A critical failure of the Bearspaw feeder main was identified as low probability in a City of Calgary assessment to prioritize asset inspections, prior to its rupture in June.
The revelation came as part of the full findings of a report into the Bearspaw feeder main break presented to the city’s infrastructure and planning committee on Wednesday.
According to the city’s asset quadrant, a process that identifies assets to be inspected or upgraded based on risk of failure, the Bearspaw feeder main fell under “high impact, low probability,” the third quadrant on the scale.
City administration noted there were no feeder mains identified as “high impact, high probability,” the first quadrant, however, some were classified in the second quadrant as “low impact, high probability.”
Ward 1 Coun. Sonya Sharp said she felt the assessment should’ve prioritized the Bearspaw line, as it carries 60 per cent of the city’s water supply.
“Now, of course it’s going to be top priority,” Sharp said.
“This is our drinking water, so I think it should’ve been high risk high probability no matter what.”
City officials noted there were preparations underway for an inspection of the Bearspaw feeder main this fall, after the assessment back in 2021, which included modelling of service disruption, and planning for emergency response and the shut down of the line.
“We can see that there was attention to this pipe, it’s not as though is was neglected over time,” Mayor Jyoti Gondek told reporters.
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“They had done some initial maintenance work and were looking to do more.”
According to a nearly 600-page report into the feeder main break, overseen by Associated Engineering, there were several factors which are believed to have led to the incident.
Those factors include microcracking, or previous damage to the outer layer of the pipe, which allowed soil contact with the pipe’s prestress wires, causing stress corrosion cracking and hydrogen embrittlement.
The report noted soil conditions near several sections of the feeder main contained high levels of chlorides, likely from salt used in de-icing materials on city streets.
“There is a likelihood that the road salt has increased chloride levels,” said Steve Wyton, the city’s head of asset management. “What we also observed though, is that there are hotspots where there’s higher levels of chloride in certain spots of the road but not in all parts of the road.”
Chloride levels near some of the concerning sections of the pipe registered higher than 700 mg/L, which is a significant increase when compared to soil testing conducted in the area back in 2014, according to Wyton.
City officials said there are plans to study the cause of increased chloride levels, while increasing soil sampling along critical feeder mains to look for chloride.
The report indicates the manufacturing and design standards applicable in when the pipe was installed in 1975 were followed, while live loading subsequent to its construction and operations of the pipe didn’t cause the rupture.
However, the report notes pressure variations were slightly outside of normal operation, but “would not be expected to trigger the rupture.”
Ward 5 Coun. Raj Dhaliwal, formerly an engineer, said he isn’t completely satisfied with the answers provided in the report and has further questions.
“We heard there was a mechanism through the pipe that failed,” Dhaliwal told reporters. “But we still haven’t heard the catalyst.”
The rupture of the Bearspaw feeder main on June 5 led to months of city-wide water restrictions, as close to 29 areas of concern were found along the pipe.
Costs for the repairs continue to be tallied, however current estimates for the original repair and maintenance on 21 hot spot are believed to cost between $35-45 million.
“We are accountable for delivering solid public service,” Gondek said. “When something fails, it’s on us.”
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