Calgary investigation into feeder main break reveals factors that degraded structural integrity

An investigation overseen by Associated Engineering has revealed some of the critical factors, but no definitive single cause, that led to the catastrophic break of Calgary’s Bearspaw South Feeder Main last June.   

The nearly 600-page report, released to city council in late 2024, points to several factors that degraded the structural integrity of the pipe and led to several months of water restrictions for Calgarians as the city conducted emergency repairs. Preliminary findings from the new report suggest that microcracking along the protective mortar outer layer of the pipe, high chloride levels in the soil at various locations, as well as severe damage and corrosion impacting the pipe’s prestress wires, all played roles in the pipe’s eventual failure. 

“The findings from this investigation have provided valuable insights into the causes of the failure and the steps needed to prevent similar events in the future,” said Steve Wyton, Calgary’s manager of asset management planning, in a statement. “Our priority now is planning for the long-term rehabilitation of the feeder main and implementing proactive measures to ensure a reliable and resilient water distribution system for Calgarians. By combining advanced monitoring technologies, contingency planning, and strategic investments, we’re taking meaningful steps to safeguard this critical infrastructure and support our city’s growing water needs.”  

While a small number of wire breaks contributed to the initial failure, the report estimates that some 200 wires broke after the rupture. The wires are protected with a 20-mm thick layer of cement mortar, and were believed to be damaged from corrosive soil interaction. The report states that stress corrosion cracking (or hydrogen embrittlement) of the prestress wires resulted in brittle wire failures. 

The report, which also utilized data from Calgary-based Pure Technologies (owned by Xylem), notes a “dramatic shift” in soil chemistry between 2014 and 2024 in the same sampling area. Whereas chloride levels were negligible a decade ago, recent samples show “significant chloride levels in the thousands” of milligrams per litre. 

“High concentrations of chlorides in the soil are not always required to cause pipe distress if there is a fluctuating water table, as fluctuating water tables can deposit chlorides on to the mortar leading to higher concentrations on the mortar itself,” the report states.  

The report notes that some elevated soil chloride concentrations may be related to road de-icing activities using sodium chloride.  

Further soil sampling is recommended in the Associated Engineering report. 

Calgary officials expect to deliver an implementation plan for the feeder main by mid-2025 to strengthen system redundancy. 

Overall findings at the site of the original break and the five “hot spots” include:  

• Microcracking: Some of the pipes had visible cracks and peeling in their mortar (outer layer of the pipe) when they were dug up, while others had mortar that was still in good condition.  

• Chloride levels in the mortar: Lab tests revealed that some damaged pipe sections had chlorides penetrate their mortar, but other pipes did not show any signs of this.  

• Pitting and corrosion: The wires that help keep the pipes strong showed severe damage, including deep pits, corrosion, and many brittle wire breaks.   

• Wire damage: There was evidence of two types of wire damage: hydrogen embrittlement and stress corrosion cracking. Both types of damage appear to be caused by chloride penetrating the mortar or small cracks in it.  

• Soil conditions: Soil testing around the feeder main showed some areas with high levels of chlorides. The cause of increased chloride levels requires further study.