Green River Bridge Pier 1 Landslide, King County, Washington

This paper will discuss how early November 2008 storms caused the retrogression of a small landslide located below the south abutment of a 75 year old, multi-pier highway bridge that spans the Green River gorge, located on State Route 169 about 30 miles southeast of Seattle, Washington. The landslide retrogression resulted in more than a half inch of movement of the abutment and a 220 day closure to construct remedial foundation support and drainage improvements. Interim stabilization efforts enacted before and after the movement of the abutment included soil nailing the undermined approach fill, installing horizontal drains, and, following the highway closure, unloading the upper slide mass by removing the 30-ft high approach fill. Permanent measures involved a secant wall of 4-ft diameter drilled shafts, tied back with 300 kip ground anchors, and the installation of a 300-ft long deep cut-off trench to intercept groundwater above the active slide limits.