June 1, 2012 by Andrew Snook
The Province of Ontario is constantly looking for new ways to meet its growing energy demands. One such venture is the Ontario Power Generation’s (OPG) Niagara Tunnel Project, a 10.2-kilometre long, 12.7-metre diameter, water diversion tunnel that will deliver an additional 500 cubic metres per second of Niagara River water from above the Horseshoe Falls to the Sir Adam Beck hydroelectric stations at Queenston, Ont.
The project will be completed by the end of 2013, and will enable OPG’s Niagara hydro stations to generate an extra 1.6 billion kilowatt-hours of clean, renewable electricity annually—enough to meet the needs of 160,000 homes.
The massive project is being built by Austrian construction firm Strabag AG and various local sub-contractors, including Dufferin Construction and Dufferin Concrete.
The tunnel runs parallel with the Niagara River and reaches depths of 140 metres below ground level. It’s being built, from the outlet canal to the intake excavation, using a two-pass tunneling method. The first pass was the excavation of the tunnel—which ran from September 2006 to May 2011, involved the removal of 1.7-million cubic metres of rock—and was performed using “Big Becky”, which at 14.44 metres in diameter is the world’s largest hard-rock tunnel boring machine (TBM). The initial rock support is comprised of swellex rock bolts, steel ribs, welded wire mesh and shotcrete, installed from the TBM and trailing gear.
The second pass is an innovative, unreinforced cast-in-place concrete liner, designed to handle the complex geological conditions in the Niagara region. The cast-in-place concrete is externally pre-stressed by high-pressure grouting between the waterproof membrane and the initial shotcrete support to offset the internal water pressure, prevent cracking and assure the expected 90 years of maintenance-free service.
Paul Moorhouse, engineering manager for Hatch Mott MacDonald and Hatch Ltd., OPG’s owner’s representatives for the Niagara Tunnel Project, says several design criteria led to using the two-pass system and the unreinforced cast-in-place concrete liner.
The fact that the liner would be 100 per cent watertight was a main factor in the decision, since some of the host rock formations could swell in the presence of fresh water and transfer high loads onto the concrete lining. Additional factors include: the corrosive nature of the ambient ground water, which led to minimizing the use of steel reinforcement in the final liner; and the need to make the tunnel as hydraulically efficient as possible to reduce head losses due to friction, which would directly affect the additional electricity generation at OPG’s Sir Adam Beck hydro stations.
The concrete liner will require approximately 300,000 cubic metres of concrete in total and is poured in two stages: first, the invert (the bottom one-third of the liner) and later the arch (the bigger of the two pours that completes the top two-thirds of the liner).
Using two separate form carriers, Moorhouse says the workers can complete 25metre long sections of invert concrete and arch concrete each day.
He says the invert concrete is more than 90 per cent complete, while the arch concrete is over 60 per cent complete.
The majority of the concrete for the project is supplied by two sources: an on-site, dedicated batch plant, supplied by Dufferin Concrete, and a second Dufferin plant located in Niagara Falls, Ont.
Bernhard Mitis, construction manager for Strabag at the Niagara Tunnel Project, says the biggest challenges his team has faced thus far are logistically related, due to the number of trucks running in-and-out of the tunnel; including those transporting the concrete to where it needs to be.
“At this point right now we have the invert concrete [liner], the tunnel arch [concrete liner] and two grouting and restoration operations.”
For the first 1.4 kilometres of the arch lining, concrete was pumped directly to the forms from the outlet portal.
“That was the limit of how far we could pump it and maintain quality,” says Moorhouse.
Since then, concrete has either been delivered by large-capacity mixer trucks through the tunnel, or gravity-fed through one of four steel drop shafts spaced along the length of the tunnel specifically for the delivery of concrete from the surface. When it reaches the tunnel, the concrete is transferred to a mixer truck for haulage to the form location. The process requires numerous tests to be performed on the concrete.
“They basically have to remix at the base of the shaft,” says Moorhouse. “They test it at the surface, then they test at the re-mixer and then they test it at the form, because the transfers can alter some characteristics of the concrete.”
Between the initial and final lining is a waterproof membrane consisting of a geotextile fleece fixed to the shotcrete with nails and Velcro discs; a vacuum tested dual-layer flexible polyolefin (FPO) membrane system (2-millimetre thick layer plus a 1.5-millimetre thick dimple layer) is used for the invert in rock formations with swelling potential and a prototype electrically tested 3-millimetre thick laminated FPO membrane is used in the tunnel arch.
Moorhouse says low-pressure cement grout is pumped between the membrane and the cast-in-place concrete lining to fill voids and imperfections within the concrete lining. A second stage of interface grouting, between the membrane and initial shotcrete lining, at pressures up to 20 bar applies a pre-stress to the concrete lining to ensure the concrete will remain in compression under all loading conditions.
With concrete placement and grouting in progress around the clock, seven days a week, and approximately 450 workers on site, Ontario’s electricity consumers are expected to benefit from completion of this clean, renewable Niagara hydropower project by the end of 2013.