During the months between December 1, 1977, and March 31, 1978, 51.7 (~131 cm) of rain fell in the area of Topanga Canyon, California. This winter rain appears to have been the underlying cause of many landslides, slump debris flows, and mudflows. One such failure occurred at 20510 Callon Drive, Topanga. This site consisted of a 3:1 northwest-facing slope covered by 4 to 20 ft (~1.2-60 m) of colluvium and soil over Topanga sandstone, shale, and basalt—apparently in place, as indicated by a local consulting firm. The area containing the site was mapped by the U.S. Geological Survey as a landslide. The landslide’s validity was discounted by the consultant geologist on the basis of his field investigation and five exploratory trenches. Lacking evidence of an existing landslide, the consultant recommended that slabs on grade be designed for expansive soil conditions, and footing be extended through the creep-prone soil to derive support in bedrock. The consultant observed that, prior to sliding, the site was affected by minor slumping in the road cut along Callon Drive.
By December 1977, the owner noticed the effects of instability. Movement worsened to the point of general failure at a rate of 2 to 4 in (~5-10 cm) per day. The site was essentially destroyed due to landslide movement, and by a mysterious fire, by March 27, 1978. Application for a building permit was made on January 16, 1976. Orders to demolish were given by June 2,1978; this constitutes a very short period of service for the structure.
Ground failure consisted of a block rotational slide, ~200 ft in length by 100 ft wide (~60 × 30 m). Its thickness is 10 to 20 ft (~3-6 m) with a 6-ft-high (~1.8 m) scarp inclined about 60°. The foot terminates on the south side of Callon Drive and above the road surface.
Two 25-ft-deep (~6 m) seepage pits were emplaced near the roadside, a location not recommended by the consultant. Instability resulted from saturation of creep-prone colluvium and soil mantle and underlying fragmental Conejo volcanic breccia, which is mixed locally with sandstone and siltstone blocks and debris from the Cold Creek Member of the Topanga Group Formation.
Three separate lawsuits ensued against the consultant, the builder, and the County of Los Angeles. Ultimate damage awarded to the owner amounted to $135,000.
