World’s Largest Hard-Rock TBM Forges Ahead at Niagara

Tunneling with a 47.2-ft diameter Robbins TBM at Canada’s Niagara Tunnel Project is more than two-thirds complete.

By June 2010, the world’s largest hard rock TBM had completed more than two-thirds of its headrace tunnel beneath Niagara Falls. The 47.2-ft diameter Robbins Main Beam TBM had excavated 4.2 miles of the 6.5-mile long conduit for Canada’s Niagara Tunnel Project in Queenston, Ont. Geologic conditions have largely determined the project’s advance, from periodic stoppages to a world-record month of 1,535 ft in July 2009. The advance rate is a landmark achievement for TBMs in its diameter range.

“We also raised the tunnel alignment by 150 ft to bring the tunnel out of the Queenston shale and into more competent rock, in order to reduce over-break,” said Ernst Gschnitzer, project manager for contractor Strabag AG. Much of the tunnel face is now in whirlpool sandstone and conditions are improving. “We are happy with the current rock conditions and ground support system, as we haven’t been short of challenges in the past.”  

Conditions in the tunnel have been highly variable, with significant over-break occurring within the first 650 ft of tunneling in Queenston shale. Crews scaled down the loose rock and adopted a newly redesigned ground support program consisting of 30 ft-long grouted spiles, 13-ft long rock bolts, wire mesh, steel straps and a layer of shotcrete. 

Four different processes are currently being done from the single tunnel opening. Crews are excavating the tunnel, performing repairs in sections of overbreak, laying invert concrete, and conducting arch lining for the upper two-thirds of the tunnel.

The finished 42-ft diameter tunnel will be fully lined with 24-in. thick continuously poured concrete and a polyolefin waterproof membrane to prevent leakage. The tunnel is being lined behind the Robbins TBM using separate invert and arch lining systems as well as a membrane laying machine. By May 2010, invert concreting had reached 3.0 miles into the tunnel, while arch lining had started up recently. 

The Niagara Tunnel Project was initiated in June 2004 by provincially owned company Ontario Power Generation. The tunnel is the third headrace under Niagara Falls, and will add up to 17,700 ft3/sec for hydroelectric generation by 2013 — enough power to service 130,000 Canadian customers.