I-405 Brickyard to SR527 Improvement Project: Hillside Retaining Walls

As part of a broader effort to reduce congestion and improve safety along a six-mile stretch of I-405, the Washington State Department of Transportation (WSDOT) undertook a major highway widening that required relocating an off-ramp and excavating at the base of a marginally stable slope. Brierley Associates served as geotechnical design consultant to design-build contractor Skanska USA and designer AECOM, taking responsibility for one of the project’s most critical components: a tiered system of two permanent anchored soldier pile and lagging walls with cast-in-place fascia.

The upper 50 feet of soil consisted of disturbed low to high plasticity clays and silts with disruption zones and slickensides throughout, presenting significant stability and constructability challenges. The lower, undisturbed soil units provided the only reliable anchor zone. Spatial constraints along the access road precluded conventional sloping, and the design had to satisfy WSDOT’s 1,000-year seismic event standard.

Brierley’s solution centered on a two-wall tiered system: an upper wall approximately 1,726 feet long and up to 28 feet high, and a lower wall approximately 1,112 feet long with a maximum height of 25 feet, with the relocated off-ramp positioned between them. Four levels of permanent ground anchors, up to 150 feet long, were installed to meet seismic demands. Upper and lower wall soldier piles were staggered to allow lower wall anchors to pass through the embedded upper wall section. In areas of disturbed glaciolacustrine deposits, a temporary soil nail wall provided stability during construction. Gravity and mechanically stabilized earth (MSE) walls using the Ultrablock system addressed access road constraints where space was limited.