Mumbai Metro structures, methods and progress 12 Mar 2020

With 17 TBMs of different types and open-cut and NATM for 26 underground stations and associated structures, construction of the 33.5km underground alignment of the Mumbai Metro Line 3 had reached more than 80% by early March 2020. Started in early 2017, S K Gupta, Director (Projects) for the client MMRC, Mumbai Metro Rail Corporation, reported that “all the TBMs have performed well” and that the station and rail tunnel works “are on track to complete all excavations by June 2020”.

Fig 1. Mumbai Metro Line 3 and its contract packages

The dense and historic urban environment of Mumbai, on the narrow north-south peninsular, demanded underground alignment for the full 33.5km of the Line 3 alignment with a transition ramp to the one surface station at Aarey and to the surface depot in the north. Line 3 is the first underground metro line for Mumbai, all other public rail services in the city are either at grade or elevated. As well as bringing metro services to the tip of the peninsula at Cuffe Parade, the new government funded line, at an investment of about INR25,000 crore or US$3.3 billion in 2011 approval values, links with the main railway and long-distance bus stations in the city and also to the main domestic and international CSIA airport terminals. In several areas, securing the land space for the open cut stations, shafts and for providing working sites has required the acquisition of private properties, and the relocation of businesses and residents caused some delays to the start of the works.

The geology is largely hard basalt and softer tuff and breccia geology with hard basalt rock at up to 125MPa in compressive strength to the south and softer breccia and tuff to the north. For the most part the hard rock excavations in rock Classes I and II are dry with groundwater ingress experienced in the fractured and jointed rock of Classes III, IV and V. Groundwater ingress, some at high volume and under hydrostatic pressures of up to 5 bar, is also experienced in mixed face conditions and through upper sedimentary overburden and in softer tuff and breccia deposits.

Congested urban environments present a great challenge for Line 3 construction

“In planing the project, the main underground works were grouped into seven packages of between INR21-30 billion ($315-445 million) in value and with between 4-6km of twin running tunnels in each package along with two or three stations and other associated works,” said Director (Projects) S K Gupta. “As well as the 33.5km of twin running tunnels and 26 underground stations, there are five rail crossovers, one at grade at Aarey in the north at the depot, and four underground at Sahar, Aacharya Atreya Chowk, CST and Cuffe Parade – two excavated as NATM caverns and two as cut-and-cover constructions. In addition, there are 91 cross passages linking the running tunnels which are excavated by NATM.”

Of the 26 underground stations, 19 are cut-and-cover boxes of normally up to 250m long x 22m-30m deep, and seven, in particularly congested areas at Hutatma Chowk, Kalbadevi, Girgaon, Grant Road, Santacruz, Sitladevi and Marol Naka, are a combination of cut-and-cover and mined NATM excavation. The stations at Cuffe Parade and BKC are up to 450m long as they include crossovers. Support of the cut-and-cover station boxes and access shafts on the line are excavations supported by secant piles, with rock and soil anchors and struts providing extra support as core excavation has progressed. Excavation has advanced using controlled drill+blast, breakers and excavators.

For three of the mined stations, at Kalbadevi, Girgaon, Grant Road, Shitladevi and Santacruz, the TBM segmentally lined running tunnels were excavated through the station zones ahead of later breakout for station platform enlargements. Mined station excavation is also by controlled drill+blast, breakers and excavators, with rockbolts and steel fibre shotcrete for immediate support and for sealing all exposed faces against water ingress and deformation. “All exposed NATM faces are sealed with steel-fibre reinforced shotcrete and grout injection is used to control local excessive water ingress,” said Gupta.

All the running tunnels on the project are designed with a 5.8m i.d. and are lined with precast concrete segmental linings reinforced with rebar cages. Most linings have straight bolted or dowelled circumferential joints while the lining design on Package 1 has knuckle joints on the circular segment-to-segment ring joints and are bolted on the longitudinal joints. The segments for all the contracts are produced a concrete casting works in Wadala, about 10km from central Mumbai, each contractor managing the fabrication of its own segment production.

Open cut station excavation supported with secant piles

Muck haulage behind all TBM drives is by rail-bound muck cars with service trains carrying in the segments for each ring of advance. Jet grouting has assisted the break-out and break-in of TBMs in zones of softer, mixed face or highly fractured or jointed rock conditions.

Ahead of selecting and ordering their TBMs, the appointed contractors of each package carried out additional site investigation. “This resulted in contractors ordering different types of shielded TBMs to cope with conditions,” said Gupta. “These include slurry, EPB and dual and multi mode machines and all have performed well.”

NATM mined stations in highly congested areas

Of the total 54km of TBM tunnelling for the project, 80% or nearly 44km is complete (at early March 2020) with 25 of the total 40 TBM breakthroughs achieved and 15 yet to occur (Table 1). Three of the 17 TBMs have completed their drives and 13 are progressing their last drives towards final break throughs. “Now, with 45km of the total 54km of tunnelling complete, and since the first TBM started in November 2017, the long-time average progress rate has been 5-7m/day,” said Gupta.

The supplier of most TBMs to the project, supplying seven of the 17 TBMs, is Terratec, delivering five new dual mode rock TBMs and two used rock EPBMs. Two new machines are supplied to the Hindustan/Moscow Metrostroy JV on Package 2 and five are to the J Kumar/China Railway Tunnel Group JV on Packages 5 and 6. The first TBM launched in November 2017 on Package 2 and has completed its work. The other six machines are into their final drives towards the last breakthrough in November 2020. The 6.6m i.d. TBMs achieved consistent progress rate averages of near or above 300m/month with bests of 102m/week and up to 20m/day.

17 TBMs are completing the running tunnels on the 33.5km underground route

On Package 3 from Mumbai Central to Worli, the Doğuş/Soma JV is using two NFM/Robbins rock slurry TBMs, one new and one refurbished. The first TBM completed its 2,073m drive from Science Museum to Worli in 16 months, after launching in September 2018, being pulled through Acharya Atrey Chowk, and achieving final breakthrough in February 2020. TBM 2 is expected to complete its drive by end of March 2020. The second 1,645m drives for the two TBMs, Science Museum to the Mahalaxmi Metro open-cut station box and on to Mumbai Central, are on the critical path of the TBM excavation programme and are expected to start their drives in April and May 2020.

The three Herrenknecht TBMs, one new and two refurbished EPBMs, are completing the 10.43km of tunnelling for Package 4 from Worli to Dharavi for the ITD-TATA-CEC JV. The three TBMs are completing the tunnels in four twin drives of from 600m from the Naya Negar open-cut launch site to Dharavi, to 2,490m, the longest drives on the project between Naya Nagar and Dadar. All three are into their final drives and are due for final breakthroughs in March, July and August 2020.

On Package 7, the L&T-SUCG JV operated three STEC EPBMs, two refurbished and one new, to complete the 7km of twin tunnelling from Marol Naka to Seepz. The three machines completed their tasks in three drives passing through MIDC and Seepz Stations to the transition ramp to the surface station at Aarey and the surface depot. The last TBM completed its drive in December 2019 and highlighted Package 7 as the first to complete all its TBM tunnelling.

Through the hard basalt rock on Package 1 from Cuffe Parade to Hutatma Chowk, the L&T-STEC JV selected two XRE Crossover TBMs from Robbins. The machines are completing the tunnels in three drives, starting at the Cuffe Parade Station box and breaking into and relaunching from the Vidhan Bhavan and Churchgate Metro Station boxes. The two machines started their third and final drives in January 2020 and are expected to complete the 649m drives in May 2020. A separate technical article about the design and performance of the two XRE Crossover Robbins TBMs is also published on TunnelTalk in March 2020.

As excavation works approach their ends, attention is turning to finishing works and M&E installations. All NATM stations, crossover caverns and cross passages are to be finished with a waterproofing membrane and reinforced cast concrete. For the Sahar crossover cavern, an in-situ steel fibre reinforced cast concrete final lining is planned. M&E contracts have been awarded and are currently in design for approval by MMRC. Also ordered from Alstom are the 31 metro trainsets of 248 cars for the new line services. These are to be manufactured and supplied for the Alsthom factory in SriCity, Andhra Pradesh.

MMRC Director (Projects) S K Gupta

“Some 55% of the project and its financial investment is now complete,” reported Projects Director Gupta. “We are now working towards the planned phased opening of Phase 1, Packages 5, 6 and 7, first, in December 2021 and of Phase 2, Packages 4, 3, 2 and 1, in June 2022. The schedule remains under continuous review and there have been some delays and challenges, including the construction of the open-cut stations and structures very close to buildings, and some adjustment to the 2011 values of the approved project that we are still working to may be needed. However, with the first set of metro trains due on site in November 2020 for commissioning trials, December 2021 and June 2022 remain our target dates for opening Phases 1 and 2 of this new and important addition to metro services in Mumbai.”