Boring Through

Unique construction techniques being tested in tunnelling projects

Tunnel development in the country has picked up pace in the past few years on account of an increased uptake of projects involving longer tunnel lengths. The growth of tunnelling activity is driven by the robust pipeline of projects in the urban mass transit, railways, roads and highways, and water supply and sewerage sectors.

A number of landmark and challenging tunnel construction projects are under execution across sectors. Indian Railways is implementing its biggest project so far – the Katra-Qazigund railway line in Jammu & Kashmir – which involves the development of a total tunnel length of 161.55 km. In the water supply segment, the Municipal Corporation of Greater Mumbai has awarded the 9.7 km Chembur-Wadala-Parel tunnel, which entails an investment of Rs 13.65 billion.

In the metro rail segment, Mumbai Metro Rail Corporation Limited awarded the construction of the 33.5 km underground length running along the Colaba-Bandra-Santacruz Electronic Export Processing Zone stretch. The tunnel construction work is estimated to entail a total investment of Rs 181 billion. Underwater tunnels are also being constructed. India’s first underwater metro tunnel, the 502 metre metro rail tunnel below the Hooghly river was completed in May 2017. The Mumbai-Ahmedabad high speed rail project also involves the construction of a 7 km tunnel under the sea. The Kaleshwaram lift irrigation project involves the construction of seven tunnels spanning a total length of 203 km. The project also involves the construction of Asia’s longest irrigation tunnel with a total length of 49.8 km (twin tunnels of 24.9 km length).

With regard to tunnel construction techniques and technologies, the tunnel boring machine (TBM) and the New Austrian Tunnelling Method (NATM) are gaining prominence for tunnelling activities in urban areas. Another advanced method which is seeing increasing acceptance is micro-tunnelling. Nevertheless, conventional methods such as the drill and blast method (DBM) continue to play a dominant role in executing tunnel construction projects in the Himalayan region and the Western Ghats as well for constructing hydro tunnels.

There is also an increased demand for high-tech equipment as geological complexities are the biggest challenge in tunnelling projects. Contractors are experimenting with new techniques and methodologies such as ring convergence measurement systems, 3D modelling, controller area network, remote blast monitoring, etc. for more challenging projects. Meanwhile, navigation systems, computerised jumbos and advanced drilling systems are being deployed for precision and better monitoring. New materials are also being deployed to improve the durability and strength of tunnels.

Construction methods and techniques

Till date, DBM has been the most prominent technique used for tunnelling across all sectors. Based on projects spanning a length of around 783 km (completed and under construction) tracked by India Infrastructure Research, around 47 per cent of the total tunnel length across all sectors has deployed the DBM technique. Sector-wise, DBM has been mostly used for hydropower projects. One of the reasons behind this is that TBMs cannot be used in challenging terrains like the Western Ghats and Himalayas. Other sectors like railways, metro and water supply also account for significant tunnel length constructed/being constructed using this technique.

In recent years, the NATM has been increasingly adopted, especially in the railway and metro sectors. While the hydropower sector accounts for the largest share, of 49 per cent (spanning a total length of 78.3 km), of total tunnel length constructed (completed projects) using this technology, the railway sector accounts for the largest share, of 75 per cent (219.92 km), of the total tunnel length currently under construction. With a pipeline of metro projects planned for implementation, the NATM is expected to pick up even further in the times ahead.

TBM, which is one of the most advanced and efficient methods of tunnel construction, is being widely used for executing tunnelling projects in congested urban areas. Based on the projects spanning a length of around 488 km (completed and under-construction projects) tracked by India Infrastructure Research, TBMs have been used mainly in the metro rail sector (43 per cent of the total length of projects tracked), followed by the irrigation (21 per cent) and water supply (20 per cent) sectors.

In the irrigation sector too, TBMs are slowly replacing the DBM construction technique that was used earlier. So far, tunnels spanning a length of more than 100 km are either being constructed or are under construction in the sector. In the water supply and sewerage sector too, the use of TBMs for tunnelling started only recently, specifically in urban areas. Heavy movement of traffic, space constraints and congestion have been the main reasons behind the use of this technology.

At present, most of the TBMs and related components and spare parts are imported from foreign manufacturers. Herrenknecht is the only company that has established a state-of-the-art TBM assembly and cutting tools manufacturing facility in India. The facility, located in Chennai, is spread over 10 acres. The TBM market in India has seen an increasing trend of contractors entering into buyback arrangements with suppliers. Since the type of TBM deployed for a particular project is subject to geological and soil conditions, TBMs once used are either revamped to cater to different projects or are sold back to the manufacturer for 5-15 per cent of the purchase price. In many cases, contractors have sold their TBMs to other contractors in the market for an agreed upon price.

Renting TBM equipment is another notable trend in the market. Contractors, depending on the pipeline of projects, geology and cost effectiveness of refurbishing the existing machinery, have been renting equipment, as many projects involve the use of different machines for short periods of time. However, most of the large companies prefer buying TBM equipment.

Besides renting, many TBM manufacturers have started offering machinery along with their services on a contractual basis. Some of the companies that have forayed into the organised rental sector are Quippo (owned by Srei Infrastructure), Gmmco (CK Birla Group), and the Sany Group. Also, Herrenknecht provides services for the installment of TBMs, rental equipment, etc.

In the next couple of years, the import of TBMs is expected to remain high with a low likelihood of TBM suppliers setting up new manufacturing or assembly units in the country. The government will need to make major alteration to the existing tax structure such that companies benefit from setting up TBM manufacturing units in India and reduce import dependence.

With the increasing complexities of tunnelling projects, monitoring devices are being increasingly deployed to examine the progress of the projects at each stage and also ensure safe operations. Systems such as the TUnIS Ring sequencing system for predictive ring sequencing, SLuM Ultra for optimal future ring sequence, the Ring Convergence Measurement System for detecting potential risks caused by deformations at the earliest possibility, and the Segment Documentation System for documenting all major processes within the production and storage of segments are being increasingly deployed to optimise project outcomes.

New construction materials and innovations

Tunnel construction requires different types of materials. The growing number of complex tunnels being constructed has resulted in conventional raw materials being replaced by advanced materials. Steel fibre-reinforced shotcrete, safer emulsion-based explosives, geosynthetics such as 5D steel fibres (for increased tensile strength), mineral admixtures, geotextile membranes, steel anchors and self-drilling rock bolts are some of the key advanced materials currently in use. Also, globally accepted microfine cement is being tested in the country to overcome issues of cracks and water seepage in tunnels. Still, the lack of awareness about suitability and the right application of these materials limit their use to a few select projects. This needs to be rectified to ensure increased application so as to prolong the life of structures.

Cement, one of the key raw materials used to provide strength to tunnel structures, has undergone significant advancements. Reinforced concrete mixed with glued steel fibres is being used as shotcrete for permanent/temporary lining, cast-in-situ lining, pre-cast segmental lining and for other flooring works. Waterproofing of tunnels is another key requirement for which geotextile membranes are being widely adopted. To improve the durability of tunnels and add to their strength, steel and fibre-reinforced polymer active anchors and steel passive anchors are also used by tunnel contractors. Apart from this, expandable friction bolts, self-drilling rock bolts, and mechanical single bolts that allow faster installation are being used to increase productivity.

At the same time, awareness about the suitability and use of materials has increased among contractors. Testing material suitability before adoption for actual construction activity is essential to avoid accidents. For this, aspects related to pre-qualification tests, energy adsorption tests, material availability and site conditions need to be taken into account.

Conclusion

Going forward, with infrastructure development at the forefront, there is great scope for tunnelling in the country in the years to come. More than 4,000 km of tunnel length is planned to be added in the next four to five years. For the successful implementation of these projects, there is a need to ensure that the factors that can slow down the execution of tunnelling projects such as safety risks, contractual issues, inadequate investigation, geological complexities, etc. are dealt with in a time-bound and effective manner.

Nikita Chhabra

 

GET ACCESS TO OUR ARTICLES

Enter your email address