Landslide mitigation using horizontal drains, Pacific Palisades area, Los Angeles, California

Abstract

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.