Rachna, Prerna head back home after boring tunnel under Hooghly

Kolkata: After eight years, it’s time for Prerna and Rachna to bid farewell to Kolkata.
The two German Herrenknecht-made tunnel boring machines—TBM S640, christened Prerna and TBM S639 named Rachna—that dug the East-West Metro tunnels under the Hooghly, joining Howrah to Kolkata, were given a formal send off at Curzon Park by those involved in the first phase of the Metro tunnelling. “I don’t think anyone has ever heard of TBMs getting a farewell.But Rachna and Prerna are special to us,” said Satya Narayan Kanwar, project manager, Afcons-Transtonnelstroy, executing agency for East-West Metro’s underground tunnelling from Howrah Maidan to Esplanade.
Afcons engaged the two TBMs in October 2011 to burrow under the Hooghly riverbed, giving Kolkata its very own Channel Tunnel. The journey, however, stopped before it began. Stuck in a route realignment impasse, the mega borers lay idle at Howrah Maidan for five years, until the nod was given to start mining from March 2016. In May 2018, Prerna and Rachna covered 3.8km to reach their destination —Esplanade—negotiating not just the riverbed, but also crumbling buildings on Brabourne Road and Dalhousie and heritage structures, like Writers’ and Currency Building.
But even after the work was done, the two TBMs could not be taken away immediately and were parked below Curzon Park. The shaft to extract the borers couldn’t be built as Army, the custodian of Maidan, delayed handing over land to the implementing agency, Kolkata Metro Rail Corporation (KMRC), which, in turn, failed to give the site to ITD ITD-Cementation, contracted to build the Esplanade-Sealdah part of the tunnel. In the absence of the retrieval shaft, only backup parts and gantries of the TBMs —each around 90m long—were disassembled and wheeled through the 3.8km tunnels and extracted bit-by-bit from the Howrah Maidan station. The main parts, each 9m long and weighing 320 tonnes, remained buried under Curzon Park.
Afcons moved the HC, demanding compensation for the machines lying idle. KMRC told the court the shaft would be ready by July 31. But it was built before that and Afcons started pulling out the machine parts with a 160-tonne capacity crane. The heaviest component (92 tonnes) was taken out first. The last part, the 94-tonne front shield of Rachna, was fished out on Sunday.
Afcons engineers, sporting saffron safety jackets and yellow helmets, lined up around Rachna’s front shield and raised a victory salute. “We share a special bond with Prerna and Rachna. We are happy they could be retrieved before the deadline,” said KMRC MD Manas Sarkar.