Twin tunnels under river built using earth balance machines to prevent settlement and manage water seepage; terming the development as Phase 2A, a social media video posted depicted visuals of trials and stations
Two tunnel-boring machines below Mithi River; (right) diagram of sediment layers above the two tracks
The Mumbai Metro Aqua Line 3 on Tuesday said it had commenced train movement ahead of Kotak BKC Metro station on the Dharavi to Acharya Atre Chowk stretch, covering 9.77 km and six stations.
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“Train movement was completed till Acharya Atre Chowk station for trials. The stretch is expected to open for commercial operations by March 2025, subject to inspection and safety certifications by the commissioner of metro rail safety,” a spokesperson said.
Terming the development as Phase 2A, a social media video posted depicted visuals of trials and stations. The final phase of the line is expected to open in Colaba by 2025 end. Once the line opens till Acharya Atre Chowk, it will take 39 minutes from JVLR to Acharya Atre Chowk.
Challenges faced
Officials said working under the Mithi River for such a project was a big challenge. It has never been attempted here under such soil conditions and hence there was no precedent of any kind. The Mithi River is crossed by a twin-tunnel section running under the river between Bandra Kurla Complex and Dharavi stations. Work was taken up with tunnel boring machines (TBMs) named Godavari 3 and Godavari 4 and the New Australian Tunneling Method, which involves sequential excavation.
From the entire 3 km twin tunnel section running between BKC and Dharavi, approximately 2 km passes under an expanded water body, including a 500-metre segment of the active Mithi River channel. To tackle this challenge, custom-designed earth balance machines were developed specifically for tunnelling beneath the water. These advanced machines are engineered to sustain natural pressure at the tunnel face, preventing land settlement, while also excelling in managing the removal of excavated material and controlling water seepage.
What is below the Mithi
The railway line is 9.57m below the Mithi river bed. Below the Mithi water flow, there is one layer of slush, followed by Grade 3/4 brecciated basalt rock, followed by Grade 1/2 brecciated basalt rock, where the machines were used.
Watertight gaskets
The system should be able to withstand more likelihood of water coming in. That's why we have designed a gasket, especially for this area. Precast concrete segment rings under special mushroom-type gaskets. To ensure that they are watertight, there is one more layer of hydrophilic gaskets that swell when they come in contact with water, making the joints further watertight.
Depth
Dharavi station which is next to the Kotak BKC station is located 18 to 18.5 metres below the ground.
Six stations
The six stations between Kotak BKC and Acharya Atre Chowk include Dharavi, Sitaladevi, Dadar, Siddhivinayak, Worli and Acharya Atre Chowk.
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