Today, at the Mumbai Central Metro Station, the Mumbai Metro Rail Corporation (MMRCL) accomplished its 42nd and last milestone on the Colaba-Bandra-SEEPZ Metro-3 circuit.
Using 558 concrete rings, the Robbins-manufactured TBM Tansa-1 completed 837 metres from the Mahalaxmi Metro Station to the Mumbai Central Metro Station upline in 243 days.
One of the longest parts of the line-3 corridor, Package-3, consists of the Mumbai Central, Mahalaxmi, Science Museum, Acharya Atre Chowk, and Worli Metro Stations.
This completes the tunnelling of the Metro-3 corridor. Tunnelling beneath Mumbai’s heritage precincts, close to ancient decaying structures, the existing Metro line, Railway lines, and a body of water with varying and, at times, challenging geological conditions, was an arduous task, according to Ms Ashwini Bhide, MD of MMRC.
The company utilised cutting-edge technologies without jeopardising employee safety. This was a tough task for the MMRC staff, General Consultants, Contractors, and Joint Ventures. MMRC’s Director (Projects), S.K. Gupta said Metro-line 3 would give Mumbai residents a swift, comfortable, and secure commute once operational.
The entire project status is 76.6% complete.
Tunnelling challenges on Mumbai’s Metro Line 3
Tunnel boring machines (TBMs) utilised to complete the tunnelling for Metro Line 3 in Mumbai were required to adopt a hybrid technology to deliver a platform tunnel in hard rock and several tunnelling firsts for India.
Two 6.65m-diameter Robbins Crossover XRE TBMs were delivered by the joint venture of Larsen & Toubro and the Shanghai Tunnel Engineering Company (L&T – STEC). The engineering obstacles were overcome with the hybrid TBM/New Austrian Tunnelling Method (NATM), high groundwater pressures, and changeable ground conditions.
For example, with two custom-built Robbins TBMs, two parallel 2.9 km long tunnel bores were driven between the Cuffe Parade and CST stations, with multiple station breakthroughs along the way. This is the first time that dual mode, crossover type TBMs equipped with a horizontal screw conveyor and high torque/high speed cutter head drives have been employed in India.
India’s hybrid TBM/NATM technology was yet another first. The approach was utilised to construct a platform tunnel in basalt rock, with NATM excavating the crown and TBM excavating the bench. Because only 25% of the cutter head is excavating the rock mass, this needed precise control over the operational parameters of the TBM. The remaining 75% of the cutter head has no contact with either rock or soil.
In addition, the joint venture contractor was tasked with driving the tunnel beneath the Mittal Towers and the historic Bhikha Behram with only 15–20 metres of cover above the TBMs. The tunnel’s route reached within 25 metres of the Arabian Sea coastline, and its invert level was 22 metres below mean sea level. This resulted in water ingress rates of up to 300 litres per minute.
On the project’s site, the ground conditions included everything from basalt to soft volcanic tuffs, shale, and breccia.
The rate of muck removal at the ground level restricted progress rather than the competence of the TBMs, even though there were several technical hurdles.
It is anticipated that once the new metro line opens in 2025, there will be a 35% traffic reduction on the roads.
The Colaba-Bandra-SEEPZ line of the Mumbai Metro is a fully underground line of 33.5 kilometres long and has 27 stations. This line is intended to connect six business districts, thirty educational institutes, thirty recreational facilities, and both domestic and international airport terminals. It will have five interchange sites for the suburban railway, in addition to one for the Monorail and one for Line-1 that runs from Versova to Andheri and Ghatkopar.
The Mumbai Metropolitan Region Development Corporation (MMRC) is the nodal body responsible for the development of the Mumbai Metro Line-3 project. MMRC is a joint venture between the governments of India and Maharashtra.
The MMRCL has set a target date of January 2024 for the completion of Phase 1 of the construction of the Mumbai Metro line 3 from Bandra to Seepz. After then, another six months will pass before operations of phase 1 can begin. Additionally, it has been suggested that the Mumbai Metro line 3 might be extended all the way to Navy Nagar and that the government could hire a consultant to conduct a feasibility study on the possibility.