The design of OWTs relies on integrated load analyses tools that simulate the response of the entire OWT (including the rotor-nacelle assembly, support structure and foundation) under combined aerodynamic and hydrodynamic loading. Despite all efforts to develop accurate integrated models, these often fail to reproduce the measured natural frequencies, partly due to the current foundation modelling. This paper presents a new foundation model for integrated analyses of monopile-based OWTs. The model follows the macro-element approach, where the response of a pile and the surrounding soil is condensed to a force-displacement relation at seabed. The model formulation uses multi-surface plasticity and it reproduces key characteristics in monopile foundation behaviour that are not accounted for in current industry practice. The basic features of the model are described and its limitations are discussed. The performance of the macro-element model is compared against field test measurements and results from FEA. The comparison indicates that the macro-element model can reproduce accurately the non-linear load-displacement response and hysteretic behaviour measured in field tests and computed in FEA. This confirms that the model can simulate the pile and soil behaviour with the same level of accuracy as FEA, but with a considerable reduction in computational effort.

Many geotechnical problems involve undrained behavior of clay and the capacity in undrained loading. Most constitutive models used today are effective stress based and only indirectly obtain values for the undrained shear strength. To match the design profiles of undrained shear strengths, in active (A), direct simple shear (D) and passive (P) modes of loading are complicated. This paper presents the elastoplastic constitutive model NGI-ADP which is based on the undrained shear strength approach with direct input of shear strengths. Consequently, exact match with design undrained shear strengths profiles is obtained and the well-known anisotropy of undrained shear strength and stiffness is accounted for in the constitutive model. A non-linear stress path-dependent hardening relationship is used, defined from direct input of failure strains in the three directions of shearing represented by triaxial compression, direct simple shear and triaxial extension. With its clear input parameters the model has significant advantages for design analysis of undrained problems. The constitutive model is implemented, into finite element codes, with an implicit integration scheme. Its performance is demonstrated by a finite element analysis of a bearing capacity problem.

In simulations of undrained triaxial tests, most soil models fail to capture the effect of post peak strain rate variation. This is due to the fact that no ''swelling'' is allowed for the viscoplastic volume strain. Imposing such restriction implies that dilative behavior cannot be modeled. Therefore, a model incorporating creep has been formulated using the so-called time resistance concept that uses a single creep parameter determined from an incremental oedometer test. The key feature of the proposed model is the introduction of the time resistance concept on the plastic multiplier rather than on the volumetric viscoplastic strain. This allows the viscoplastic volume strain to be either positive or negative depending on whether the state of the soil is on the ''wet'' or ''dry'' side of critical state line. The proposed model is based on an existing elastoplastic model for structured soft clay (S-CLAY1S). The paper gives a description of the constitutive model and the numerical scheme used in the implementation of the model. Capabilities of the model are illustrated with simulations of oedometer and triaxial tests. Results from such analyses show that the model is able to capture essential features of soft clay behavior.

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Time-dependent settlements of thick in situ clay layers are normally analysed based on results of thin laboratory specimens. However, the time used to complete primary consolidation is significantly different for laboratory specimens and in situ soil layers. Two totally different cases, referred to as creep hypotheses A and B, have been used as a basis of discussion to assess the effect of creep during the primary consolidation phase. Several laboratory and field experiments have been conducted to study the effect of soil layer thickness on the time-dependent compressibility of a soil layer. Some of these tests seemed to support hypothesis A, others hypothesis B, and in some cases showed a behaviour between the two. As a result this question has continued to be a controversial topic among researchers, and remains to be an issue that needs to be resolved. In this study, some relevant experimental investigations from the literature are thoroughly studied and critically reviewed, and also explained consistently using the isotache concept. This work indicates that the isotache approach can capture the main characteristics of the time-dependent compressibility of clays during both the primary and secondary consolidation phases. It is also shown that the misuse of the isotache concept, as reported in the literature, may give a confusing picture of reality. Based on the considered data, it is demonstrated that the measured time-dependent compressibility of clays agrees well with hypothesis B.