Landslide Mitigation at Eden Canyon Road in Alameda County, California

The Eden Canyon Road landslide is a deep-seated, 150-m (500-ft) long, 50-m (160-ft) wide landslide that affected approximately 50 m (160 ft) of roadway. The slide, activated by heavy rainfall during winter 2006, was consequently evaluated through field exploration, including drilling and sampling of test borings, downhole logging of 60-cm (24-in) diameter borings, and installing an inclinometer. The slide plane was estimated to be at a depth of 12 m (40 ft) below the roadway. Upslope, numerous areas of seepage were observed within the landslide area and slow seepage was observed from the sidewalls of the borings. Groundwater was observed in the exploratory borings at depths of about 6 m (20 ft). The active landslide was associated with the reactivation of a portion of an older, dormant landslide. Complete repair of the active landslide was ruled out for several reasons. Mitigation included horizontal drains up to 185 m (600 ft) long and a gravity buttress. The gravity buttress was an innovative system of counterfort subsurface walls of overlapping drilled shafts that were filled with cast-in-place concrete. The drilled shafts were approximately 1 m (3 ft) in diameter and extended from about 1.5 m (5 ft) below the roadway surface to a depth of at least 3 m (10 ft) below the base of the landslide.