Contractor Afcons Infrastructure is celebrating achieving breakthrough on its second tunnel drive for Line 1 on the Chennai Metro in India.
The contractor completed the twin 1km bore between the Washermanpet district and Chennai International Airport on schedule despite difficult ground conditions.
Afcons used a Robbins mixed ground earth pressure balance tunnel boring machine (TBM) to excavate the 6.65m diameter tunnel through a mix of granite – up to 150MPa in strength - and sand, silt and clay with boulders of up to 300mm in diameter. The TBM was equipped with disc cutters as well as soft ground tools and Afcons has reported that the drive was achieved without any water leakage or settlement at the surface.
The first bore was started in January 2012 and the unexpectedly hard rock conditions caused high cutter consumption rates and slowed the advance. Robbins worked with Afcons to carry out geological face mapping and give technical advice that helped to keep work on the second bore, which started in February last year, on schedule and reduce cutter consumption rates.
“We experienced very high water pressure in this alignment, as the water table in Chennai is just 1.5m below ground and the strata above the crown included silty sand, clay and weathered rock,” said Afcons senior manager Gopal Dey. “It was very important for us to maintain the earth pressure to reduce the inflow of water, and to avoid any settlement on the surface with proper grouting. Despite the challenges the TBM was able to complete a section below the Koovam River without any water flowing into the tunnel.”
The completed tunnels will form part of the 32km long Line 1 route, which has 17 stations and is due to open in 2019.