A force resultant model for spudcan foundations in soft clay

A site specific assessment is required prior to deployment of a jack-up rig at each new location to ensure the platform's ability to withstand the design load conditions on site. During preloading in soft clays, the spudcan foundations penetrate deeply. Soil backflow around the spudcan occurs, sealing the cavity. While it is generally believed that backflow provides benefit in terms of increased stiffness and bearing capacity under combined loading, no comprehensive guidance is available to date. This may result in undue rejection of rigs. The importance is therefore clear in removal of excessive conservatism through thorough understanding of the foundation performance where backflow occurs. If the additional capacity can be accurately defined and confidently relied on, jack-ups should no longer be rejected for sites they can safely work on. In this thesis, the behaviour of buried spudcans in soft clay soils under combined vertical (V), horizontal (H) and moment (M) loading is investigated in detail. On the basis of the accumulated evidence, a force resultant model is developed for predicting the load-displacement response of spudcans in soft clays. A complementary numerical and experimental research approach has been adopted to develop the force resultant model. The elastic stiffness of the spudcan under VHM loading is studied by finite element analyses, taking account of the influence of increasing shear modulus with depth, backflow and embedment depth. The bearing capacity surface of the spudcan in the VHM load space is first estimated numerically, assuming "wished in place" foundations with complete backflow. Centrifuge experiments are then performed to evaluate the VHM bearing capacity surface with realistic simulation of the large deformation installation process. As expected, the bearing capacity surface that is indicated by the experimental results is smaller than that predicted by the wished in place numerical analyses. However, considerably larger bearing capacity than previous results in stiff clay without backflow is confirmed. Further numerical analyses are performed with explicit consideration of the spudcan installation effects on the VHM bearing capacity surface. These also expand the database to include a wider range of soil sensitivities and embedment depths. The plastic displacement flow rule is evaluated from dedicated centrifuge experiments. These model components (elastic stiffness, VHM yield surface and plastic flow rule) are combined with an existing method for predicting the load-penetration response (hardening law) of spudcans in soft clay to propose a plasticity model that describes the soil-foundation interaction in terms of force resultants and corresponding displacements. The force resultant model is implemented numerically in FORTRAN and can readily be incorporated in a general structural finite element program, which allows integrated soilstructure analyses to be performed. Such integrated analyses of plane frame jack-up structures under quasi static push-over load are performed. Backflow is shown to result in significantly higher push-over capacity as well as a stiffer system response of the jack-up. The proposed model has significant implications for jack-up site specific assessment in soft clay.