THE NORTHRIDGE BLUFF LANDSLIDE: A MAJOR COASTAL BLUFF FAILURE PLACING COASTAL DEVELOPMENT AND ENGINEERED FACILITIES AT RISK, DALY CITY, CALIFORNIA

The Northridge Bluff Landslide, located on a 140m high coastal bluff in Daly City, California, failed catastrophically during December 2003 as a deep-seated rock landslide that disaggregated into a flow as it descended the steep bluff and subsequently advanced 100 m into the surf zone. The bedrock that forms the bluff is composed of steeply dipping, poorly indurated fine sandstone, siltstone, and claystone of the Pliocene Merced Formation. Following the initial failure, bedrock above the head scarp continued to creep down slope, locally progressing to rapid rockslide and rockfall failure. Failure on the upper portion of the slope is structurally controlled by a steeply dipping bed of sheared claystone that marks one margin of the landslide and two prominent joint sets that acted as “sliding” and “releasing “ surfaces along the other margins. We followed the sheared claystone eastward from the bluff, allowing us to better define the potential for regression of the landslide toward upslope properties. The risk posed by this bluff landsliding was not fully recognized or evaluated prior to development of the bluff top. As a result of the landslide, a critical City storm drain system, a new community church, and a portion of a public park were placed in jeopardy. Using both qualitative geologic judgment and quantitative engineering analyses, we developed a Landslide Hazard Map that defines a Zone of High Risk along the margins of the landslide area. We employed rigorous rock mechanic and slope stability analyses to define the likely limits of future upslope regression of the landslide boundaries. In the high risk areas, a series of alternative remediation measures to mitigate the landslide risk to both public and private properties were developed.