Mumbai Coastal road: Challenges in digging the twin tunnels

The making of the country’s first undersea tunnel, set to be opened to vehicles next month, has had its share of challenges, from conceptualising it on paper to its execution, but this was a near disaster they encountered right at the beginning.

To complete all civil works pertaining to the twin tunnels, forty men worked day and night for more than two years with a Chinese TBM — Mavala, to dig through complex geological strata, geopolitical tensions and even navigate through pandemic outbreak.

Named after Chhatrapati Shivaji Maharaj’s mountain warfare experts belonging to the Koli and Kunbi community, TBM – Mavala was manufactured by China Railway Construction Heavy Industry company (CRCHI). Assembly of the mammoth machine and initial digging too has had its share of challenges and was impacted by Covid lockdown.

Weighing over 1,700 tonne and 12 metres in height, the massive TBM arrived at Mumbai’s Jawaharlal Nehru Port from Shanghai in April 2020, from where it was transferred to PDP via 17 trucks after it was dismantled.

Since the TBM was imported from China, the contract stated that a delegation of engineers from TBM’s parent company CRCHI was to visit India and train MCRP engineers to operate the machine while digging the first 500-metre stretch.

The tunnel is set to be inaugurated in the second week of February. (Express photo by Amit Chakravarty)

However, owing to the pandemic and skirmishes between Indo-Chinese forces in Galwan Valley, the visit of Chinese engineers was deferred by a year.

“Since the Chinese couldn’t arrive in time, Indians started work on their own and finished mining the first 400 metres all by themselves,” said Namkak Cho, the tunnel expert from the Project Management Consultant (PMC), M/s Yooshin Engineering Co.

The BMC, along with PMC staff and contractors assembled the TBM and brought it to the launching site at PDP in a 200-wheel self-propelled transporter and freight wings. Civic officials said it took them three months just to assemble the entire TBM from the day it arrived and in December, 2020, the TBM with a tri-colour themed cutterhead was lowered 20 metres to begin boring.

Digging of the first tunnel began in January 2021 from PDP, and the breakthrough of the first tunnel was achieved in January 2022 at Marine Drive. The boring of the second tunnel began in April 2022 from the Marine Drive end and the final breakthrough at PDP was recorded on May 30, 2023.

Of the total length, around 900 metres of the tunnel would run beneath the Arabian Sea as per the plan, while 1 km would pass through Malabar Hills. This posed another challenge as mining the tunnels through this complex topography and geology included cutting through hard geological elements like breccia, basalt and shale, as well as components including mixture of sand and clay found in seabed.

Of the six cross passages, two are for vehicular crossing and four for pedestrians. (Express photo by Amit Chakravarty)

Officials said the depth under the seabed was very shallow and the gap between the crown of the tunnel and the seabed was very small. “Due to the presence of weathered rocks, sea water would leak on the TBM, to deal with which we would spray slurry continuously to prevent saline water leaking from the gaps,” an official said.

In one such instance, the TBM’s main-bearing seal made of rubber malfunctioned, resulting in the project coming to standstill for three months. The function of the bearing that broke down was to protect TBM’s main gearbox and prevent hard rocks and stones from entering the machine. “The rules state that one set of spare parts for the machine should be housed at the site that would not take more than 24 hours to reach. In this case, we were not anticipating the malfunction therefore we didn’t keep any spare parts either with us,” Cho said.

Mining works stalled for three months and the TBM became operational only after a bearing was imported from Italy. The overall manpower inside the TBM would include a maximum of 10 people from BMC, 20 people from PMC and five more people from the Chinese maker of the TBM.

With a diameter of 11 metres each, the twin tunnels are the widest cylindrical tunnels in the country. (Express photo by Amit Chakravarty)

Civic officials recall that often due to frequent impacts with rock-solid structures underground, the spokes in the cutterhead would slow down. Behind the cutterhead lies a chamber that collects sediments and rocks that would fall like debris during mining. These collected sediments would be transported out through a suction pipeline to prevent accumulation of debris in the tunnel.

Simultaneously, through another pipe-line system, the TBM would continuously spray slurry on the surface of the tunnel to prevent any cavity that would cause a landslide. Slurry is a thick liquid-like chemical component made by mixing bentonite with water.

The 2.07 km twin tunnels are part of the 10.58 km MCRP, a high-speed vehicular corridor being built by BMC at a cost of Rs 13,983 crore. The tunnels originate from Marine Drive in south Mumbai and extend till PDP at Breach Candy. The TBM – Mavala had a working diameter of 12.19 metres, making it the largest TBM to ever be used in India’s history of infrastructure works.