LANDSLIDE DAMAGE WITHIN BIG SANTA ANITA CANYON, LOS ANGELES COUNTY, CALIFORNIA

The record rainfall of 2004-2005 caused significant damage to Santa Anita Canyon Road, located north of the City of Arcadia. Forty slope failures occured along the canyon and Santa Anita Dam access roads. Two of the largest failures caused closure of the canyon road for 11 months. These failures affected the residents of the canyon and all who use it. Studies by this student author, who resides in the canyon, were undertaken to determine the cause of the failures and to determine possible mitigation measures. The three largest failures (known as the Arcadia, Dam Access and Pine Slides) occurred within the City of Arcadia, Los Angeles County, and United States Forest Service jurisdictions. Adverse geologic conditions and excessive rainfall appear to be the main cause of the failures. The failures involved either bedrock or colluvial sediments and occured along joints, intrusive dikes or along clay seams. Recent fires, fractured bedrock, steep topography and blocked drainage also contributed to the failures. Proposed mitigation for all of the failures involves the combined use of simple removal, retaining structures, Geogrid, gabions, soldier piles, and rock netting. The Arcadia failure occurred along clay seams in colluvium. This failure was repaired first as it undermined the road, thereby limiting access to the upper canyon. A combination of a retaining wall, Geogrid, and gabion structures were utilized to rebuild the collapsed road bed. Both the Dam Access and Pine Slides failed along bedrock joints. The dam access slide was removed using manual labor undertaken by the residents and completed using a front end loader. Removal of this slide was critical in order to gain vehicular access in between the Arcadia failure, Santa Anita Dam and Pine failures. Removal of the Pine slide was not completed until December 2005. Repairs of the Dam Access Road and Pine Slide are still in the design phase. Proposed repair of the dam access road involves the use of retaining structures, soldier piles and rock netting. The retaining structure adjacent to the Pine slide may need to be extended northward. The Big Santa Anita Canyon slope failures had both significant economic and personal impacts on the residents and jurisdictions involved.