Abstract:
The primary goal of the I-405 Brickyard to SR527 Improvements Project was to alleviate congestion and enhance safety by expanding the express toll lane system and constructing new direct access interchanges. Brierley developed the design of the Hillside Retaining Walls, one of the project's most critical components. These walls were necessary to widen the highway and relocate an off-ramp, which required excavation at the base of a marginally stable slope. The two walls were constructed as a tiered system with the off-ramp positioned between them. The upper wall, approximately 1726 feet in length and up to 28 feet in height, retained a 27-degree slope with a vertical height of about 50 feet. The lower wall, approximately 1,112 feet long, reached a maximum height of 25 feet.
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The walls were designed as an anchored soldier pile and lagging wall with a permanent cast-in-place fascia. The upper 50 feet of soil consisted of disturbed low to high plasticity clays and silts, with evidence of disruption zones and slickensides throughout. The wall was designed to withstand a 1,000-year seismic event. To meet the seismic demands and ensure flexibility, four levels of permanent ground anchors were installed. These anchors were up to 150 feet long and spaced 6 feet apart horizontally through hole pockets in the permanent soldier piles. The bond lengths for the permanent ground anchors ranged between 15 and 40 feet and were anchored in the lower undisturbed low to medium plasticity clays and silts. Extensive creep testing was performed to verify the design bond strengths.
The project also required the installation of a temporary access bench to facilitate the installation of the soldier piles. An access road was also constructed using gravity and mechanically stabilized earth (MSE) walls where there was insufficient space for sloping.