Third bore complete for Chennai Metro project 16 Nov 2017

On Tuesday, October 31, 2017, a 6.65m diameter Robbins EPBM broke through for the third time on the latest 1.8km section of the Chennai Metro project for contractor Afcons Infrastructure Ltd after achieving advance rates of 80mm/minute in challenging conditions. The metro will provide further transportation routes for a rapidly developing city.

6.65m Robbins EPBM breaks through at Chennai Metro, India

Robbins EPBMs have been used on multiple tunnel drives before and as they are built one third heavier than other machines on the market and are designed for 10,000 hours of usable life, they are ideal for long distances.

“Robbins is known for designing resilient machines,” said Gopal Dey, Afcons Senior Manager. “For this project, we needed a machine that was specially built to excavate mixed ground from soft clay to hard rock. The Robbins EPBM has unique design features that made this machine ideal for the geology.”

After completing the original twin tunnels, the TBM was refurbished in preparation for the latest section. During refurbishment, the inner seal greasing system was changed from auto to manual mode and the foam nozzle system was modified. These changes optimized the machine for excavation in the highly variable mixed face conditions encountered throughout the project.

Celebrating rates of up to 80mm per minute in mixed ground

The TBM started the third tunnel drive on February 3, 2017 and faced a major challenge from the start. “After exiting the launching shaft, the TBM erected a few rings before having to cross a live railway track, which meant there would be a possibility of ground settlement,” said Manivannan Venkataraman, Afcons Director. “At this stage, we had to closely monitor TBM face pressure and advance the machine at a uniform rate to ensure proper primary grouting.” The geology consisted of clayey sand and about 60m of mixed ground.

“We chose this machine for the project due to the underground geology in Chennai, which continuously changes from soil to mixed ground,” said Dey. “We also liked that Robbins EPBMs are designed with active articulation to allow the machine to turn or steer with ease.” During active articulation, thrust cylinders react evenly against the entire circumference of the tunnel lining, even in a curve—a design that eliminates the problem of tunnel lining deformation. “This feature enabled the machine to negotiate through well-controlled conditions,” added Manivannan.

With all three tunnels, Robbins field service staff were on hand to assist and support as required. During the course of each bore, Robbins advised how best to manoeuvre through challenging ground and best operate the machine.

This latest tunnel connects multiple stations along the metro line, starting from Washermenpet through to Sir Theagaraya and to Kurukpet.