Monday, July 22, 2019

Contractor suggests moisture miscalculation the cause of unstable Yukon school

Structural problems at the Ross River School are likely a result of a miscalculation of the soil’s moisture content when the building was constructed in 1999, according to the company that installed its underground cooling system at the time.

“We believe that the soil was much drier than was anticipated and so the degradation goes deeper … and that’s what causing the movement of the school,” said Bill Watt, vice-president of Arctic Foundations of Canada Inc.

The building was constructed on permafrost. If soil is moist, it takes longer to thaw out because it holds more latent heat — heat that allows water to turn to vapour. If it’s dry, though, there’s a lack of latent heat and the thaw extends deeper into the ground.

Temperature monitoring equipment has shown that despite a thick layer of frost under the school in the winter, in the summer, it thaws out, said Watt. “And it thaws out to a much greater depth than was ever anticipated. The only explanation for that is that the soils are much drier than what was anticipated.”

Two geotechnical reports by EBA Engineering Consultants Ltd. completed in 1998 show that moisture content readings were taken from 30 soil samples. To determine ground conditions, the company drilled four holes into the permafrost and excavated one test pit to determine the potential for frost heaves.

Both reports recommended a thermosyphon system be installed to freeze the ground in the wintertime, when heat from the building would otherwise cause it to thaw.

The Ross River School has been plagued with structural issues, including cracks in the walls and sloping floors, since shortly after it opened in the fall of 2001. In 2015, it closed for five months and the whole building was relevelled.

In May of this year, after a series of earthquakes, an engineering company assessed the school and recommended it be relevelled again this summer, at an estimated cost of about $1.2 million.

This month, two engineering firms stated in a memo to the Yukon government that the building is structurally safe to occupy, but “continued foundation movements (heaving and settlement) have affected the functionality of the school” and “further remediation is needed as part of a long-term strategy to ensure the continued structural safety of the school.”

At the time of construction, Watt’s company was hired to install the system underneath the school. He describes thermosyphons as a “passive refrigeration device,” because they don’t require power.

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