Robbins EPB surmounts Chennai Metro Challenges
Mar 04, 2016
Recent breakthrough marks triumph over Mixed Ground Conditions
Contractor Afcons Infrastructure Ltd. reflected on the breakthrough: “We are really proud of our executing team, who have maintained a high standard of quality. We didn’t record any water leakage or settlement at the surface, and we have demonstrated a high standard of safety in the tunnel during construction,” said Mr. Gopal Dey, Sr. Manager for Afcons.
The TBM was launched on its initial drive in January 2012 from a 28 m (92 ft) deep starting pit. Challenges began nearly from the outset. The TBM bored into mixed face conditions that contained varying strengths of granite, from weathered to hard granite of 150 MPa (21,700 psi) UCS. The unexpectedly hard rock caused high cutter consumption rates and slowed advance.
With the data gleaned from the geological investigation, Robbins was able to advise Afcons on the optimal operating parameters to get through the difficult conditions, including cutterhead RPM, thrust pressure, penetration rate, and cutterhead torque. The parameters also resulted in a reduced cutter consumption rate.
The TBM was launched on its second tunnel in February 2015. Conditions were just as difficult as the first drive, but now the team approached it with experience: “We experienced very high water pressure in this alignment, as the water table in Chennai is just 1.5 m (5 ft) underground and the strata above the crown included silty sand, clay and weathered rock. 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,” said Dey.
The TBM broke through into a receiving shaft, utilizing a unique setup for the second time that had the machine emerging under water. “These were the first breakthroughs in India under wet conditions in the retrieval shaft, which is 30 m (98 ft) below the ground level. The retrieval shaft was filled with Bentonite slurry 10 m (33 ft) from the base slab in order to arrest water entry from outside the diaphragm wall,” explained Manivannan.
The completed sections of tunnel will now be commissioned as part of Line 1, a 32.1 km (19.9 mi) long route in total with 14.3 km (8.9 mi) underground and a total of 17 stations. The southeastern Indian city of Chennai is a rapidly growing technological and industrial center with a population of more than 8.2 million people and a high need for alternate means of transportation.
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