Why did Calgary's feeder main break? A new report shares the answer

The final report on the Bearspaw feeder main break was presented to a city committee on Wednesday, along with a price tag for the repairs, which is in the neighbourhood of $35 million to $45 million.

The City of Calgary's infrastructure and planning committee received the nearly 600-page report during its regular meeting.

An independent engineering firm's report for the city indicates microcracking to the outer layer of the pipe put soil in contact with reinforcement wires underneath, which caused the wires to corrode.

This weakened the pipe's ability to withstand pressure, causing it to burst.

The report, produced by Associated Engineering, said design and operations of the feeder main did not contribute to its failure, but "several failure mechanisms" and "soil conditions" did.

"The cause of the June 5 failure was the breakage of a sufficient number of pre-stress wires in a pipe segment causing a loss of pressure-resisting capability in the pipe," the report said.

"The pre-stress wires are wrapped under significant tension around the pipe's outer concrete core. The wires are protected with cement mortar.

"Upon their exposure after the event, approximately 200 wires were observed to have failed."

The city now uses new monitoring technologies.

Another idea is to reduce the amount of chlorides in the soil.

When the pipe was created and installed, there was no code accounting for salt, which is now used on icy roads.

"We need to do a further study but in the meantime, if we know that the materials we're using for de-icing are causing issues, we shouldn't be using them. So, there are many, many more things to unpack from the information we've just gotten," Calgary Mayor Jyoti Gondek said.

"There was a change in the amount of chlorides in the soil from 10 years ago to now, so possibly what they can do is review how they are salting the roads. I don't know if that's changed over the past 10 years," said Ron Hugo, mechanical engineer with the Schulich School of Engineering.

Hugo says getting a better understanding of the water table--and strategically opening and closing pump stations to create a gentler flow of water--are some ways the city might be able to mitigate risks.

The city aims to release its plan to strengthen the system by the middle of 2025.

The break flooded a section of northwest Calgary, leading to a local state of emergency.

Water restrictions were put into effect for all Calgarians, including residents in surrounding communities that relied on water produced at the Bearspaw treatment plant.

The restrictions were lifted in the fall and all repairs to the feeder main were wrapped up last month.

The final cost for the repairs is still being finalized, but the report estimates the cost of the original repair plus five "hot spots" is $20 million to $25 million.

The cost of 21 subsequent repairs is between $15 million and $20 million.

The cost to repair the 1,500-millimetre segment of the feeder main is also being finalized.

"Administration will return to the infrastructure and planning committee in 2025 with a plan, and related costs, related to the implementation of redundancy and rehabilitation investments as related to this feeder main," the report said.

The report also said that after 29 separate repairs to the feeder main, it is now "operating within acceptable structural integrity limits."