THE OCEAN TRAILS LANDSLIDE: DEFINING SAFE ZONES ALONG HIGH COASTAL BLUFFS, RANCHO PALOS VERDES, CALIFORNIA

On June 2, 1999, a large ancient landslide reactivated on the newly developed Ocean Trails Golf Course property in Rancho Palos Verdes, California, resulting in the destruction of the 18th fairway and a call by City officials to re-evaluate the area's geologic conditions. The ensuing subsurface investigation and landslide repair, which included a 500 meter long by 30 meter deep buttress keyway, provided insights into the subsurface geology and cast doubt on the accuracy of previous investigation techniques. The Ocean Trails Property is situated on a prominent marine terrace at the southernmost side of the Palos Verdes peninsula, approximately 3 kilometers east of the famous Portuguese Bend landslide. This marine terrace stands approximately 30 meters above the modern wave-cut platform and has a very steep seacliff resulting from wave erosion at the shoreline angle.

The area is underlain by the Altamira Shale member of the Miocene Monterey Formation, which consists of thin-bedded marine siltstone, sandstone, and shale, with isolated bentonite interbeds. Bedrock materials are overlain by a thin marine terrace deposit, which is topped by 6 to 12 meters of fluvial deposits. Large areas of the coastal bluff are underlain by seaward dipping strata, resulting in ideal conditions for translational rock block landslides along the thin, low-strength bentonite beds that daylight in the seacliff. Defining the subsurface location and inclination of the weak bentonite horizons is critical to assessing the potential for landsliding and establishing prudent setbacks.

Geologic exploration began in the 1960s, including aerial photograph analysis, geologic mapping, trenching and drilling of widely spaced and relatively shallow large- and small-diameter boreholes. Three large landslides were identified along the seacliff and building setbacks were established early on. Geologic exploration in the late 1980s and 1990s included large- and small-diameter boreholes and geophysical surveys, which helped to establish a new geologic model featuring two well-defined bentonite beds extending through the project area. Exploration following the 1999 failure, including 36 large-diameter boreholes, revealed more bedrock complexity than originally interpreted and resulted in a dramatically refined geologic model.