From February, the Mumbai Metro Rail Corporation (MMRC) will begin tunnelling 12.5m below the Mithi riverbed as Mumbai’s first underground Metro — the Colaba-Andheri-Seepz line — goes underwater.
The proposed 33.5km Metro-3 line will be the second metro in the country to have tunnels under a riverbed. The first is being constructed in Kolkata under the Hooghly River.
The MMRC will construct two tunnels of 1.18km length each with tunnel-boring machines, currently used towards Vidyanagari, and a 170m tunnel using the New Austrian Technology Method (NATM). The tunnels will be between Dharavi and Bandra-Kurla Complex (BKC). The south end of BKC station will also be underwater.
To accomplish this engineering feat of constructing under a riverbed, the contractors will adopt several safety methods. The soil and broken rocks above the tunnel will be strengthened to reduce permeability and ingress of water. An “umbrella” of steel bars will be created above the excavation area for safety and there will be a constant check on soil/rocks during excavation.
SK Gupta, director (projects), said, “The challenge is to create a safe curtain for the work to go on and also resist water pressure. Another engineering challenge is that the rocks here are weaker. Our designers, consultants and contractors have worked on this plan for long to make it possible”
With the water levels raising in the river every monsoon, Gupta said more precautions would be taken to control water pressure.
The BKC station is also a crucial intersection point as it is being planned as a three-line station with two main platforms and a cross-over line, which will connect to either side of the mainline.
The cross-over line is being constructed using NATM. This will be the first metro to use NATM underwater.
The MMRC has completed close to 17km of tunnelling in the city with all 17 tunnel-boring machines put to work. The MMRC on Tuesday awarded the contract for automated fare collection (AFC) sysin all of its proposed 27 underground stations to a consortium of M/s. ASIS Elektronik ve Billisim Sistemleri A.S., Turkey and Kalindee -ASIS joint venture.
The AFC system will also follow the integrated ticketing system tem that is being implemented by the Mumbai Metropolitan Region Development Authority for seamless travel in the city.
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