Geotechnical Analysis Of A Submarine Slump, Eureka, California

Abstract A large submarine slump covers more than 150 km2 of a gentle (1 W) slope offshore of Eureka, Calif. We have conducted a detailed geologic and geotechnical investigation of the slump and the surrounding area to delineate the geometry of the slump, determine its cause, and quantitatively evaluate the sediment properties that led to the failure. Our study included an effort to test procedures for using short-core samples in such an evaluation. Cyclic and static triaxial and one-dimensional consolidationtests were performed on gravity-core samples from both within and outside the slump; all samples showed some degree of overconsolidation. We used a normalizedstrength- parameter approach to estimate the strength at the failure surface below the level of sampling. A stability analysis based on a pseudo static infinite-slope model shows the slump probably to be earthquake induced. This analysis also gave values of the sedimentparameters leading to failure at the slump site and to stability at surrounding sites. Owing to the complex variation in sediment properties, subtle changes in properties may determine the stability or failure of thesediment mass. Introduction Slumping (defined as rotational movement along a discrete failure surface) is common to many continental margins of the world and can be an important mechanism of downslope sediment transport (Cook and others, 1981). With the expanded economic exploitation of offshore areas, mass transport of sediment by slumping and other means has become an important consideration in engineering design. Because of the hazard that such mass transport poses to engineering structures, a need exists both for identification of active or potential sediment failures and for quantitative assessment of the potential formovement. Both a geologic and geotechnical (soil mechanics) approach to the study of submarine slumps is important for complete assessment. Interpretive sub bottom acoustic profiling and side-scan sonography are required for identification of zones of past and present instability, whereas measurements of sediment strength and other characteristics are needed to diagnose failure mechanisms and stability factors. From a combined approach, an understanding of submarine mass transport can be obtained. Studies conducted in 1977 and 1978 by the U.s. Geological Survey identified several large areas of unstable sediment, including the one discussed herein, on the continental margin off northern California (Field and others, 1980). In 1979, a combined geologicgeotechnica 1 a na lysis of a large slump a nd the surrounding sea-floor west of Eureka, Calif. was conducted. The study was designed to evaluate the geometry of the slump, the mechanism of failure, and the relative stability of adjacent areas.