CHENNAI: The Chennai Metro Rail Limited (CMRL) achieved a major tunnelling milestone with the Corridor 3 downline tunnel boring machine (TBM) 'Adyar' completing its breakthrough at RK Salai station, the third consecutive TBM breakthrough achieved under Phase-II in as many days.

After starting at Royapettah station on December 29, 2025, the machine took 277 days to break through the other end at RK Salai station, during which 'Adyar' had to negotiate several critical structures along Royapettah High Road, including Peters Bridge, the Employees' Provident Fund Organisation (EPFO) building, Andhra School, and several shop lots.

The tunnel was constructed through silty sand with an overburden of approximately 9 to 13 metres.

As many as 11 cutterhead interventions were carried out during the tunnelling operation.

Despite the challenging urban environment and the presence of existing structures along the alignment, the drive was completed without any major ground settlement or adverse impact on existing infrastructure, said CMRL.

The latest breakthrough is part of a series of three TBM milestones achieved by CMRL in quick succession.

On September 29, the Corridor 3 downline TBM 'Kaveri' achieved a breakthrough at Government Kilpauk Medical College (KMC) Hospital station and Corridor 5 downline TBM 'Maruthan' broke through at Villivakkam Bus Terminus Metro station the next day.

With 'Adyar' completing its drive on Thursday, CMRL has achieved a hat-trick of TBM breakthroughs in three consecutive days.

The tunnelling work for the 12-km underground section of Corridor 3 between Kellys and Taramani is being executed by Larsen & Toubro with eight TBMs deployed for the works.

The tunnelling between Royapettah and RK Salai began in September 2024 with 'Bhavani' on the upline, which completed its breakthrough in October 2025, while the downline TBM 'Adyar' has now completed its drive.