Massive Niagara project sees end of tunnel
"Big Becky," the largest hard rock tunnel boring machine in the world, resumed its underground journey on December 27. The Niagara Tunnel Project TBM is drilling a third tunnel to divert water from Niagara Falls to the Adam Beck Generating...
“Big Becky,” the largest hard rock tunnel boring machine in the world, resumed its underground journey on December 27. The Niagara Tunnel Project TBM is drilling a third tunnel to divert water from Niagara Falls to the Adam Beck Generating Station at Queenston, southern Ontario.
The giant TBM had been halted for nearly three weeks in December to allow crews to repair a 2-inch crack in the machine’s centre spine beam. The workers also used the shutdown as an opportunity to do other maintenance work on the machine.
The December stoppage was a minor delay compared to other setbacks that the project suffered in its earlier stages. Work began in 2005, but the tunnelling design-build contractor, Strabag AG of Austria, soon unexpectedly encountered poor rock conditions. The Queenston Shale above the tunnel was fracturing vertically, even though, according to the project web site, none of the test boring samples had uncovered the problem. The fracturing rock slowed progress to such a snail’s pace that in 2008 Strabag decided to change the alignment and Ontario Power Generation, the project owner, is reportedly footing the bill. The tunnel was originally intended to be completed in 2009 for $600 million but is now scheduled to be completed in 2012-2013 for approximately $1 billion.
The massive Big Becky, manufactured by Robbins Company, is roughly four storeys high, 150 metres long and weighs around 4,000 tonnes. It works 20 hours a day, carving a tunnel that is 14.4 metres in diameter — one and a half times larger in diameter than the Chunnel between France and England. The material it is removing is enough to fill 100,000 dump trucks.
The water will enter the new tunnel by dropping down a 100-metre shaft drilled just upstream from the Horseshoe Falls. Much of the route then follows alongside two existing tunnels dating from the 1950s and which run directly below the city of Niagara Falls along Stanley Avenue. The tunnel runs around 90-150 metres below ground, with an alignment determined by the need to avoid the glacial silt of the ancient buried St. David’s Gorge.
By late December the TBM was past the 9-kilometre mark on its 10-kilometre journey and will emerge in a coffer dam built near the Falls. Every two or three metres or so, the TBM halts so that workers can reinforce the walls with steel ribs, wire mesh and rock bolts, then spray on shotcrete. Far behind the TBM, workers are installing the permanent concrete lining, with the invert (bottom half) past the six kilometre mark.
Once completed, the new tunnel will carry about 500 cubic metres per second of water, adding about 1,600 gigawatt-hours to the Sir Adam Beck’s station’s annual power generation capacity. The flow of the Niagara River at the falls is about 6,000 cubic metres per second. One third of the water is allowed to flow over the falls to keep it as a tourist attraction, while the remaining two-thirds is available for power generation, split between the U.S. and Canada.
Hatch Mott MacDonald and Hatch Acres are consulting engineers on the tunnel project for the owners, Ontario Power Generation. Morrison Hershfield, and ILF Consulting Engineers (of Germany) have also helped in the design. Morrison Hershfield won a 2008 Canadian Consulting Engineering Award for the tunnel Intake Structure. To see the article about that project, click here.
To visit the Niagara Tunnel website, click here.