East Lake Sammamish Parkway Stabilization Project

Summary

East Lake Sammamish Parkway (ELSP) is located along the eastern shore of Lake Sammamish and extends between SR-202 to the north and I-90 to the south. A segment of the roadway has experienced settlement in the last 7 years, and this project plans to stabilize the settling embankment. The ELSP Roadway Stabilization Project is located approximately 800 to 1,600 feet south of the intersection with NE Inglewood Hill Road. The segment is a Minor Arterial road comprised of a single lane in each direction. The roadway in this location consists of a 10- to 15-foot-tall embankment with a steep slope along the western side and a wetland along the east side. Underground utilities including a sewer main, water main, and groundwater culverts are present within the embankment.

Background

Significant and ongoing surface settlement and cracking has been observed at this segment, specifically in the last 7 years. During this time, the asphalt shoulder of the southbound lane has settled up to 6 inches and has pulled away from the southbound lane along the fog line. The roadway deterioration has required extensive repair efforts and costs for the City. Geotechnical investigations indicate the settlement is caused by the consolidation of loose fill and possible degradation of underground utility pipes within the embankment. In addition, the embankment was likely built on top of a wetland or area of seepage. As such, the settlement is expected to continue as the embankment fill continues to consolidate. The photos below show the existing conditions of the roadway.

Existing roadway conditions, looking north from south end of project limits.

Existing roadway conditions, looking north from middle of project limits.

What’s the Latest?

The City is currently working to evaluate two design options for repairing the embankment, determine the best value concept for future design and construction, and complete a 30% design of the preferred alternative. A summary of each design option is provided below.

Slope/Embankment Stabilization

This option includes stabilization of the embankment and western slope using retaining walls installed to the west of the shoulder and existing sewer. The walls would be constructed using soldier piles and lagging, tangent piles, or other similar techniques.

Embankment Reconstruction

This option includes excavating and rebuilding the entire embankment and rebuilding it with structural fill. This would require construction of retaining walls on one or both sides of the embankment to reduce the embankment footprint. In addition, the culverts and the sewer line may require reconstruction in the process. Existing water and gas utilities would need to be supported or replaced as part of the construction process.