Cool Crossings

May 2010 Vol. 65 No. 5

With typical daily temperatures barely topping 5 degrees Fahrenheit, winters in Calgary, Alberta, can easily hinder trenchless operations. Liquids in machinery can freeze solid, while expanding groundwater can potentially warp rails and other equipment in launch pits.

Recently, Edmonton, Canada sub-contractor In-Line Contracting needed a durable, cold-resistant solution to effectively dig through the mudstones and clays common to the area, while maintaining the strict line and grade requirements needed for a gravity sewer.

The 690-foot-long crossing for a trunk sewer upgrade involved the launch of a self-propelled boring machine, known as a Robbins Rockhead, in December 2009. Crews covered the top of the pit each night and pumped heat into the 20-foot deep launch pit to prevent fluids from freezing and water from expanding.

Project background

The new pipeline, known as the Confederation Sanitary Sewer Trunk Upgrade, is set to increase capacity to overused lines in northwest areas of the city. “The city of Calgary has been growing quite rapidly over the last number of years, so all of the pipes need to be larger. This is all part of an ongoing project to upgrade the city’s infrastructure,” said Bill Stace, crew foreman for In-Line. The new line replaces an old trunk sewer installed in 1905 that is currently over capacity, particularly during heavy rain events.

The entire pipeline, being construction for the city of Calgary’s Water Resources Department, is approximately 1.25-miles long and ties in with existing trunk sewers in the region. From the crossing site, the trunk sewer will tie in with an existing 66-inch diameter sewer line, which will combine with other gravity sewers. All of the material is then conveyed to the city’s largest treatment plant at Bonnie Brook. By December 2009, nearly the entire pipeline had been completed, except for a 690-foot crossing below 32nd Avenue, a busy thoroughfare just outside the downtown area.

Core samples of the crossing area found clay with layers of fractured, hard mudstone 20-feet below the surface. The alignment of the tunnel was lowered in the hopes of excavation in comparatively stable siltstone bedrock. “We opted for a trenchless crossing because open cut of the main road, 32nd Avenue, was not an option. The ground was also shown to be somewhat unstable, and not conducive to open cut tunneling,” said Ali Rafih, superintendent for general contractor Volker Stevin.

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