Curious spectators peered over the edge of the retrieval shaft. The tunnel face, large enough to accommodate four stories, seemed dwarfed by the shaft’s size until you notice small figures in safety orange walking around the base.
Later the area would fill with dust as the 14.4m diameter Robbins TBM made its final breakthrough into the warm May sunshine on Friday, May 13.
Big Becky, as it was named by a group of local school children, had been launched in summer 2006 to bore the 10.4km tunnel from the Sir Adam Beck Generating Complex. When the Niagara Tunnel is finished it will increase hydroelectric power supply for owner Ontario Power Generation (OPG), enough power to service an additional 130,000 Canadian customers.
The TBM had been excavating a 300m grout tunnel up to the intake structure, after an initial breakthrough in March. The May ceremony included speeches from several local politicians, the CEOs of Strabag and OPG and Ernst Gschnitzer, project manager for Strabag.
Gschnitzer thanked the client for its fast decision-making, particularly when the project hit bad ground, and project manager Hatch Mott MacDonald, for “sharing the headache.”
The Niagara Tunnel met challenging rock conditions early on in its drive through the area’s Queenston shale. Large rock blocks started to fall from the crown before support could be put in place, and overbreak exceeded 3m at times.
While the first 2,300m were expected to be problematic, Strabag eventually proposed changes to the alignment after difficulties with fractured rock slowed progress rates down to two to 3m per day.
“At that point we were boring downhill and every couple months you were getting into a different type of rock,” says Mike Kolenich, Robbins’ project manager. “After about 1,000m—the machine went down at about a seven per cent grade for about 1,000m—when they started to level out that’s where they started experiencing bad ground. They knew that they would need some ground support but not to the extent that they’ve had to do, and not for the entire duration of the tunnel.”
In 2008 the alignment was raised by about 45m to get into more stable ground, and with that overbreak was greatly reduced. Progress rates improved up to 10 to 15m per day last year. July 2009 saw the project meet two records for TBMs 11m in diameter or larger: 468m in one month, and 153m in one week, according to Robbins.
It was also the first time 20in cutters were used, Kolenich explains, to get better wear and because it seemed like a good project to test them out.
Back at the breakthrough ceremony Gschnitzer saved his biggest thanks for the all-Canadian workforce used on the project. “You did a really impressive job when you produced 20 to 30m a day. But you really did a much more impressive job when you produced half a metre or one metre a day, because you did it under conditions which were just unbelievable.
“When I was here and watched you working in these conditions I was speechless. It was unbelievable how you handled this project at its technical limits.”
There are still two years to go before the tunnel can be put into operation. Approximately 30 per cent of the continuous concrete lining has been completed already.
The finished 12.8m diameter tunnel will be fully lined with both 600mm thick cast in place concrete and a polyolefin waterproof membrane to prevent leakage.
Dismantling for Big Becky will take place over the next few months. Many of the pieces will be moved out of the tunnel by self propelled modular transporters or SPMTs, in addition to the large capacity cranes. The cutterhead—which was the first to use 20in cutters—will stay in Niagara Falls and will be displayed as a legacy to the project.
“It’s a good sense of accomplishment,” Kolenich says. “This whole project was a pretty big engineering feat as well as mechanical and civil.”
