Prevedal Road Repair

During the winter of 2017 near the town of Lyman, Washington, heavy rains, steep slopes, and poor subgrade conditions caused a landslide along Prevedal Road in Skagit County. The landslide caused a portion of the existing narrow one-lane road to slough off with the downhill embankment. LDC partnered with Robinson Noble, Inc., a Geotechnical firm, to coordinate engineering efforts and establish several design alternatives to address slope stability at the slide location and minimize risk of future erosion with a new storm drainage system.

The final design included a reinforced slope wall, which used geosynthetic layers to rebuild the failed embankment and support the roadway. Due to the steep angle of the storm drainage system penetrating the wall, a sleeve was installed within the wall layers to aid with both wall and storm drainage constructability. Stormwater conveyance was designed to replace the previous culvert in this location that had discharged directly onto the downhill slope instead of discharging to the already scoured slope. The newly installed drainage system collects roadway and ditch runoff from the uphill portion of the road and pipes it to the bottom of the downhill slope through welded HDPE pipe, which discharges to a diffuser tee and gabion basket system.