Chennai Metro Rail Limited’s TBM Adyar has completed the downline tunnel breakthrough at RK Salai station, marking the third consecutive tunnelling milestone achieved under Phase-II in three days and advancing work on the underground Corridor 3 through central Chennai.
The tunnel boring machine began its drive from Royapettah station on December 29, 2025, and took 277 days to reach the other end at RK Salai station. During the operation, Adyar passed beneath and around several critical structures along Royapettah High Road, including Peters Bridge, the Employees’ Provident Fund Organisation building, Andhra School and multiple shop lots.
The tunnel was constructed through silty sand at an approximate depth, or overburden, of 9 to 13 metres. CMRL said 11 cutterhead interventions were carried out during the drive. Despite the dense urban setting and the presence of existing buildings and infrastructure along the alignment, the tunnelling was completed without any major ground settlement or adverse impact on existing infrastructure, according to the metro rail agency.
The breakthrough follows two other TBM milestones recorded by CMRL in quick succession. On September 29, the Corridor 3 downline TBM Kaveri broke through at Government Kilpauk Medical College Hospital station. The following day, Corridor 5 downline TBM Maruthan completed its breakthrough at Villivakkam Bus Terminus Metro station.
Together, the three breakthroughs reflect the scale of simultaneous underground construction underway across Chennai Metro’s Phase-II network. The latest drive forms part of the 12-km underground section of Corridor 3 between Kellys and Taramani. Larsen & Toubro is executing the tunnelling work, with eight TBMs deployed for the project.
The Royapettah–RK Salai section has now seen breakthroughs on both tunnel tracks. Tunnelling between the two locations began in September 2024 with the upline TBM Bhavani. That machine completed its breakthrough in October 2025, while the downline drive by Adyar has now been completed.
The sequence places attention on the construction challenges involved in expanding metro infrastructure beneath established urban neighbourhoods. The section includes busy roads, existing public and private structures, and shallow underground conditions, making control of settlement and protection of surface infrastructure central to the tunnelling operation. CMRL’s next project milestones will determine how these completed tunnel drives are integrated into the wider Corridor 3 construction programme.