Date of incident: November 2023
Notice of incident number: 2023166920006
Employer: Construction company

Incident summary
During a seismic and structural upgrade of a church, workers were filling two brick chimneys with concrete. As concrete was being poured into one of the chimneys, the chimney collapsed and struck the scaffolding that was installed around it. The scaffolding collapsed, and three workers were seriously injured.

Investigation conclusions

Cause

• Concrete pressure exceeded what chimney could withstand. The pressure of the liquid concrete that was poured into the existing chimney structure was higher than what the structure could withstand. The pressure inside the chimney was calculated to have been as high as 124.1 kPa (18 psi) after 3 m to 4 m (10 ft. to 13 ft.) of concrete was poured into it. The pressure resulted in the progressive collapse of the chimney, which caused the scaffolding to collapse.

Contributing factors

• Chimney was not adequately supported. Sprayed concrete (shotcrete) had been applied to parts of the existing chimney structures and essentially acted as formwork. However, the section of the chimney that was being filled with concrete at the time of the incident did not have shotcrete or another type of support applied to it. Furthermore, the chimney was divided into two vertical chambers, and the brick wall that divided the chimney was not given additional support.
• Lack of testing and engineering instructions. The employer who was responsible for the concrete work did not perform any calculations, modelling, analysis, or testing to determine if the existing chimney structures could resist the pressure of the concrete during the infill. The employer had been provided with structural drawings that instructed that the chimneys be filled with no more than 1.2 m (4 ft.) of concrete without approval and inspection by a specialty professional engineer. The employer was responsible for retaining an engineer for instruction related to temporary works, including formwork, to ensure the chimneys were stable and strong enough to withstand the loads being imposed on them. A letter of assurance from a specialty professional engineer was required but was not obtained.
• Inadequate instruction and supervision. The employer did not provide its workers with adequate instruction related to the work that they were assigned. Additionally, the employer did not provide an experienced supervisor to oversee the work being done on the day of the incident. Instead, the employer assigned a worker to a supervisory role without providing the worker with adequate information, instruction, or training on the responsibilities of a supervisor.

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