Mumbai: On 24 September 2018, the Wainganga 1, a tunnel boring machine (TBM) deployed for constructing Mumbai’s first underground Metro corridor, recorded the project’s first breakthrough.
It drilled its way through rocky strata of basalt, breccia and tuff for 259 days, boring about 4.6 metres every day on an average, to complete its drive of 1.26-km and see the light at the other end, at the proposed Terminal 2 station of the Chhatrapati Shivaji International Airport.
The then chief minister Devendra Fadnavis reached the spot to witness the feat, which the Mumbai Metro Rail Corporation (MMRC) described as “the result of blood and sweat expended by a team of nearly 250 engineers, technical and mechanical workers together”.
The achievement was seen as significant, as until about a decade ago, when the corridor was still in the planning stages, many officials and experts had expressed concerns over the feasibility of building an underground corridor in a crowded, complex city like Mumbai with the ground strata comprising soft marine clay and hard basalt rock. They said it would be a very expensive and time-consuming venture.
Nearly three years since that first breakthrough at Terminal 2, 16 more tunnel boring machines like the Wainganga 1 have finished creating almost 53 km of tunnels under Mumbai for the fully underground Colaba-Bandra-Seepz Metro.
Overall, tunnelling is 97 per cent complete and there are just three more breakthroughs to go, which authorities at MMRC hope to complete by this year’s end.
The journey was, however, full of unique challenges requiring boring close to the coast, under dilapidated buildings, under the existing suburban railway corridor, the elevated Metro corridor, and Mumbai’s infamous Mithi River.
S.K. Gupta, director (projects) at the MMRC, told ThePrint, “Those who doubted the feasibility of tunnelling a Metro corridor in Mumbai were not wrong. Mumbai is certainly different from other cities. It is confined to a narrow width, it is densely populated, it is congested, old, and it has another unique character of seven islands brought together by reclamation. We don’t have a combination of these issues in any other Indian city.”
He added, “The presence of the sea also affects us because water can come through fissures and folds, and the boring was not too far from the sea.”
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The showpiece underground Metro
Sreedharan, who has earned the moniker of ‘Metro Man’ for effectively steering the Delhi Metro project and advising on other lines across India, had once described Mumbai’s Colaba-Bandra-Seepz line as “the most difficult urban project ever undertaken in India”, MMRC’s Gupta recalled while speaking to ThePrint.
The 33.5-km Colaba-Bandra-Seepz line, popularly known as Metro 3, is considered to be key as it proposes to connect the tip of south Mumbai to the city’s north, linking business districts, including the Bandra-Kurla Complex. The metro line will also provide connectivity to the Chhatrapati Shivaji Maharaj International Airport.
While the project cost has not been officially revised from the original Rs 23,136 crore, government sources say it has already swelled nearly 40 per cent to about Rs 32,000 crore.
The fate of the project currently hangs in balance with the Uddhav Thackeray-led Maha Vikas Aghadi (MVA) insisting on moving the car depot for the line from its original location, the ecologically sensitive Goregaon’s Aarey Milk Colony.
The location of the car depot for the metro project has been a bone of contention between estranged allies — Thackeray’s Shiv Sena, which leads the MVA government, and the opposition BJP, which is in power at the Centre.
The Thackeray-led state government had decided to move the car depot to a plot in Kanjurmarg, but the BJP-led Union government stalled the move, claiming ownership of the land. The matter has been in the Bombay High Court since December last year, and the MVA government is now scouting for other land options to relocate the car depot.
Meanwhile, in terms of sheer engineering, the project has overcome several major challenges to reach a stage where tunnelling in six out of the seven contract packages is 100 per cent complete. The stretch that remains is between Mahalaxmi and Mumbai Central, and is likely to be completed by the end of the year unless there are delays due to the Covid-19 pandemic, Gupta said.
An MMRC spokesperson told ThePrint, “Overall, 79 per cent civic work and 68 per cent station work has been completed. Presently, track installation work is in progress.”
A senior state government official who did not wish to be named said that the question over the location of the car depot will not impact the project.
“The Metro project is still at the stage of civil work. Finishing work on the stations, systems, testing, commissioning of rolling stock etc will take some time, and we will be able to get an alternate car depot ready. We will take a final call on the car depot issue soon,” the official said.
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Tunnelling under old buildings, railways and water bodies
Tunnelling for Mumbai’s first underground Metro started in 2017 when the MMRC launched its first tunnel boring machine. Tunnelling altogether involved the use of 17 TBMs and about 100 labourers and roughly 15 engineers working in two shifts per machine.
“There were many difficult stretches. Sometimes the geology would make our life difficult and sometimes, the sensitivity of the structure above,” Gupta said.
In South Mumbai’s Kalbadevi, Girgaum, Grant Road areas, for example, the geology was difficult and the structures above were also very old and precarious, he added.
The geology in Worli was such that the TBMs would constantly get choked and require frequent cleaning. While tunnelling under the operational Versova-Andheri-Ghatkopar Metro corridor, the MMRC had to plan the alignment such that the underground Metro tunnel falls in between the foundations of the existing Metro line.
Tunnelling under the Mithi river was another challenge. “The geology below the river was not very conducive for us. We also had a water head above so the risk of water gushing in is high. It didn’t happen, but we had to account for all such eventualities,” Gupta said.
The geology immediately below the water body was made of slush and basalt rock. The MMRC deployed a special ‘earth pressure balance’ TBM, the Godavari, for tunnelling beneath the water body and controlling the ingress of water.
“We got a different gasket design because there was more water likely to impact the work. We had a method statement ready for every eventuality. We could not afford to get stuck below the Mithi river,” Gupta said.
Gaskets are used to make the individual pre-cast segments in tunnelling watertight.
For seven stations, the MMRC had to resort to the New Austrian Tunnelling Method, typically useful in areas where there is not much space to launch a tunnel boring machine, or come out on the other side.
The Covid-19 pandemic was an added dimension to the challenge of tunnelling under Mumbai. Gupta said, with work being in confined spaces, underground, on a number of occasions tunnelling had to be stopped because even if one worker tests positive the entire team had to be quarantined.
Earlier this month, the MMRC team made its 39th breakthrough at the proposed Mahalaxmi Metro station.
Gupta said, “Three more such breakthroughs and we will be 100 per cent done tunnelling under Mumbai.”
(Edited by Amit Upadhyaya)
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