|Client:||Sunshine Coast Regional District|
|Location:||Pender Harbour, British Columbia|
Based on an order from the British Columbia Ministry of Environment, the Sunshine Coast Regional District (SCRD) was required to close the Pender Harbour Landfill by the end of 2015. XCG was retained by the SCRD to assist with the final closure and the transition of the site to a transfer station facility.
- The landfill began operation as a natural attenuation landfill in the 1960s, with no formal design and operation criteria or performance goals. Thus, the landfill has performance limitations when compared to an engineered landfill.
- A number of retrofitted environmental controls were added over the years, including stormwater diversion ditching and piping, perforated leachate collection pipes, and a leachate treatment wetland. The condition/status of this infrastructure was unknown by the SCRD.
- The landfill is characterized by a steep (approximately 1.5 vertical to 1 horizontal) eastern slope, heavily vegetated with bushes, shrubs, and trees.
- Low permeability soil (as required for the ministry prescribed final cover) is unavailable in the area.
Prior to undertaking the detailed design of the landfill closure, XCG completed a number of studies and tasks. Existing infrastructure, including leachate and stormwater pipes, were flushed and video inspected to ensure their proper functionality.
A landfill performance assessment was undertaken to evaluate the performance of the attenuation zone of the landfill, and to identify gaps in the hydrogeological database for the site. It was found that the Site was performing well, despite design limitations.
A geotechnical assessment was undertaken on the eastern slope of the landfill, with a particular focus on closure of the landfill. The results of the landfill slope stability analysis indicated that the highest risk of instability to the landfill would result from the removal of existing vegetation which would reduce the veneer stability and likely result in short-term erosion of the cover. As such it was recommended that measures to control surface water be implemented to improve long-term stability, including ditches and pipes to divert water away from the eastern slope of the landfill. The vegetation and existing cover on the eastern slope of the landfill would remain in place for the final closure.
XCG prepared a closure plan to facilitate final closure of the landfill. The plan included landfill final contours, final cover design, soil balance, erosion and sediment control, surface water management, leachate management, landfill gas management, proposed end use, and a post-closure environmental monitoring program.
The regulatory prescribed final cover system includes 1 metre of low permeability 10-5cm/s soil. As this type of material is not native to the sunshine coast, XCG proposed an alternative cover design. Using a hydrological evaluation of landfill performance model, XCG demonstrated that the proposed cover system exceeded the performance of the ministry prescribed cover, offering a cost effective design and reducing long-term environmental risk.
As recommended by the geotechnical assessment, the final closure design incorporated stormwater diversion ditching along the top of the steep eastern slope of the landfill to improve long-term stability. Stormwater will be diverted by way of ditching and pipes to the existing leachate treatment wetland.
Based on the closure plan, XCG undertook detailed design of the final closure of the landfill, including preparing detailed design drawings, specifications, form of tender, agreement and general conditions. Construction was undertaken in the fall of 2015, with XCG providing construction oversight services.
Post-construction services included as-recorded drawings, and a post-closure care plan for the landfill. XCG also prepared a transfer station operations manual for the new transfer station facility.