After three years of hard-fought tunneling, contractor Afcons overcomes challenging mixed ground conditions and high cutter wear to record second and final breakthrough of its Robbins EPB on Contract UAA-01 of Chennai’s new metro system.
The 1,027m long return drive into the shaft at Chennai Central station means that the contractor now only has two partially completed drives remaining.
Gopal Dey, Senior Manager for Afcons, said: “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.”
The 6.65m diameter Robbins EPB was designed to excavate granite, sand, silt, and clay with boulders up to 300mm in diameter. The specialized design utilized a combination of 17in disc cutters as well as soft ground tools. Small grippers located around the shield allowed for cutterhead stabilization in harder ground, with the additional benefit of enabling retraction from the face in difficult conditions.
The TBM was launched on its first drive in January 2012 from a 28m deep launch shaft. Challenges arose from the outset: the TBM bored into mixed face conditions that contained varying strengths of granite, from weathered to hard granite of 150 MPa UCS. The unexpectedly hard rock caused high cutter consumption rates and slowed advance rates.
As a result of the unexpected geological conditions, Robbins India provided a geologist to carry out full face mapping, daily, for the whole of the first drive, in both hyperbaric and open mode conditions. The data was then used to provide a comprehensive geological record for the second drive, enabling Robbins to advise Afcons on the optimal operating parameters to get through the difficult sections, including cutterhead RPM, thrust pressure, penetration rate, and cutterhead torque. The parameters also resulted in a reduced cutter consumption rate on the return drive.
“The Robbins Field Service team extended very good services to us, particularly in the mixed face and full face rock when they deployed their geologist for face mapping,” said V. Manivannan, Executive Vice President fof Afcons. “This helped us to understand the strata ahead of us, and based on this the TBM advance rate and operating parameters were decided.”
The TBM was launched on its second tunnel drive 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 on this alignment, as the water table in Chennai is just 1.5m under the ground and the strata above the crown included silty sand, clay and weathered rock,” explained Dey for Afcons.
“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,” he added. 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 a best daily advance rate of 12.6m, and a best weekly figure of 62m.
The TBM broke through into the receiving shaft on January 27 (2016) by utilizing, for the second time, a setup that involved the machine emerging under water. “These are believed to be the first breakthroughs in India under wet conditions in the retrieval shaft, which is 30m below the ground level. The retrieval shaft was filled with Bentonite slurry 10m 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 the 32.1km of Line 1, which includes 14.3km of underground alignment and a total of 17 stations. Project owner Chennai Metro Rail Limited (CMRL) is about to retender the much-delayed Contract UAA-02, which was left half-finished following a contractor walkout by Metrostroy of Russia In May last year, and for which both TBMs have suffered extensive damage following an inundation of the tunnel during the severe floods in the city in December.