Repair of Storm-Damaged Slopes, Lower Mount Wilson Road, Los Angeles County, California

Abstract

The lower 4.2 km (2.6 mi) of Mount Wilson Road provides exclusive vehicle access to Los Angeles County Fire Department facilities at Henninger Flats in the San Gabriel Mountains north of Pasadena, California. Severe storms in January 2005 rendered the road impassable by debris sloughing from above and soil sliding below the roadway; a fiber-optical communication line buried in the road was not broken. Slope damage ranged in scale from localized shallow sloughing to major slope movements up to about 90 m (300 ft) wide and 120 m (400 ft) in vertical height. Repair measures considered to restore vehicle access were very expensive and likely to be damaged by future slope movements. Henninger Flats is the head of a megaslide in Precambrian metamorphic rocks that encompasses more than 2.8 km2 (1 mi2). Typical safety factors required for Los Angeles County projects could not be attained with reasonable cost; therefore, repairable deformation that is passable by field vehicles was selected as the repair criterion. The two worst locations adjacent to Pasadena were repaired in 2009 with gabion baskets and welded-wire reinforced soil systems. Preliminary designs were developed for material quantities; field engineering was used to manage specific problems encountered during construction. Once the largest slides near the bottom of the road were repaired, fire department maintenance crews used the county's fire-fighting equipment to clear the road, providing vehicle access to Henninger Flats. Slide debris stability improved during the 4-year period between storm damage and road repair, contributing to the success of the project.