Abstract Large host landslides commonly encompass smaller, secondary landslides; hence the term “compound landslide.” Secondary landslides differ significantly from their host landslide in that secondary slides (1) have smaller volume; (2) often have greater surface exposure; (3) are more readily saturated by water infiltration; (4) require a smaller driving force to initiate movement; (5) have greater frequency of movement; and (6) their capacity for movement can be either independent of the host and other adjacent secondary landslides, or induced by adjacent landslides. Multilevel flow systems (perched water tables) commonly form within compound landslides due to the relatively low permeability of slip-surface gouge, which may slow recharge from the overlying secondary landslides to the host landslide. Movement-inducing pore-fluid pressures can often be reached more rapidly in secondary landslides. Controlled sources of water, for example, from irrigation and septic waste-water injection systems, can supplement vastly natural recharge from rainfall, artificially maintaining saturation in near-surface secondary landslides. The progressive movement of a host landslide can generate a family of precursive failure surfaces in the landslide’s interior that can coalesce to form secondary landslides. Evolution of local topography contributes to the morphology and slip direction of secondary landslides to the extent that stress trajectories within a host landslide are perturbed by changes in the shape of the ground surface; for example, increased slope gradient along a landslide’s toe is a common cause of secondary landsliding. The non-plane strain nature of compound landslides, exemplified by the complex interactions between host and secondary landslides, often precludes the meaningful application of general, plane strain, mechanical models in assessing the hazard potential of a specific compound landslide. Rote stability analyses of compound landslides based upon assumed homogeneous, isotropic, linear-elastic behavior throughout the slide prism fail to consider the intrinsically more unstable (and thus more hazardous) secondary landslides. Geologists involved in landslide exploration must be aware of the characteristics of compound landsliding; those involved in mitigation must consider the hydrologic and dynamic implications of secondary landsliding in order to conduct the requisite field investigations and make appropriate design recommendations, which must be individu-ally suited to each compound landslide.

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.