A massive eight-year construction feat to bore a 41-foot-wide, 6.3-mile-long tunnel deep beneath hard rock under the City of Niagara Falls in Ontario, Canada, was successfully completed this March. Undertaken by Ontario Power Generation (OPG) to divert water from the Niagara River and carry it downstream to the 2,080-MW Sir Adam Beck generating complex, the C$1.5 billion project now propels additional water to the hydroelectric plant at a rate of 17,660 cubic feet per second (Figure 1).
|1. A massive undertaking. Ontario Power Generation and Austrian firm STRABAG in March completed construction of a 41-foot-wide, 6.3-mile-long tunnel deep beneath hard rock under the City of Niagara Falls in Ontario. The project was begun in 2005 to divert water from the Niagara River to the 2,080-MW Sir Adam Beck generating complex. Courtesy: Niagarafrontier.com|
The project has been in the planning for nearly a century. When the Sir Adam Beck Niagara Generating Station Number 1 (then known as the Queenston-Chippawa plant) was opened in 1921, it was accompanied by plans for two canals leading from the Welland River to the power station. After the first canal was built, however, the second canal plan was abandoned. In the 1950s, after Ontario’s power demand surged, work began on a second generating station (Sir Adam Beck Number 2) at the site as well as on twin water tunnels 45 feet in diameter at a depth of 330 feet. The two tunnels, completed in 1955, and the original 1921 hydro canal divert a total of 63,566 cubic feet of water per second to the two hydroelectric generating units.
When work began on the newest Niagara Tunnel, Austrian firm STRABAG Co. was forced to route the tunnel to bypass the glacial silt of the buried St. David’s Gorge and to maintain a safe separation from the existing tunnels (even though they run on a mostly parallel route) at a depth of 459.3 feet (Figure 2). But this depth proved cumbersome, as some workers explain on a site dedicated to the project (http://www.niagarafrontier.com), because it was predominantly Queenston Shale (mudstone). “The reddish-purplish shale is fractured and has resulted in many roof-line rock falls slowing the boring operation,” the site says. “Although test boring samples were conducted in preparation for this project, none uncovered the vertical fracturing in the rock strata that the tunneling crews [experienced].”
|2. A rough road. To bypass the glacial silt of the buried St. David’s Gorge and to maintain a safe separation from an existing 1921-built hydrocanal and 1955-built twin tunnels, the new Niagara Tunnel, completed this March, burrowed at a depth of 459.3 feet in rock strata predominantly consisting of Queenston Shale (mudstone). Source: OPG|
In 2009, the difficult rock conditions forced OPG and STRABAG to revise the project’s schedule. According to Ontario’s Ministry of Energy, the project that employed 580 people during the peak of construction was completed nine months ahead of the revised schedule and nearly C$100 million under budget.
—Sonal Patel is POWER’s senior writer.