US Hwy 95, Bonners Ferry Landslide
Bonners Ferry, Idaho
Improvements to a tortuous, three-mile stretch of US Highway 95 near Bonners Ferry, Idaho included crossing an ancient landslide with a keytrench embankment; however, a massive and rapid earthflow occurred during construction. The 500-foot-wide slide involved an estimated 400,000 cy of material, undermined the highway, and 1,200 feet down slope it buried and scoured a county road and Union Pacific Railroad tracks.
Landslide Technology was retained by Idaho Transportation Department for emergency response to evaluate slide causation and assist with recommendations to remediate the slide area. Subsurface conditions consist of layered clayey silt, silt and sand with natural water contents close to the Liquid Limit. The cause of the flow slide was determined to be a combination of factors including pre-existing marginal stability, high groundwater pressures with local artesian conditions, and liquefaction during keytrench excavation.
To proceed with highway construction, remedial measures included a rock shear key across the earthflow. Since the slide debris was sensitive to liquefaction, the remainder of the keytrench was not constructed and the embankment foundation was improved by capping the slide debris with a pervious sand layer and installing wick drains to dissipate excess water pressure. An added benefit of the wick drains included improvement of the liquefaction resistance of the slide debris in the event of an earthquake.