Date of incident: September 2015
Notice of incident number: 2015165420049
Employers: Excavation company; municipal government

Incident summary
As part of an upgrade to a sewer treatment plant, workers were constructing a lift station (a large underground cylindrical structure to move wastewater from lower to higher elevations). The excavator contractor was using a dewatering and shoring method that involved an open steel caisson driven into the soil as a barrier to prevent groundwater and soil from entering into the sides of the excavation. Crushed stone was placed around the outside perimeter of the caisson. A worker standing on the crushed stone was pulled down and trapped in the ground when a quicksand condition developed under his feet. The worker was rescued by his supervisor and co-workers. He sustained injuries.

Investigation conclusions

Cause

• Work process created quicksand condition. When the hydrostatic pressure differential between the outside and inside of the caisson became too great, a quicksand condition was created. The worker was pulled down with the slurry of gravels and wet soils in the subsidence and became trapped.

Contributing factors

• Pre-work for site failed to identify key facts. The city and the contractor relied on data in two geotechnical reports. These reports did not identify the presence of a confined aquifer under artesian pressure directly under the proposed lift station. Instead, the information that was provided created an impression in the minds of the project planners that beyond the coarse gravel layer was a continuous silt layer suitable for the construction. A lack of understanding about the true nature of the groundwater systems was a factor in this incident.
• Owner failed to provide all information in its possession. The information contained in a third report would have been useful for a more complete understanding of the subsurface conditions in the local area where the lift station was being built. This information was not made available to the contractor for this construction project.
• Dewatering did not control hazard created by hydrostatic pressure. There were several forms of dewatering under way at the site. However, the quicksand condition would not have occurred if the ground around the site had been effectively dewatered and the hazard presented by a high hydrostatic pressure differential had been eliminated.

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Additional resource:

Audio Slide Show: Worker Pulled into Ground by Quicksand Condition