metro Phase I are some of the key projects that have been completed using the NATM. Some of the other tunnel projects completed using the NATM technique are the Jammu-Udhampur-Katra-Qazigund-Baramulla Railway Line Project, Kasara Ghat-Igatpuri Tunnel Project, Budni-Barkhera Third Line Project, etc.
Some challenges faced in this method are the need for constant quality assurance and quality control due to the absence of pre-fabricated support. Besides, this method requires a higher level of labour monitoring since it involves a significant amount of manual work.
Microtunnelling
Micro-tunnelling is a trenchless construction method utilised for installing pipelines beneath highways, railroads, runways, harbours, rivers and environmentally sensitive areas. This method is useful for laying large-diameter gravity sewers in cities where open cut installation is difficult. It is also employed for installing product pipelines in areas where the soil condition does not permit horizontal directional drilling and long individual crossings across rivers.
Mumbai was the first city in India to experiment with microtunnelling for the Mumbai sewage disposal project. Other cities such as Delhi and Kolkata have also used this technology. In a recent development, in order to address waterlogging issues, the Secunderabad division of South Central Railway employed the microtunnelling method using TBM. This method facilitated the installation of pipes through the yard and crossing railway lines. Furthermore, micro-tunnelling will be employed to connect new sewerage channels in areas like Dahisar, Kandivli, and Bandra in Mumbai.
The micro-tunnelling method also faces geotechnical and hydrogeological uncertainties due to high dependence on soil conditions. Besides, this method of tunnelling is not appropriate for low strength pipes due to the jacking procedures involved. Obstructions in the soil profile may pose challenges during the cutting procedure, thus hampering the tunnelling process.
Opportunities galore
Going forward, DBM is expected to remain the dominant technique for tunnelling. This is because it is a more flexible method with the ability to withstand greater challenges compared to other advanced mechanised techniques, which require greater uniformity during project implementation. In terms of sectors, the NATM is expected to remain the dominant method of tunnelling in the railway sector. Microtunnelling holds significant potential for increased deployment in tunnel construction, particularly for water supply and sewerage tunnels. However, this method will require investments in training the workforce to operate advanced technologies. Further, the high cost of domestically manufactured TBMs may result in a continued reliance on imports from other countries for tunnelling machinery for a significant period of time.
