Compressional Behavior of High-Void Ratio Marine Sediments

ABSTRACT Consolidation tests performed on a large number of marine sediments obtained by the R/VALAMINOS, Texas A&M Oceanographic Research Vessel, in the Gulf of Mexico indicate that high-void ratio marine clay sediments exhibit a linear void ratio-pressure relation in contrast to the non-linear relation ordinarily observed in clay soils. It is believed that use of this linear relation will provide (1) a more accurate evaluation of the preconsolidation pressure of marine sediments, (2) a more precise determination of the compressibility of the upper five meters of such sediments, and (3) a better understanding or the actual settlement of engineering structures placed on sea bottom. INTRODUCTION A reliable determination of the supporting capability of ocean-bottom sediments is required for the rational design of offshore engineering structures In addition to factors such as function, shape and size of the proposed structure and its foundation, the allowable pressure on sediments depends mainly upon parameters characteristic of their mechanical response. The mechanical response of sediments to superimposed loading is usually analyzed for two independent conditions. First, the structure should not break into the sediments: this is a condition related to sediment shear strength. Second, the structure should hot deform nor settle excessively: this is related to the deformability and compressibility of the supporting sediments. Since compression settlement should not be great enough to damage the structure and/or its function, it is important that the compressibility characteristics of the underlying sediments be known as accurately as possible. This article deals with the compression of clay sediments due to vertical consolidation. In the consolidation process, the compression for volume change of saturated clays can take place only as water is forced out of the voids between the particles. CONSOLIDATION OF CLAY SOILS In a laboratory compressibility test developed by Terzaghi [7J, a clay sample is encased in a ring and sandwiched between two porous stones. A vertical pressure, applied to the sample, places the water in compression so that initially the pore water carries all the vertical pressure. As water leaves the voids, the pore pressure decreases and the intergranular pressure increases, causing a compression of the-sample. When the compression of the sample has virtually ceased all the applied vertical pressure is carried by the soil particles and the pore pressure is practically nil. The vertical pressure is then increased (usually doubled) and this is repeated for the range of pressures under investigation. Since the compression of the soil sample is caused by the pressure actually carried by the soil particles only, this pressure is called the "effective pressure, p." Results of laboratory compressibility or consolidation tests performed on soil samples are usually presented in the form of void ratio-effective, pressure curves. The void ratio e, of a soil is the ratio of the volume of voids to the volume of solids. since, in the consolidation test, the cross-sectional area of the sample-remains constant, the void ratio e, is also the ratio of the height of voids to the height of solids, and the height of solids remains constant throughout a given consolidation test.