Four firms have bid for construction of 21 km underground stretch for Phase-II project of Chennai Metro Rail (CMRL).

Gulermak, Tata Projects, Larsen & Toubro (L&T) and Shanghai Tunnel Engineering Company (Singapore)-Woh Hup (joint venture) have submitted their bids.

While the entire Phase-II project covers a length of 118.9 km, CMRL will build the project by giving out contracts in sections.

Initially, two tenders were floated for construction of tunnels between Venugopal Nagar in Madhavaram to Kellys (nine km) and then from Kellys to Taramani (12 km). After technical and financial bids are finalised, a contractor will be chosen to build the tunnels in a few months.

Two tunnel boring machines (TBMs) each will operate in these two stretches — one for onward and another for return tracks.

Though the TBMs can bore at a speed of five-10 mtr a day on an average, geological conditions will determine actual working speed. It could take nearly four years to finish tunnelling in this stretch.