Mumbai: water tunnel network
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A backgrounder
As in 2024
Richa Pinto, July 19, 2024: The Times of India
From: Richa Pinto, July 19, 2024: The Times of India
Not many know that Mumbai started building big tunnels and using tunnel boring machines (TBMs) much before the Capital. It's just that Mumbai's tunnels until now were meant for moving water, not people.Quietly, without media frenzy, the city's water tunnel network crossed the 100km milestone on June 21, with the completion of a 9.7km underwater tunnel from Ghatkopar to Parel via Wadala.Although China, Japan and many other countries use water tunnels in cities, only New York City with a 111km water tunnel network is ahead of Mumbai now.
WHY DIG SO DEEP?
Mumbai's water tunnels run 100-110m below the surface - unlike pipelines that are at a depth of 3-5m - and cost almost Rs 100 crore per km to build. For instance, the 9.7km Ghatkopar-Parel tunnel cost Rs 943 crore. Wouldn't it be cheaper to stick to pipelines then? Maybe, if you could find the space to lay more of them. But additional municipal commissioner Abhijit Bangar said tunnels have several advantages.
Bangar, who holds charge of Brihanmumbai Municipal Corporation's (BMC) water supply projects department, said, "Because the tunnels are deeper, the chances of contamination and theft are significantly reduced. Currently, Mumbai loses close to 35% of its water due to some of these issues. Tunnels can help reduce this loss marginally."
WATER TUNNEL
There's also a strategic reason for pushing the water network deep underground: "As an international city, Mumbai faces the risk of sabotage. But it is much more challenging for anyone with such intentions to tamper with infrastructure 100m underground."
It's also eco-friendly in a way because when water from lakes and streams drops 100m to enter the tunnels, "gravity facilitates easier and more efficient water supply".
DECADES OF TUNNELING
The first tunnel to carry water to Mumbai was built outside the city in 1955. It was 6.7km long and transferred water from the Modak Sagar reservoir to Bel Nullah in Thane district.
In 1963, the first underwater tunnel was built within the city using the then revolutionary new Austrian tunnelling method (NATM) that requires controlled blasting. This 5.8km tunnel ran from Mulund in the east to Kandivli in the west and had a diameter of 2.5m (8.2 ft). By 1983, a tunnel boring machine (TBM) had arrived in the city to build the 3.8km tunnel from Mahalaxmi Racecourse to Malabar Hill. "It was the first time in the entire country that a TBM was used," says a BMC official.
The Mumbai region's water tunnel network kept growing. During 1983-91, a tunnel was built under Kasheli creek in Bhiwandi. Between 1995 and 2000, a 4.9km water tunnel was built between Ruparel and Mahalaxmi Racecourse.
The last major tunnel before the Ghatkopar-Parel link was built in 2018 between Powai and Vervalli, a distance of 2.2km.
A MEGA-PROJECT
The Ghatkopar-Parel water tunnel completed last month is on a different scale, though. It was built in two phases. Boring of the Ghatkopar-Wadala stretch was finished by September 2022, and the 5.3km Wadala-Parel stretch was started a month later. An official said work on this project went on during the pandemic.
The TBM used to dig this tunnel was assembled in China and lowered into the earth at Ghatkopar's Hedgewar Garden. Since then, work has been carried on in three daily shifts. "Alignment is of utmost importance in a project like this," said an official, adding, "We need to ensure that the tunnel is brought out at the anticipated site at the same depth. So, over 100 labourers are at work at any point of time across the three shifts."
While the boring has been finished, the Ghatkopar-Parel tunnel will start transporting water only by April 2026
ONGOING WORKS
Other tunnel works are on meanwhile. There's the Rs 618-crore Ghatkopar-Trombay water tunnel which measures 5.5km. Another tunnel measuring 3.2km is being built between Powai and Ghatkopar. Once commissioned, these will carry treated water to various parts of Mumbai.
Two other long tunnels are on BMC's drawing board. There's a plan to build a 7.1km tunnel from Kasheli outside the city limits to Mulund Octroi Naka. Another tunnel measuring 14.1km will fetch water from Yewai Master Balancing Reservoir in Thane district to Kasheli.
MANY CHALLENGES
Building any tunnel, even with a TBM, is not easy. Sometimes, you run into weak rocks or cavities that need to be filled up first. "A TBM cannot push by itself. It must get a firm grip around it. So, if there's a cavity, we fill it up to let the TBM get traction," an official explained.
Falling rocks and seepage pose other risks to the machine and its crew. In the Ghatkopar-Parel tunnel, for instance, much time will be spent on lining the walls with concrete to prevent seepage.
But the bigger challenge now comes from people digging deep bore wells within the city. "Nobody should dig a bore well in the city without BMC's permission. Maps of water tunnels are available in each administrative ward and permissions for bore wells are granted only after confirming that the site does not lie above a tunnel," an official said. "Still, there have been instances of people digging deep and piercing our tunnels.”
