Over the past few years, tunnelling methods have evolved on the basis of requirements in the construction sector. Tunnel boring machines (TBMs) have revolutionised the tunnelling industry by making the process safer, more economical in creating underground space and opening up the possibility of creating tunnels where they were not feasible before. The tunnelling methods and the type of TBMs used for underground metro projects depend on the geotechnical characteristics of the ground, the level of the water table, the maximum settlement allowed at the surface, etc.
Of late, the cutting-edge New Austrian Tunnelling Method (NATM) has started gaining prominence in the metro sector, particularly in cities that typically have space constraints. NATM is commonly used on both sides of underground metro stations to create a safe opening for TBM launch and outbreak. 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 NATM. Meanwhile, the drill- and -blast method is a conventional method of tunnelling that is useful for hilly areas. The cut- and -cover method is economical and useful for constructing tunnels with shallow depth, but it is highly dependent on the availability of right of way.
Key projects using TBM and NATM
Mumbai Metro Line 3: TBM and NATM
In a notable development, tunnelling work on the 832.5 metre-long Mahalaxmi-Mumbai Central stretch of the Mumbai Metro Phase I Line 3 Project (Colaba-Bandra-Santacruz Electronics Export Processing Zone [SEEPZ]) was completed in June 2022. For the 33.5 km-long project between Cuffe Parade and SEEPZ, 17 TBMs were used. TBM tunnels were widened by NATM to create platform tunnels and cross passages.
Pune Metro: NATM and TBM
The underground section of the Pune Metro is being constructed using NATM. The Mandai and Budhwarpeth metro stations have been designed using this technology due to the scarcity of space for construction. Also, TBM Mula II has achieved a breakthrough at the Budhwar Peth metro station.
Bengaluru Metro Rail Project Phase II: TBM
Tunnelling work on the 209 metre long Tannery Road-North Ramp stretch of Bengaluru Metro Rail Project Phase II was completed in August 2022. TBM Vindhya completed works after six months of underground drilling.
Kanpur Metro Project: TBM
The Uttar Pradesh Metro Rail Corporation recently launched TBM machines, named Nana and Tatya, for the Kanpur Metro project. The TBMs, which will be used to construct the 5.6 metre diameter underground tunnels, are around 80 metre-long and weigh about 450 tonnes.
Delhi Metro Phase IV: TBM
The tunnelling stretch of Delhi Metro Phase IV was completed at Krishna Park Extension on the Janakpuri West-RK Ashram Marg corridor with a TBM breaking through after boring a 1.4 km-long tunnel. The tunnel has been constructed at a depth of approximately 14-16 metres. The tunnel has been built with the earth pressure balancing method and the concrete lining is made up of precast tunnel rings.
Ahmedabad Metro: TBM without D-wall
In a recent development, Afcons has launched a TBM without a diaphragm wall (D-wall) in the Ahmedabad Metro’s underground project (UG-1). The company is constructing two underground stations, twin tunnels of about 3.5 km length in total, a cut- and -cover section as well as a ramp connecting the elevated and underground sections of the project. To ensure the smooth functioning of the TBM, an in-house calibration unit has also been developed for the earth pressure sensor.
East West Howrah Metro Station project: River tunnel
The underwater tunnel corridor for the East West Howrah Metro Station Project is being developed 33 metres below the riverbed and will establish metro connectivity between Howrah and Kolkata. Of the 16.6 km long east-west stretch, a 520 metre-long section will reportedly be under the riverbed. About 80 per cent of the development work of the East West Howrah Metro Station is already complete and full-fledged service is expected to start in 2023.
Metro rail projects are expected to generate about Rs 700 billion worth of opportunities for construction companies over the next five to six years. Competitive intensity is expected to remain moderate in the near to medium term, with the incumbents expected to benefit the most.