Kolkata: TBM Durga will make her breakthrough on Friday, exactly a year after it started boring a tunnel below St Thomas’ Boys School, Kidderpore. She is due to start cutting through the final barrier opening into the Victoria Memorial station around 10 am.
A breakthrough for a tunnel-boring machine (TBM) is the climactic milestone when the machine’s massive rotating cutterhead grinds through the final barrier of rock, soil or concrete to emerge into an open reception shaft or cavern inside an underground station. The breakthrough marks the successful completion of the tunnel drive.
Sibling TBM Divya, the second machine digging the underground section, should finish boring and building the stretch by the first week of Aug, marking the end of the first phase of the corridor’s underground tunnelling.
TBM Durga started digging the Purple Line metro tunnel from Kidderpore to Victoria Memorial on July 10, 2025, flagging off the corridor’s 5km underground construction. Exactly 365 days later, the much-delayed 14km Joka-Esplanade corridor is all set to achieve the first “breakthrough”, next to the iconic Victoria Memorial. After years of delay, the green signal for the Purple Line’s Victoria Memorial station came in Feb 2022. The line, which will eventually extend both north and southwards, now runs an elevated 8 km from Joka to Majerhat. Work is on to complete the last elevated station, Mominpore, beyond which is Kidderpore — the first underground station.
Now, the Purple Line’s first underground tunnel will be linked to the under-construction station opposite Victoria Memorial. TBM Divya began burrowing the second tunnel from the same shaft at Kidderpore on Oct 11, 2025. The two TBMs (parts of which were imported from Germany’s Herrenknecht), assembled in Tamil Nadu, are one of the first “make-in-India” endeavours in the country.
When TBM Durga cuts through the specially built wall below the Maidan, opposite Victoria Memorial, it will have completed its first phase of tunnelling, across 1.7 km from Kidderpore. It will have to be relaunched to start tunnelling the second phase — from Victoria to Park Street — covering 900m because the Victoria station is still under construction. Usually, breakthroughs happen when an underground metro station is structurally complete.
In case of the Purple Line, the exception is being made because the project, conceived in 2009, has hit several hurdles and missed multiple deadlines because land (site) was unavailable. “We had to start work as and when land was available,” an official explained.