More than two years after 35 men and a giant Chinese Tunnel Boring Machine began cutting though complex geological strata, India’s first undersea twin tunnels in south Mumbai are close to completion.
Set to open in November, the tunnels start near Girgaon (ahead of Marine Drive), extend north under the Arabian Sea, Girgaon Chowpatty and Malabar Hill, and end at Breach Candy’s Priyadarshini Park. The 2.07-km tunnels are a part of the Rs 12,721-crore Mumbai Coastal Road Project (MCRP) being built by the Brihanmumbai Municipal Corporation (BMC).
The 10.58-km MCRP links Marine Drive to the Bandra-Worli Sea Link and is just one part of the Coastal Road project. The high-speed coastal road aims to cut down the 45-minute commute from Girgaon to Worli during peak hours to just 10 minutes.
The tunnels, which have a diameter of 12.19 metres, run 17-20 m below sea level. A nearly 1-km stretch lies under the sea. The tunnels hit peak depth at Malabar Hill at 72 m. Resembling the shape of the Queen’s Necklace — the famous C-shaped Marine Drive promenade — the tunnel entry and exit points will have fiberglass facades.
There will be six cross passages inside the tunnel — four for pedestrians and two for motorists. Each tunnel has three lanes of 3.2 m each. Namkak Cho, a senior tunnel engineer with South Korea-based Yooshin Engineering Company, a project management consultant (PMC), said two lanes in each tunnel will be operational, while the third will be used in an emergency or if the vehicular density increases.
Another highlight of the project is the use of the largest ever tunnel boring machine (TBM) in the history of India. Named Mavala in honour of Shivaji’s warfare experts from the Koli and Kunbi community, the TBM weighs more than 1,700 tonnes and is nearly 12 m tall. While the boring work started in January 2021, work on assembling and launching the TBM began a year ago. Manufactured by the China Railway Construction Heavy Industry Company Ltd (CRCHI), Mavala is divided into three different sections.
Mantayya Swami, MCRP chief engineer, said, “The TBM’s first section is the cutterhead with high-powered spokes. The second is a seal which acts as a chamber that collects falling sediments and rocks. The third is a control room located inside Mavala. The entire operation is controlled from there.”
Mavala arrived at Mumbai’s Jawaharlal Nehru Port from Shanghai in April 2020. Seventeen trucks were deployed to transfer the dismantled TBM to Priyadarshini Park. Civic officials said it took nearly three months to assemble it. The TBM was then shifted to the launch site, located a few metres away, from where it was lowered underground to dig through the basalt, breccia and shale. In December 2020, the TBM was lowered 20 m below the ground level.
Stating that Mavala would mine around 7-8 m per day, Cho said, “In conventional tunnel digging, one can mine a maximum of 5 m daily. However, Mavala was able to dig up to 8 m per day. We would hit 20 m on some days, all thanks to the area’s geology.”
After year-long mining operations, Mavala made its first breakthrough from the Girgaon end in January 2022. “Earlier, we had planned to dismantle the TBM after the breakthrough, bring it out, reassemble it and restart mining for the north-bound tunnel from Priyadarshini Park. This process would have taken six months. We decided to rotate it using a turntable instead,” Swami said.
Boring work for the second tunnel started in April 2022. The BMC expects to achieve its breakthrough by the end of May, since only 140 m of mining work is left.
The team faced multiple challenges, including lockdown and geopolitical tensions with China. Cho recalled how the arrival of engineers from China, who were supposed to visit the Mumbai site to train engineers on how to operate the TBM, was delayed due to the pandemic and political tension between India and China. The Chinese engineers arrived a year later.
“Engineers from China were supposed to arrive with the TBM to train Indian engineers for the first stretch of 500 m. However, they couldn’t come to India. So the Indians mined the first 400 m all by themselves,” Cho said.
There were other challenges too. Swami said, “We had completed around 8 m of mining from Priyadarshini Park when we realised that the seabed above us was on the verge of collapsing. We immediately sprayed slurry on it to seal the cavity. Other challenges we faced included the shallow depth under the seabed, the small gap between the tunnel’s crown and the seabed, and seawater leaking on us due to weathered rocks. We would spray slurry continuously to prevent seawater from leaking through these gaps.”
The tunnel’s proximity to the centuries old Malabar Hill water reservoir, located under Malabar Hill, was another problem for the officials. This reservoir is the primary water source for south Mumbai. A single crack in it would have hit water supply to suburban Mumbai.
The biggest stumbling block for the team was the malfunction of a bearing seal (a rubber material from the TBM) in February. Cho said a bearing seal prevents external materials from entering the gearbox. The malfunction brought the project to a halt for three months. “According to rules, a set of spare parts for the machine should be kept on site or at a distance that is not more than 24 hours away. This malfunction was unexpected, so we didn’t have a spare,” Cho said, adding that the TBM would have malfunctioned if mining was not stopped temporarily.
“We had to import the bearing seal from Italy. The installation is complete and test runs resumed from April,” Cho added.
Ashwini Bhide, the Additional Municipal Commissioner who is spearheading this project, said 93 per cent of the tunnel works are complete.