Mammoth 650-tonne tunnel boring machine completes epic 1,600-km journey from Tamil Nadu to Kolkata; here's why

KOLKATA: A giant tunnel boring machine (TBM), 90m long and weighing around 650 tonnes, almost entirely made in India, has reached Kolkata after a 1,653 km journey from Tamil Nadu's Alinjivakkam. Despite the ongoing impasse over land for construction of the Purple Line's Kidderpore station, Metro has decided to proceed with the underground tunnelling from Kidderpore to Park Street.

TBMs are earth pressure balance (EPB) machines used to excavate tunnels through hard rock, wet or dry soil, or sand. The superior technology allows burrowing and building tunnels with precast segment rings. The Purple Line's TBMs will first build 1.7 km from St Thomas' to Victoria. The second drive will span 950 m from Victoria to Park Street. The northern end between Park Street and Esplanade will be built using the cut-and-cover method.
Around 5 km of the 14 km Metro corridor, which will connect Joka and Esplanade via 12 stations, is underground. But there is uncertainty whether Kidderpore station will be built at all, as land at Kolkata Police's Bodyguard Lines is not available yet.
Meanwhile, contractors L&T, awarded the Rs 2,447 crore underground package by Rail Vikas Nigam Ltd (RVNL), the railways undertaking tasked with building most of the city's Metro lines, has started work in full swing. L&T commenced work on this package in Sept 2023, with Victoria station. In July 2024, it started casting tunnel ring segments in its Sarkarpool (near Mahestala) unit. The unit is likely to produce around 3,000 segments to feed the TBMs to build the section's twin tunnels, which will have an inner diameter of 5.8 m.
The TBM, dismantled and loaded onto 17 trailers, will be directly unloaded in the shaft built inside St Thomas' Boys School and launched from there.
On Feb 20, Metro GM P Uday Kumar Reddy and RVNL's chief project manager Vipin Kumar travelled to Alinjivakkam to witness the second TBM's factory acceptance test (FAT). Unlike the East-West Metro project, where all three pairs of TBMs were manufactured in Germany, shipped to Delhi, and ferried by road to Kolkata, most of the two new TBMs were built and assembled in Tamil Nadu.
"Except for four parts - the cutter head (which digs through soil), manlock (which holds workers to access the inside of tunnel borer), material lock (which removes the excavated material hydraulically), and propulsion system (comprising hydraulic cylinders which supply power) - most of the two TBMs were indigenously built," an official said. "Though S-1410A and S-1411A are from German manufacturer Herrenknecht AG's TN unit, they were not totally built/assembled outside India," he said.
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