With significant momentum in the infrastructure space, developers increasingly need to execute tunnelling projects. Tunnels are required to be constructed across various projects in the railway, road, metro, water supply and sewerage sectors. While several technologies exist in the market today, there has been an increased focus on the deployment of tunnel boring machines (TBMs) for constructing such tunnels, especially in India’s urban centres.
India Infrastructure Research analysed a total of 1,204 tunnels for methods and techniques of tunnel excavation. While conventional technologies dominate the tunnelling industry, there has been a significant shift in tunnelling techniques from conventional technologies such as drill-and-blast and cut-and-cover to more mechanised technologies such as TBM and the New Austrian Tunnelling Method.
The rise in the deployment of TBMs for tunnelling projects can be attributed to the increase in tunnelling activities in congested urban environments. The need to provide adequate infrastructure to the country’s urban centres has increased the number of metro, water and sewerage projects being executed. TBMs facilitate such tunnel construction and are best suited for projects involving longer tunnels in congested areas.
TBMs essentially comprise circular plates which are equipped with disk cutters. As the machine is deployed, the plates rotate, enabling the disk cutters to cut through the rocks. The machine is suitable for use in both hard rock and soft ground geologies. Reasonably consistent geology allows for extended and continuous use of the machine for tunnel construction.
At present, there are 61 key tunnels, totalling a length of 405 km, which are being constructed using TBMs. Of the technologies used for construction in all the projects analysed, TBMs comprise a share of 18 per cent in terms of tunnel length and 5 per cent in terms of the number of tunnel projects undertaken.
TBMs have emerged as the preferred technique for metro tunnel construction and account for 85 per cent of the total length and 71 per cent of the number of metro tunnels constructed. It is also popular for the execution of irrigation and water supply projects. In this sector, 56 per cent of the total length and 89 per cent of the total number of all tunnels analysed have been constructed using TBMs. While the technology is also used in hydropower and road projects, its share among the tunnel technologies used is marginal. Drill-and-blast is usually the preferred technique for such tunnel projects.
Considering the current ongoing tunnelling projects, TBMs account for 32 per cent and 11 per cent of the total length and number of tunnel projects respectively. Moreover, TBMs are being deployed for the execution of all ongoing metro projects in the country.
How TBMs fare vis-à-vis conventional techniques
The use of mechanised tunnelling techniques such as TBM changes the cost composition of tunnel projects. Mechanised processes usually involve higher costs of transportation (about 6 per cent of the total project cost) as compared to those for non-mechanised techniques (where the cost of transportation is marginal, at only about 1 per cent of the total project cost). At the same time, using TBMs also results in a steep increase in the cost of equipment deployed. While only about 14 per cent of the total cost for projects using conventional techniques is towards equipment, the figure stands at 24 per cent for mechanised technologies.
At the same time, deploying TBMs in hilly and difficult regions continues to be challenging owing to the high cost of equipment and its transportation. In certain cases, flooding and mucking impact these machines, leading to time and cost escalations for projects.
However, while certain costs do increase with TBM deployment, it also has higher advanced rates and thus requires lower construction time. The use of this technology also involves lower traffic disruptions, less debris creation, and fewer adverse environmental impacts, as compared to conventional technologies.
Owing to the immense benefits of quicker project completion, and less debris and traffic management issues, TBMs have emerged as the preferred technology for the construction of tunnels in congested areas. Going forward, more and more developers are expected to use this technology for the execution of water supply, sanitation, irrigation and metro tunnel projects.