The underground stretch from Madhavaram to Kellys is being taken up first. The first tunnel boring machine (TBM) that arrived from China more than two months ago has already been lowered into the construction site. The two 6.61m TBM machines manufactured by Terratec in China will be used to build about 9-km long twin tunnels on line -3's Venugopal Nagar – Kellys’ section. One TBM is being pressed into service in the initial tunnelling operation at Madhavaram and drill toward Kellys to construct an underground section.
The TBM at Madhavaram will take months to reach Kellys, and the length of time would primarily rely on the geological conditions and then the second TBM will be deployed on the corridor and the contractor gives a gap of couple of weeks between deploying the two TBMs operating with a space of 100m or more is considered better.
At a later stage, two more TBMs will begin work between Kellys and Taramani. Around 26.7 km of Corridor 3's 45.81 km stretch from Madhavaram to SIPCOT and 10 km of Corridor 4, a 26-km route connecting Light House and Poonamallee, will be in underground service. Another 5.8 km underground section of Corridor 5 connects Madhavaram and Shollinganallur spanning up to 47 km.
The Phase II project will require 23 TBMs, and the additional equipment is expected to arrive soon and then the underground work will gradually spread to other parts of the city.