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US-20 Highway Realignment and Landslide Stabilization, Eddyville, Oregon
The Oregon Department of Transportation (ODOT) launched a design-build project to realign six miles of US Highway 20 in the Oregon Coast Range. The alignment required soil and rock cuts up to 200 feet high, embankment fills up to 150 feet high and multiple bridges, all through ancient landslide terrain and faulted bedrock. During construction, several of the ancient landslides were reactivated by increased loading from fill placement and removal of lateral support during cut excavation. ODOT retained CCI-LT to assist with a redesign of the alignment to mitigate the reactivated landslides. Our team’s work tasks included landslide hazard and geologic reconnaissance, landslide interpretation and modeling, static and seismic stability analyses, designing landslide mitigations, developing PS&E documents, and engineering support during construction. CCI-LT assembled a multidisciplinary design team to complete the design of the new alignment. We prepared multiple design packages including Design Acceptance Phase (DAP) level and final PS&E. Work tasks included risk-based and constructability analyses for selection of appropriate landslide mitigations, and seismic analyses to quantify resiliency of mitigation systems. Our designers utilized performance-based design criteria, with concurrence from ODOT and the Federal Highway Administration (FHWA) and developed a project-wide geotechnical risk assessment and risk register. Project stakeholders selected a solution utilizing earthworks and large culverts to stabilize landslides as opposed to a bridge-centered solution to span the unstable ground. |
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Designers utilized multiple tools to stabilize reactivated landslides. Key design objectives included dispersing collected groundwater, movement-tolerant landslide mitigations, and creating wildlife corridors.
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