In this paper the pull out capacity of bucket foundations in soft clay, subjected to inclined loading, is considered. The results from three dimensional finite element analyses are used to construct interaction diagrams of vertical and horizontal ultimate forces. It is shown that normalising the results with respect to the maximum vertical force (obtained from an analysis involving only vertical pull-out) leads to a unique interaction diagram which is Independent of the foundation dimensions. The implication for design is that once the vertical pull-out capacity has been evaluated, it is then possible to determine the capacity under any inclined loading, without additional complex three dimensional analyses.
The use of bucket foundations and suction anchors for offshore installations has become widespread. Such foundations have been used where the seabed soils are either of a granular nature and/or consist of soft clay. To date, most of these foundations have been subjected to compressive loading. However, considerable interest is now being directed to the situation where the forces applied to these foundations involve tension, whereby the foundation is being pulled from the sea-bed. The resultant force involves both vertical and horizontal components. One of the main design considerations for such foundations is the assessment of then pull-out capacity.
In this paper the short term pull-out capacity of bucket foundations in soft clays is considered. Results from a parametric study, involving three dimensional finite element analyses of bucket foundations, in which the diameter, skirt length, sod-structure adhesion and inclination of loading were varied, are presented. The soil was assumed to be an isotropic soft clay with an undrained shear strength increasing with depth. This study forms part of a larger research project in which the effects of sod anisotropy on the behaviour of offshore foundations are under investigation.
A lightly overconsolidated soil (OCR=l. 1) has been assumed and a form of the Modified Cam clay constitutive model has been used to simulate its behaviour. The yield and plastic potential surfaces are given by a Mohr Coulomb hexagon and a circle respectively (Potts and Gens (1984), Gens and Potts (1988)). Basic details of the model are given in Appendix A and the material properties used in the present investigation are given in Table 1. The latter were based on data from recent site investigations performed at a soft clay site
The bucket foundations were assumed to be loaded relatively quickly and therefore undrained conditions were assumed in the clay. It may be noted that the parameters in Table 1 are essentially effective stress parameters. It is not possible to input directly the undrained strength, or its change with effective stress. However, the undrained strength can be derived from the basic parameters as indicated in Appendix A. The undrained shear strength distribution with depth, which is consistent with the material parameters given in Table 1, is shown in Figure 1. The undrained strength plotted refers to triaxial compression conditions. As can be seen, it vanes linearly with depth, giving Su/svN=0.33