Anisotropic Bounding-surface Plasticity Model For Cyclic Behaviors Of Saturated Clay And Its Application

The embedded offshore structures, such as the pile anchor, suction caissonanchor, the suction bucket foundation and the drag anchor, which need to withstandunder the extremely complicated ocean environment, are often deeply embedded inthe seabed to keep their stability and functionality. Inevitably, the embedded offshorestructure and its surrounding soil are subjected to the long-term, low-frequency cyclicloading caused by the winds, waves and currents. Hence, the evolvement of the cyclicbehavior of the seabed soil could induce the great change of the bearing capacity ofthe embedded offshore structures. With regard to the problem, a dynamic model forassessing the cyclic bearing capacity of the embedded offshore structures is presentedin the paper.Firstly, within the critical state framework, a new bounding surface plasticitymodel without elastic region suitable for capturing the cyclic shakedown anddegradation of saturated clay is developed. By taking the stress reversal point as thegeneralized homological center, the generalized hardening rule is proposed. Withmovement of the generalised homological centre, at lower stress amplitudes, thecyclic process ends at a steady state, and cyclic shakedown is reached. At higher stressamplitudes, a damage parameter related to the accumulated deviatoric plastic strain isincorporated into the form of the bounding surface, which is hence able to contract tomodel degradations in stiffness and strength. The model requires the critical stateparameters and the hardening modulus parameters, all of them have clear physicalmeanings. For the monotonic loading, only the former is needed, for the cyclicloading, the latter should be calibrated by the laboratory tests related with the dynamicshear modulus and the damping ratio. The developed model is validated throughundrained isotropic cyclic triaxial tests in normally consolidated and overconsolidatedsaturated clay under both one-way and two-way loadings. Both cyclic shakedown anddegradation are well reproduced by the model.Secondly, by describing the initial anisotropy and stress-induced anisotropyseparately, i.e., the former is taken into account through adopting an inclinedbounding surface at the start of shearing loading but without further rotation in the subsequent shearing event and the latter is considered by the kinematic role of thegeneralised hardening rule, the proposed model is improved to reflect the effects ofinitial anisotropy. A series of simple monotonic loading test and complicated cyclicloading tests are carried out. By compared with the analytical and experimental results,the rationality and efficiency of the assumption are verified. Thus, without increasingany model parameter and complexity, the functionality of the isotropic, kinematic androtational hardening rule are included in the newly proposed bounding surface model.Furthermore, the new anisotropic bounding surface model is generalised to3-D stressspace by the Von-Mises criterion method and the transformed stress method based onthe SMP criterion, respectively. In addition, The comparative analysis of the twomethods is systematically carried out to compensate for the lack in the study ofgeneralising the2-D model to3-D stress space for the constitutive model for cyclicbehaviour of saturated clay.Thirdly, the proposed bounding surface model in the paper is integrated by theimplicit return mapping and the sub-stepping integration schemes respectively andimplemented into ABAQUS. Two necessary modifications have been made for theimplicit and the sub-stepping integration schemes when applied to the boundingsurface model without elastic region. Because there is no yield surface in the type ofthe model, which the conventional implicit algorithm returns the stress state back to,or the sub-stepping integration corrects the drift of the stress state to. The applicabilityof the two integrations to the type model without the yield surface is enhanced.Furthermore, the comparative studies on the accuracy, robustness and efficiency of thetwo integrations for this kind of the bounding surface model are carried out both onthe soil element level and a coupled analysis of a rigid square footing on a normallyconsolidated saturated clay. In addition, the procedures of the two integrationalgorithms are given in detail, which can be highly reproduced. It should be noted thatthe model is a representative of a class bounding surface model with vanishing elasticregion which includes the typical model characteristics, i.e., the isotropic, kinematicand rotational hardening, the integrations for the model does provide a guidance forthe similar work.Finally, due to that the existing criterion of the ultimate bearing capacity can not suitable for different types of load-displacement curves, a new criterion is establishedbased on the systematical analysis on all types of load-displacement curves. Byadopting the bounding surface model which has implemented in ABAQUS, the cyclicbearing capacity of the embedded offshore structure (the Vertically Loaded Anchor,VLA) is investigated. It shows that VLA has three typical types of displacementresponses under cyclic loading, i.e., the cyclic shakedown, the cyclic hypo-shakedownand the cyclic degradation. The stiffness of the coupled embedded offshorestructure-soil system shows the similar characteristics correspondingly. A criterion fordetermining the cyclic bearing capacity of the embedded offshore structure isproposed. The effects of the mean value and amplitude of the cyclic loading as well asthe number of cycles are investigated. The procedure for assessing the cyclic bearingcapacity is developed, which can be used for further evaluating the stability of theembedded offshore structures.