A slope failure compromised the integrity of a critical stormwater storage facility at a petroleum storage terminal within northern Alberta. Through periodic ground inspections and a review of LiDAR and aerial imagery, significant lateral deformation of a lined stormwater pond was identified. The potential for pond overflow and uncontrolled stormwater release into the natural environment due to compromised discharge outlets and liner integrity posed an operational and environmental risk. A fast-tracked review and assessment of both the historical and as-built construction details, a good understanding of the regional geology, and geotechnical investigation and instrumentation were required to identify remedial measures that could be implemented to restore the pond prior to the onset of major storm events.
Geotechnical investigation and assessment showed weak zones in the bedrock at shallower depths than were historically encountered at the site. Spatial constraints, uncontrolled fill placement in close proximity to the pond, expansion of the pond following original construction, and compromised subsurface drainage were identified as factors that may have contributed to the failure. Assessment of remedial options considered impacts relative to both local and global factors of safety. The selected remedial option was a combination of slope unloading, shear key construction, and improved drainage with the design and construction occurring within a timeframe of 3 months to minimize the time that the pond was out of service.
The paper highlights the challenges associated with the slope unloading and shear key construction that were required to control the ground movements. An approach involving sequential excavation and replacement was adopted to manage the risks associated with shear key construction. In addition, directional drill methods were incorporated to install some of the drainage measures due to the risk associated with an open cut approach. The project was fast-tracked to minimize exposure to high precipitation and limited stormwater storage capacity.