This paper describes a technique for relating the strain rate fields and soil forces in the soils in prototype problems or around in situ measurement devices to the fundamental soil strength properties from laboratory tests. The strain-rate field is obtained with continuum mechanics principles from the deformation patterns which are based on model tests. The soil strength variation with strain-rate is described with a generalized power law. The soil forces are predicted with the use of the Bound Theorem. Applications to the interpretation of vane shear tests and mud flow forces around piles are described.
The magnitude of forces due to mud flow around piles is an important design consideration for offshore structures and systems for the Gulf of Mexico. Three examples of situations in which mud flow forces can influence the design are the following. The first is the movement of sub sea soils (mudslide) in a zone near the mud line, which can be triggered by rapid deposition of new sediments on the sloping seafloor or by wave-induced pressures during severe storms. This soil movement will impose a force on fixed structures located in its path, which must be taken into account in the design. The second is the long term behavior of anchor piles subject to large lateral loads. The pile-soil interaction in this case depends on both the consolidation characteristics of the soil, as well as its un drained creep behavior. A third example is the interaction between sub sea pipelines and soils in mudslide susceptible areas.
Traditional methods for predicting ultimate soil forces on piles rely on plasticity theory, which assumes rate independent materials. To supplement these, laboratory and model tests have been performed to study the shear rate effects on soil strength, in situ measurements of strength index, as well as soil forces around pipes. In recent years, research sponsored by the American Gas Association at Texas A&M University has dealt with models to describe the visco elastic and history dependent behavior of soils.
This paper takes an alternative approach to the problem by concentrating on problems in which large deformation has occurred, and the soil resistance can be approximated as a nonlinear (non-Newtonian) viscous material. The strain-rate field can be derived from the deformation pattern, which can in turn be based on model tests. The soil forces are predicted with the use of the Bound Theorem. The results can be presented in a form similar to the traditional approaches. However, the strain-rate variation, and consequently the strength variation, in the problem are accounted for. The relationships between the strain-rate field in complex problems, such as mud flow around piles or vane shear tests, and conventional laboratory tests are also established.