Guy T. Houlsby

Advanced finite-element analyses are increasingly being used in geotechnical design, and central to these analyses are reliable and accurate constitutive models to represent soil behaviour. These models need to address response across a wide range of conditions, including for instance cyclic loading for offshore applications. Existing advanced constitutive models for sand have limited capability for modelling cyclic loading, are often complex and rarely respect the laws of thermodynamics. HySand_base, a new constitutive model for sand under cyclic loading, is presented. Its concise, rigorous and simple formulation within the hyperplasticity framework is given. Each element of the formulation is explained through the progressive construction of the model, and the improvements brought by each additional feature are illustrated for drained monotonic, undrained monotonic and undrained cyclic tests. HySand_base uses 14 material parameters. It is a density- and pressure-dependent multi-surface plasticity model, rooted in critical state theory, with yield surfaces based on the fusion of Matsuoka–Nakai type surfaces with consolidation surfaces. Non-associated plasticity is adopted, with volumetric plastic strains arising from two mechanisms: anisotropic dilation and consolidation.

We describe an algorithm to be used for the numerical integration of plasticity models in finite element or finite difference codes. The algorithm is simple to code and implement. It is presented here for a generic form of plasticity model encompassing both single and multiple yield surfaces, and it is readily adaptable to more complex models. Three examples are given to demonstrate the efficacy of the algorithm in controlling integration errors.

Clays exhibit complex mechanical behaviour with significant viscous, non-linear and hysteric characteristics, beyond the prediction capacity of the well-known modified Cam Clay (MCC) model. This paper extends the MCC model to address these important limitations. The proposed family of models is constructed entirely within the hyperplasticity framework deduced from thermodynamic extremal principles. More specifically, the previously developed MCC hyper-viscoplastic model based on the isotache concept is extended to incorporate multiple internal variables and to capture recent loading history, hysteresis and smooth response of the material. This is achieved by defining an inelastic free energy and an element that implements a bounding surface within hyperplasticity, resulting in pressure dependency in both reversible and irreversible processes with a unique critical state envelope, and only eight material parameters with a readily measurable viscous parameter. A kinematic hardening in the logistic differential form in stress space is derived that enables the proposed model to function effectively across a wide range of stresses. Based on this kinematic hardening rule, the current stress state acts as an asymptotic attractor for the back or shift stresses whose evolution rates are proportional to their current state.

Monopiles supporting offshore wind turbines experience combined moment and horizontal loading which is both cyclic and complex – continuously varying in amplitude, direction and frequency. The accumulation of rotation with cyclic loading (ratcheting) is a key concern for monopile designers and has been explored in previous experimental studies, where constant-amplitude cyclic tests have shown rotation to accumulate as a power-law with cycle number. This paper presents results from laboratory tests in dry sand, which explore the rotation response to constant and variable amplitude, unidirectional and multidirectional cyclic loading. The tests are designed to inform model development and provide insight into key issues relevant to monopile design. Unidirectional tests show behaviour consistent with previous studies and provide a basis for interpreting more complex tests; multidirectional tests provide new insight into the monopile response to multidirectional cyclic loading; and multi-amplitude storm tests highlight salient features of the response to realistic loading. Tests are conducted in both very loose and dense sand, where the behaviour is found to be qualitatively similar.

Located in between the Flores and Adonara islands, Larantuka Strait has one of the strongest tidal currents in Indonesia. A bridge 810 m long is planned to be constructed across the strait to connect the islands. The Palmerah Tidal Bridge project, commenced in 2019, is planned to be the first combined tidal energy/bridge project worldwide. By combining the installation of turbines with bridge construction, the project is intended to achieve certain economies. However, the financial viability would depend on the power production at this site. This study is an independent assessment regarding the tidal energy resources, estimating the average power available at possible array locations in the area of the Palmerah Tidal Bridge. This paper examines alternative locations for turbine deployment along the strait as well. Moreover, it also discusses the flow-field change and its effects on the Indonesian throughflow in the region. This paper discovers that the average available power from this site is lower than the output that has been predicted for this project. However, if the prediction is the maximum (as opposed to average) power available for the subsequent stages, then this study would prove that the numbers are achievable.