Abstract Slope movements, including types of landslides and extremely slow soil creep ( Varnes, 1978 ), occur throughout the United States and within many national parks. The collection of vital signs of regional landslide information, referred to as monitoring, is not only scientifically useful, but is beneficial for assessment of landslide hazards and risk, which is in turn important for regional operations and planning. Different types of slope movement, such as fall, topple, slide, spread, and flow, can occur in a variety of materials and degrees of slopes. Specific types of landslides ( Fig. 1 ), such as rockfall, earth slump, and debris flow, can occur depending upon the types of geologic materials and movement ( Cruden and Varnes, 1996 ). A landslide can be caused by one or more of several factors, of which geological, morphological, physical, and human factors are the most common. The term landslide trigger refers specifically to an external stimulus, such as intense rainfall, rapid snowmelt, earthquake, volcanic eruption, or stream or coastal erosion. These stimuli initiate an immediate or near-immediate landslide movement by rapidly increasing shear stresses or porewater pressures, by ground acceleration due to seismic activity, by removing lateral support, by reducing the strength of slope materials, or by initiating debris-flow activity. Most landslides with recognized triggers are caused by precipitation: rainfall, snow meltwater, or combinations of both. In rock masses, rain and meltwater penetrate joints and produce hydrostatic pressures. In soils, the increase of pore-water pressures reduces shear resistance ( Schuster and Wieczorek, 2002 ).

Abstract Over the past four decades, ongoing deformation of an 18-m-thick peat deposit within the flat-lying Mercer Slough has resulted in damaging deflections, and near-collapse in three cases, of pile-supported Interstate 90 bridges and a major water line on the east side of the slough. The peat is partially underlain by a dense sand unit, which includes a highly pressurized aquifer that produces artesian flow 1–2.5 m above the ground surface. Inclinometers on the east side of the slough show the peat flowing toward the structures and then apparently directed west along the interstate centerline. Large displacements recorded in several inclinometers near the center of the slough suggest a length of deforming peat that approaches 600 m, which is likely initiating retrogressively. Potential causal mechanisms include poor engineering characteristics of the peat, presence of high hydrostatic pressure transmitted within and beneath the peat, seasonal water-level variations of Lake Washington and induced hydraulic gradients within the peat, dredging of the Mercer Slough channel, puncturing of the underlying aquifer by numerous pile foundations, and fill placement along the eastern margin of the slough. The peat is flowing around the pile/shaft foundations; however, excessive lateral loads are still being applied to the foundations in a poorly understood and unpredictable manner. The most severe deflections have occurred in the outermost structures where the peat is primarily flowing transverse to the structures.

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.