75 Avenue Storm Outfall and Slope Remediation
Client: The City of Grande Prairie |
Location: Grand Prairie, Alberta |
Type: Slope Stability and Remediation |
Project Budget $2.3 million |
ParklandGEO undertook a detailed slope stability evaluation after a visual inspection found evidence of a large landslide on the east bank of the Bear Creek was causing a large storm sewer drop manhole to tilt precariously to the point of toppling, and other distress to a gabion outfall structure. The investigation included borehole drilling, installation of instrumentation including slope inclinometers and piezometers, detailed modeling and geotechnical design. Remediation was recommended as abandonment of the storm line was not practicable.
The slope exhibited multiple slope failure mechanisms, with a large rotational slide acting as the main mode of failure triggered by toe erosion by the creek. Complicating the remediation plan was that the native clay soils exhibited a low residual strength that could not be relied up once the material was re-used as engineered fill. To account for this loss of soil strength, the remedial plan included installation of a 5 m wide gravel shear key, a geo-grid reinforced clay engineered fill, a drainage blanket to minimize piezometric pressures, and comprehensive erosion control protection measures utilizing A-Jacks concrete armouring and concrete revetment mats, along with re-vegetation.
ParklandGEO prepared design drawings and a tender package, which included over 15,000 m3 of common excavation and engineered fill, 1,900 m3 of gravel fill, about 1,000 m2 of A-Jacks and geosynthetic erosion control matting, and 40,000 m2 of biaxial geogrids. ParklandGEO also obtained all necessary permits for work in and adjacent to the creek from the various regulatory authorities.
A detailed construction plan was needed to minimize the risk of working beside a partially frozen creek, and to ensure that silt and other deleterious materials did not enter the creek, during or after the remedial program.
