I-405 Brickyard to SR527 Improvement Project - Hillside Retaining Walls-PPT Presentation

PPT Presentation

Pdf Summary

The I-405 Brickyard to SR 527 project widens the highway and relocates an off-ramp to improve corridor capacity and safety. The work requires retaining walls on a steep, landslide-prone hillside while maintaining highway operations.<br /><br />The site has a slope of about 27 degrees and more than 50 feet of vertical relief. Excavation at its base posed significant risk because the slope was already marginally stable. The upper 50 feet consist of disturbed glaciolacustrine clays and silts, with slickensides and evidence of historic movement. Stronger, undisturbed soils below the disturbed zone provide the ground-anchor bond zone. Design also had to address plastic clay behavior, limited construction space, and WSDOT’s 1,000-year seismic requirements.<br /><br />The selected solution is a tiered system of anchored soldier pile and lagging walls, with the relocated ramp between them. The upper wall is approximately 1,726 feet long and up to 28 feet high; the lower wall is about 1,112 feet long and up to 25 feet high. Both use multiple levels of permanent ground anchors, bonded in undisturbed soils, and permanent cast-in-place fascia. The upper and lower soldier piles are staggered to let lower-wall anchors pass through the upper wall’s embedded zone without interference. Extensive creep testing verified anchor bond strengths in the plastic clays.<br /><br />Construction sequencing includes an access bench, soil-nail stabilization, coordinated installation of the staggered piles, and staged excavation with anchors installed level by level. The design-build team integrated construction sequencing into the permanent design, helping reduce redundant temporary works and costs. The project demonstrates the value of staged analysis, anchor testing, and close designer-contractor coordination for complex retaining systems in seismic, landslide-prone terrain.

Keywords

I-405 Brickyard

SR 527

highway widening

off-ramp relocation

retaining walls

landslide-prone hillside

soldier pile walls

ground anchors

seismic design

construction sequencing