Afcons officials said it would take about a month to complete the retrieval.

At the front of the machines are cutters that cut through the soil.

Each machine is 100 metres in length but a large portion was disassembled and taken to Howrah Maidan, from where the parts were pulled out. The remaining portions of the two machines — around 10m in length — could not be taken to Howrah Maidan because their diameter is more than that of the tunnel.

The machines that dug the East-West Metro tunnels under the Hooghly river will be pulled out from underground at Esplanade, where they had been lying for more than a year.

The machines travelled 3.8km from Howrah Maidan to Esplanade and are waiting to be taken out at 23m under the surface. “The extraction starts on Saturday. The rear portion of the machine that had burrowed the tunnel through which the Howrah-bound trains will pass will be pulled out first,” said an official in the construction company Afcons, which has built the twin tunnels between Howrah Maidan and Esplanade.

Once fully functional, the East-West Metro will connect Salt Lake Sector V with Howrah Maidan and will pass under the Hooghly. In the first phase, trains will run along a 5.5km elevated corridor between the Sector V and the Salt Lake stadium stations.

“The preparations for retrieving the tunnel-boring machines are complete. The machines will be pulled out through a shaft built at Esplanade adjacent to the site where the East-West station is being built. A 160-tonne-capacity crane will be deployed to pull out the machines, each of which weighs 313 tonnes and has six components. The heaviest component is the front shield and it weighs 92 tonnes,” said an engineer involved in the project.

One of the machines, as it looked a fortnight ago (Pradip Sanyal)

Cables have been attached to a tunnel-boring machine for extraction (Pradip Sanyal)

The shaft through which the machines will be pulled out (Pradip Sanyal)

“The diameter of the cutter’s head is 6.38m, while that of the tunnel’s inner ring is 5.5m,” an official said.

The two machines had reached Esplanade in April 2018. Railway officials said the machines could have been pulled out much before but unavailability of land at Esplanade caused the delay.

“The machines, once retrieved, will be sent to our facility in Nagpur for overhauling,” an Afcons official said.