Abstract The storm of January 18,1993, triggered or reactivated an older landslide that destroyed nearly a dozen homes in the Castellammare mesa area of Los Angeles, Cali-fornia. The catastrophic slope failure was the end result of the 1992-1993 rains and more than 100 years of local development that included construction and maintenance of coastal and local roads, construction of hillside homes, and maintenance of the mesa. Residential development started in the mid-1920s. Unimproved lots were sold and res-idences constructed, starting in 1926, without consideration of geologic hazards. The presence of large ancient landslides was determined from mapping started in the 1950s. However, lot by lot development continued without proper consideration of regional hazards. Remedial repairs were conducted by the city of Los Angeles, Caltrans, and private home owners in a non-integrated fashion, generally in response to catastrophic failures. Significant slope failures were observed during the rains of 1938,1941,1952, 1969,1978, and 1983. By late 1992, the region had deteriorated significantly because of slope creep and poor maintenance. Site reviews during and following the rains of Jan-uary 18,1993, indicated the need for (1) an understanding of regional geologic hazards and geotechnical problems associated with the development on and maintenance of a relatively large, paleolandshde; (2) an integrated, geotechnical approach to the devel-opment of an area of small, individually owned lots; and (3) a single government agency working with the home owners’ association to coordinate geotechnical studies, local government improvements, development and maintenance of private lots, and the maintenance of streets, utilities, dewatering wells, and other infrastructure.

Abstract Horizontal drains have been successfully used for a number of years in many areas for improving slope stability within landslide and/or landslide-prone areas. One such application of their use is discussed relative to a more than 66-m-high (220+ ft) 2:1 (26°) southwest-facing cut slope located in the Pacific Palisades area of the city of Los Angeles, California. The slope is somewhat unusual in that the geologic structure of the bedrock (i.e., siltstone) appears to be neutral and/or favorable relative to the orientation of the slope surface. However, the slope underwent minor downslope movement (i.e., sagging, creep) following the heavy rains of 1978. Manifested mostly by cracks within paved terrace drains and/or downdrains, the movement appears to be related to local perched ground water and a northwest-southeast-trending fault zone that cuts across the southern part of the slope. Horizontal drains are currently being used to improve slope stability by reducing the amount of ground water, especially within the vicinity of the fault zone. Ground water was encountered in 11 of the 16 horizontal drains; initial flow rates were up to 5.71 (1.5 gal) per minute. Calculations suggest that the dewatering of the slope could increase slope stability to a factor of safety of more than 1.5.