Numerical Analyses For Bearing Capacity Behavior Of Offshore Foundation Under Complicated Conditions

The objective of this paper is to investigate bearing capacity behaviors of offshore foundations under complicated conditions. In the study, through numerical calculation and theoretical analysis, relationship between bearing capacity, failure mechanisms and footing/subsoil interactive behaviors, strength character of subsoil and loading conditions is investigated in details. Numerical analysis methods have been established for calculation of bearing capacity of foundations under combined loading. Based on the methods, parametric studies are performed, and diagrams and simplified formulae for bearing capacity of foundations are obtained, which can be used to develop and improve the design of offshore foundations. Meanwhile, according to subloading theory, ALPHA model is modified to be able to simulate complicated deformation behaviors of soil before yielding. Based on the model, pseudo-static finite element procedure for calculation cyclic bearing capacity is established. Using this analyzing procedure, case studies are performed for bearing capacity of homogeneous foundations under cyclic loading. The main investigations consist of the following parts.1. Combining the fully backward Euler method and sub-incremental technique, an error-controlled implicit constitutive integration method is presented. By introducing pseudo yielding function for the apex stress area, a returning map scheme of two projection directions is established to remove the apex singularity and zero stress point in the yield surface, according to associated flow rule. Based on the two methods, a user material subroutine for ABAQUS is complied, which overcomes the difficulty of ABAQUS in analyzing cohesionless-frictional geomaterial. Furthermore, the returning map scheme of two projection directions is extended to constitutive model with non-associated flow rule, in order to account for different dilatation. Based on this algorithm, a user element subroutine is compiled in the framework of ABAQUS adopting second-order zero-thickness interface element. Cases study show that the user element subroutine can be used to simulate the interactive behaviors, in the case, the contact bodies are discretizd as second-order element, while the contact algorithm based on contact pair built in ABAQUS can not.2. For strip footing in homogeneous subsoil, the bearing capacity factors and embedment factors are obtained through finite element calculation. In the calculation, soil strength characters and footing/subsoil interactive behavior (smooth contact, frictional contact or fully coarse contact) is considered in details. The factors are presented in the form of calculation diagrams, and compared with the published results. Conclusion can be draw that the factors obtained by finite element method are more credible and can be used in foundation design.3. Finite element analysis methods are established for calculation of bearing capacity of foundations under combined loading. Based on themethods, the bearing capacity of strip footing on homogeneous and two-layered saturated clay is calculated. The effects of footing/soil interactive behavior, soil strength profile and combined loading on the failure envelopes and failure mechanism are analyzed. Simplified equations for critical thickness ratio of top layer and vertical bearing capacity are established for layered foundations. Failure loci in two or three dimensional load space are presented. The loci and formulae can be used to develop and improve the design of offshore foundations.4. The ALPHA model is modified according to subloading surface theory, and the initial anisotropy induced by consolidation is considered in the model. A semi-implicit constitutive integration algorithm for the modified model is proposed. And a subroutine embedded in ABAQUS for pseudo-static method is compiled based on the integration algorithm. Using the subroutine, simulations for triaxial tests are implemented, and the results have been compared to the rusults from modified Cam Clay model. The comparison shows that the modified ALPHA model can predict plastic deformation before yielding and other complicate deformation behavior than modified Cam Clay model. Finally, finite element analyses for homogeneous foundation are performed to investigate the cyclic bearing capacity behaviors.