I-90 Snoqualmie Pass East Improvements

Interstate 90 is the primary east-west transportation corridor across Washington State, climbing from Seattle through the Cascade Range toward Spokane and beyond. Projections by the Washington State Department of Transportation (WSDOT) made it clear that sections of I-90 would need to be widened and redeveloped to keep up with the region’s growth. GeoEngineers assisted WSDOT with one of the most challenging phases of this effort—a 15-mile stretch of I-90 that climbs toward Snoqualmie pass west of the mountains between Hyak and Easton.

WSDOT’s goals for the project included six travel lanes, road protection to reduce avalanche risks and a wildlife overpass to allow animals to safely cross the highway. Atkinson Construction led the project team, with Jacobs Engineering providing designs and GeoEngineers serving as geotechnical subconsultant.

The original project design called for an extended snow shed to cover all six lanes through a high-risk avalanche corridor—an expensive and difficult-to-maintain solution. GeoEngineers worked with Atkinson and Jacobs to develop an alternative concept. Instead of protecting the highway with a snow shed, which would require significant annual upkeep and repair, engineers suggested building two elevated bridges that would allow avalanches and rockslides to pass harmlessly underneath the interstate. Although construction costs for both concepts were similar, the bridge’s much simpler maintenance requirements would save WSDOT an estimated $650,000 per year.

GeoEngineers’ biggest challenge was understanding the complex subsurface conditions along much of the proposed highway alignment. The route curved along a slope on the north shore of Keechelus Lake, and the geology included inconsistent bedrock depth and rock ridges. During the geotechnical investigation, the GeoEngineers team also discovered non-structural fill left over from blasting on the uphill side of I-90 during the highway’s original construction. These conditions made it challenging to determine geotechnical recommendations for the bridge’s drilled shafts. In some areas, drillers encountered up to 60 feet of sidecast fill material before finding competent rock.

Approach

• Because of subsurface variability and complexity, GeoEngineers recommended additional drilling exploration at the site of each bridge pier. Supplemental explorations following the design phase helped give the team confidence in the depth and strength of the rock.
• GeoEngineers delivered geotechnical recommendations for the two new I-90 bridges and approach embankments. This included calculating landslide loads for the bridge and a seismic analysis.
• The team analyzed the impact of blasting during construction on the existing bridge and soil nail wall and helped develop safe rock blasting plans.
• During construction, GeoEngineers staff observed the installation of soil nail walls, ground anchors and drilled shafts to verify that contractors followed project plans.
• GeoEngineers also assisted with designs and specifications for temporary shoring systems to support construction, including an MSE wall to support an existing WSDOT bridge during construction and a large (up to 40 feet tall) soil nail wall to support I-90 during construction and provide site access.

Results

GeoEngineers overcame complex and variable subsurface conditions to deliver geotechnical recommendations for this important regional transportation project. With the completion of the I-90 Snoqualmie Pass East Project, travelers can expect a safer, more efficient six-lane freeway and far fewer closures due to avalanches and rock slides.