As requested by the City of Grand Forks, Golder Associates Ltd. from Kelowna, a consulting firm specializing in environmental-related areas, conducted a field reconnaissance of reported sinkholes that developed on the Granby Mine Slag Piles located at the north end of Grand Forks.
While the slag pile is mined by Pacific Abrasives & Supply Inc. under an exclusive contract, the City of Grand Forks is the owner of the slag pile properties.
City officials were called to the property on July 12 to investigate two large sink holes and other forming crevices, with employees of Pacific Abrasives.
Monitoring of these depressions continued over the weekend, with noticeable increases in size.
On July 16, city staff enlisted the services of a geotechnical engineer to determine the potential risk of the situation and to professionally determine the cause of the depressions. An overview of the engineer’s report was received by council at its regular meeting on July 23.
In his report, geotechnical engineer Andrew Van Dyk stated that, “The size of the depression at the ground surface due to the soil pipe collapses are expected to be proportional to the subsurface cavity that existed prior to the collapse.
“Based on the size of the collapse, it is expected that they developed over several years and brought to a critical state by the increased groundwater seepage resulting from the high precipitation received this year.”
He referenced the fact that part of the Pacific Abrasives operation includes a settling pond, which is contained by a 3.048-metre (10-foot) berm along the west, south and north sides.
He added, “Soil pipes develop over time and remain undetected for several years. At least four seepage areas into the pond were reportedly observed during the pond construction.
“It is reasonable to suspect that other cavities may exist in the area.”
Following the recommendations of the geotechnical report, council authorized city staff to proceed with a geophysical survey followed by a subsurface borehole investigation in order to determine if there are cavities beneath Granby Road.
The cost of the survey is estimated to be a maximum of $10,000, which will be funded from the Slag Sales Revenue Reserve Fund.
Concerned citizens wanting more information contact City of Grand Forks CAO Lynne Burch at 250-442-8266.
– Submitted by the City of Grand Forks