Granite Falls Alternate Route

(Snohomish County, WA)  Landau Associates provided geotechnical engineering services for Snohomish County’s 2.1-mile bypass (alternate) route for State Route 92 (SR 92) that curves north and east of downtown Granite Falls, connecting to the Mountain Loop Highway north of Gun Club Road. The alternate route features two 12-ft wide travel lanes with wide paved-to-gravel shoulders and guardrails. The alternate route also includes turning lanes at key intersections. The original route for SR 92 passed through downtown Granite Falls. Its two narrow, substandard lanes (with failing pavement sections in some areas) were unable to safely handle existing and projected traffic volumes and were impacted by numerous access points and intersections with inadequate turning lanes.

Landau Associates investigated subsurface soil and groundwater conditions along the alignment and identified any likely causes and lateral extent of local areas of slope instability, conducted slope stability analyses, developed alternative slope stabilization methods, provided design information for the identified slope stabilization methods, and provided preliminary cost estimates for selected slope stabilization methods. Landau Associates also developed preliminary foundation recommendations for a possible bridge to span an unstable ravine. In addition, design-level recommendations were provided for the project, including developing geotechnical recommendations to support design of several tied-back soldier pile walls to support the proposed roadway near the top of an unstable slope.

The Granite Falls Alternate Route project received an Engineering Excellence Award from the American Council of Engineering Companies (ACEC) – Washington.

Photo 1: City Of Granite Falls