The Bonnybrook bridge over the swollen Bow River gave way beneath a Canadian Pacific Railway train (TSX:CP) on June 27 as the city was trying to recover from high water that had washed over many neighbourhoods just days before.
Six tank cars carrying a petroleum dilutant teetered on the failing bridge. They were unloaded and removed over two days and never went into the river.
"Unprecedented flooding of the Bow River was a major factor in this bridge failure. The bridge handled several major floods for over a century, but the river was not to be denied last June," said George Fowler, a civil engineer who conducted the investigation.
"Intense, unprecedented floodwater flow had attacked the shale, sandstone, bedrock and clay pier foundation — eroding and undermining it."
Fowler said the bridge, which was built in 1897 and expanded in 1912, had been properly inspected by Canadian Pacific Railway.
"Inspections conducted on the Bonnybrook bridge exceeded regulatory requirements during the flood. Visual observations of the rail and track alignment, and service, would normally detect deviations. However, in this case, such inspections did not provide warnings of the sudden bridge failure," said Fowler.
"This bridge has been around since 1897 and in that time it survived several significant flooding events," he added. "There was no reason to believe this event would be any different."
Fowler said the true extent of the damage to the foundation wasn't known until repairs were underway two months later.
Railways fall within federal jurisdiction and are responsible for their own inspections. Fowler said CP has revised its bridge inspection practice and its inspector training program and is investing in research into early detection of erosion at railway bridges.
CP dismantled the Bonnybrook Bridge in the autumn of 2013 and work on a new span was completed in April of this year. The damaged bridge pier has been replaced and reinforced with a new foundation.
An official with CP Rail welcomed the final report. President and chief operating officer Keith Creel said he agrees with the safety board that the City of Calgary's command structure worked well to get the site secure and to help remove the derailed cars.
"Our relationship with the first responders of the City of Calgary allowed us to co-ordinate efforts to work quickly and to safely remove the cars from the bridge," said Creel.
"CP is grateful to the Calgary Fire Department and other first responders and thanks them for their skill, effort and commitment to public safety."
Follow @BillGraveland on Twitter