St. Johns Dike Erosion and Seepage Cut Off
St. Johns Landfill was the primary landfill for the Portland area for over 50 years. The site is adjacent to the Columbia Slough and the Smith and Bybee Lakes Wildlife Area, which are separated from the landfill by a protective levee. Over the years, tides and currents have eroded segments of the levee. Extensive loss of wildlife habitat and local slumps along the shoreline raised concern about a catastrophic slope failure.
Cornforth Consultants was retained by Metro Regional Services to investigate and repair the levee slopes. The complex nature of the project required evaluations of soil erosion, failure mechanisms of the levee slope, strength behavior of soil and rock, slough channel hydraulics, environments for native vegetation, multiple uses of geotextile products, and regulatory permitting. In addition to design, Cornforth Consultants provided the Engineer's construction cost/estimate, plans and specifications, and quality control during construction.
The design involved graded rockfill in the tidal zone at the toe of the levee, and compacted silt reinforced with a variety of geotextiles and native plants in the upper slope. This design differs from other projects in the Pacific Northwest in that the upper slope is capable of sustaining native vegetation without irrigation. The engineered design of the slope provides adequate stability to keep refuse safely within the landfill.
As the native plants grow, aesthetics and water quality improve along with wildlife habitat and scour protection. The growth of vegetation and the project have been very successful.
This levee restoration won an ACEC Grand Award for Engineering Excellence in the State of Oregon.