MUMBAI’S FIRST underground metro connecting Colaba-Bandra-SEEPZ will make the city the second in the country, after Kolkata, to have an underwater metro rail connection. The line will run under the Mithi river between Bandra Kurla Complex and Dharavi stations for a length of 1.2 km. This will be a unique engineering feat and will require expertise from the Mumbai Metro Rail Corporation (MMRC) along with their contractors and consultants. The necessity of tunnelling under a water body and the geological characteristics of the area also make it one of the major engineering challenges before the metro implementing body.
“It is normal to have some amount of water entering into the tunnel during construction, but when there is a water body above the tunnel the water seepage is much higher. Also the rocks here are weak making tunnelling a major engineering challenge. Apart from developing a competent design beforehand, our engineers and designers with geologists will be present at the site and they will have to take decisions on the spot based on the situations,” says S K Gupta, Director (Projects), MMRC.
Certain special measures will also be taken during excavation to safeguard against challenges. These include strengthening the weak and broken rock to improve the geological structure and reducing the permeability.
Apart from the main lines connecting the two stations, a stabling line will also be laid at a depth of 12-13 metres from the river bed. This line will be used for stabling and reversing of trains which will end at the BKC station.
While the tunnels for the mainlines will be excavated by Tunnel Boring Machines (TBM), the two tunnels for the stabling line with the cross packages connecting it to the mainline will require the use of the New Austrian Tunnelling Method (NATM). This is also the first time that NATM technology will be used underwater.
TBMs are huge machines that will be lowered at some distance from the river with the help of a heavy duty crane through shafts and will drill twin tunnels below the riverbed. NATM technology is generally used to create tunnels of smaller lengths. “These tunnels will only be 165-m long and TBMs are not used for such short lengths. Moreover, it will become difficult to remove the machine from the tunnel. Machinery used in NATMs are smaller and can be easily pulled out of the tunnel,” adds Gupta.
Despite the difficulties involved in boring tunnels underwater, MMRC has chosen BKC station for the stabling line considering the traffic loads and the bigger space available there.
“We could manage so much space only at this station. The adjoining stations like Dharavi, Shitaladevi, or Vidyanagari do not have as much area to accommodate a third platform. This was possible only here,” says Gupta.
While the construction of these tunnels poses a major challenge, MMRC assures that post construction it will be perfectly safe for travel.
“The challenges are only during the construction phase and once the tunnels are ready they will be perfectly safe,” he says.
He further assures that the existence of a metro line under the river will have no effect on the river or its ecosystem. “The lines are running way below the river bed and so will have no effect on the river,” adds Gupta.
However, environment activist Janak Daftari has raised worries over the impact the tunnelling activity will have on the river ecology.
“Construction activity underwater will definitely have an impact on the river’s ecosystem. While it is difficult to say how adverse the effect will be as it is an unexplored territory, it is certain that it will further deteriorate the river,” says Daftari.
The construction of Kolkata’s East-West Metro Corridor is underway and TBMs are set to tunnel below the Hooghly river soon. The entire corridor is expected to be completed by next year while Metro 3 is slated to be completed only by 2021.