Offshore Soil Sampling and Geotechnical Parameter Determination

Abstract

In a study aimed at increasing the value of geotechnical analyses of piston and gravity core samples of marine sediments, nine core sampler systems were compared with each other and in-situ measurements. Conclusions are drawn regarding the extent of coring disturbance to be anticipated with each system and the applicability of two methods for correcting this disturbance. Introduction Geotechnical parameters are those characteristics of soil and rock that bear directly on the performance of engineering structures. They are the properties that determine whether a foundation will settle excessively, a pile will experience a bearing capacity failure, an anchor will pull out, or a slope supporting a structure will fail. On land, the list of pertinent geotechnical parameters is increasing steadily as more knowledge is gained about the stress-strain behavior of soil and rock and as computer codes are developed to handle increasing complexity. In the ocean we remain, in many ways, at a primitive level, designing structures so that their support anchors and foundations will not fail catastrophically. Less concern is shown during structural design toward small-scale deformations and settlements. The structures are built to withstand any smaller movements. There is, therefore, in the study of marine geotechnology, a greater interest in the soil properties that enter into the prediction of failure loads than there is on land. These failure properties generally can be summarized as shear strength, with different classes being delineated according to whether pore-water movement occurs during shear. if there is no water movement, as is generally the case in the short- and intermediate-term loading of clays, the critical parameter for use is the undrained shearing strength. If there is water movement as is generally the case with sands, the critical parameter is the drained strength, described by the friction angle. Because the risk of failure usually is much greater with clays than with sands, the undrained shearing strength of clays and other fine-grained materials can be taken as the most important single geotechnical parameter of marine soils.Undrained shearing strength has not been a simple property to measure in the hostile marine environment. Either a test device must be placed on or in the bottom to measure the property directly (in-situ test) or a sample must be brought back and tested for its strength in the laboratory. The in-situ test usually is an expensive and complicated venture involving a heavy piece of machinery equipped with electronic gear that must be maintained in one position on the seafloor for extended periods of time. Even then the soil has not been seen or felt directly, and a sample usually is required.Sampling has many problems as well. Sampling devices are somewhat crude lengths of pipe, topped with a heavy weight and dropped into the bottom. A piston attached by a line to the support vessel may be used to ride up inside the pipe and maintain the length of soil within the pipe at a level near the amount of pipe penetration into the seafloor. JPT P. 891＾