The undrained shearing strength of fine grained soils is usually the most important geotechnical-parameter needed to predict failure load for seafloor supported structures. It can be obtained by testing gravity or piston core samples, but the value may differ from the in place strength through coring disturbance. Nine different corer types were used at six seafloor sites in water depths ranging to 5460 m. The corers varied from conventional oceanographic piston corers to corers specially designed to reduce disturbance. Soils included near-shore silts and clays and deep ocean oozes and clay. Comparative in-place strength measurements were made at four sites. The improved corer samples produced strengths as high as twice those measured on conventional samples, an indication of reduced disturbance. In some plastic soils, cored sample strength equalled in-place strength while in less plastic soils cored sample strengths were significantly less. Two procedures, one involving residual pore water pressure measurements and the other involving fabricating the strength profile through triaxial testing, are given to correct for sampling disturbance. These methods lead to estimated in-place strength values that are usually within plus or minus 20 per cent of the actual in-place strength.
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 pullout, or a slope supporting a structure will fail. On land, the list of pertinent geotechnical parameters is steadily increasing 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 but with less concern 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 can generally be summarized as shear strengths 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 in predicting the ultimate structural load 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. Since the risk of failure is usually 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.
