Determination of Compressibility of Marine Sediments from Compressional-Wave Velocity Measurements

ABSTRACT Knowledge of the compressibility of marine sediments is required for the design of adequate foundations for offshore installations. The determination of compressibilities of sediments by standard laboratory tests necessitates expensive sampling. Empirical relationships between void ratios, coefficients of compressibility and seismic velocities have been established by comparing laboratory consolidation tests with simultaneous compressional wave velocity determinations for clay muds representative of four environments of the Gulf of Mexico. Charts developed from these relationships enable a rapid and economical determination of the sediments compressibilities over large water-covered areas, from in-situ velocity measurements. INTRODUCTION The design of adequate foundations for offshore structures and underwater installations requires the determination of the bearing capacity of the sea floor. The effectiveness of certain anchorage systems depends upon the holding capacity of the sediments. Submarine slope stability and potential danger areas susceptible to mass movements such as slumping are related to the compatibility of the sea floor topography with the engineering properties of the sediments. The mechanical response of sediments to loading is usually analyzed for two conditions, considered independently. First, stresses imposed by loading should not cause the sediments to break or fail along surfaces of rupture: this is a condition related to sediment shear strength. Second, compression settlement or decompression uuheaval should not be great enough to damage the structure and/or its function: This is a condition related to sediment compressibility. Therefore, the shear strength and compressibility in addition to bulk unit weight and water content, are the most important engineering properties of marine sediments. These properties can be determined either directly or indirectly. The direct methods rely on current technology developed by Soil Engineers: undisturbed samples are tested in the laboratory for their engineering properties. In the shallow water of the coastal shelves, boring depths can be of several hundred feet. In deep water, sediment investigations are limited by the shallow penetration depths from which samples can be obtained by coring or dredging. Samples retrieved by recent deep water drilling are usually disturbed, and cannot be tested in the laboratory for the accurate measurement of their engineering properties. Although indirect methods are still in the experimental stage, seismic methods appear to be the most promising, because they enable the investigation of engineering properties of sediments over large water-covered areas economically. However, a direct theoretical relationship between seismic properties cannot be anticipated and must be determined empirically. Seismic properties are defined within a limited range of small pressures compatible with the propagationof elastic waves causing small, transient and recoverable deformations in the sediments, whereas engineering properties? deal mostly with large, permanentand irreversible de-formations resulting from the application of the much larger pressures imposed by engineering structures.