On January 29, 1982 a deep-seated landslide occurred on the coastal bluffs, south of the glider port, in Torrey Pines City Beach. The landslide resulted in the movement of 1.4 million cubic meters of landslide debris. The basal rupture was believed to have occurred at an elevation of approximately 140 feet (depth of 195') within the upper few meters of Ardath Shale. This basal rupture occurredapproximately 90 to 100 feet below the rupture surface of previously recorded landslides. A detailed study of the bluffs at and adjacent to Black's Beach was performed by the San Diego City Parks and Recreation Department as part of general planning for Torrey Pines City Park. The study included a field-mapping program and analysis of stereo-pair aerial photographs. The purpose of their investigation was to map general geologic conditions and prepare a map showing areas of potential danger to the public(Vanderhurst, McCarthy, Hannan 1982). The largest landslides that have occurred in the area of study (Figure 1) are coherent block-glide landslides (Figure 2). This type of landslide is made possible by the presence of a medium of low shear strength, within which slip (basal) surfaces can develop at the sole of slides. Block-glide landslides are characterized by a lack of internal fracturing within the translated blocks. The purpose of this investigation is to use analyses of Atterberg Limits to locate beds/planes of possible weakness, low shear strength, and/or high plasticity within the landslide-prone bluffs.
