Recent breakthrough marks triumph over Mixed Ground Conditions
On January 27, 2016, a Robbins mixed ground EPB broke through at Chennai Metro, finishing up a challenging second drive that saw the full gamut of difficult conditions. The 1,027 m long second drive for the machine was part of Lot UAA-01 on Line 1 of the city’s metro, consisting of two parallel 1.0 km (0.6 mi) tunnels running from the Washermanpet area towards Chennai International Airport. 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 6.65 m (21.8 ft) diameter Robbins EPB was designed to excavate granite, sand, silt, and clay with boulders up to 300 mm (12 inches) in diameter. The specialized design utilized a combination of 17-inch diameter disc cutters as well as soft ground tools. Small grippers located around the circumference of the machine’s shield allowed for cutterhead stabilization in harder ground, while additionally reacting the forces needed to pull the cutterhead back from the face in difficult conditions.
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.
A crew of Robbins Field Service personnel and engineers assisted Afcons in remedying the problem. Robbins India provided a geologist who carried out face mapping for the whole of the first drive, in both hyperbaric and open mode conditions on a daily basis. The data not only assisted the crew in operating the TBM, but also provided a comprehensive geological record for the second drive. 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.
Contractor Afcons was pleased with the help they received: “The Robbins Field Service team extended very good services to us, particularly in the mixed face & full face rock when they deployed their Geologist for face mapping. This helped us to understand the strata ahead of us, and based on this the TBM advance rate and operating parameters were decided,” said Mr. V. Manivannan, Executive Vice Presidentfor Afcons.
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. Despite the challenges the TBM was able to complete a section below the Koovam River without any water flowing into the tunnel. The machine achieved up to 12.6 m (41 ft) in one day and 62 m (203 ft) in one week.
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.
