The bank protection measures of waterways shall become more environmentally friendly in the future including the use of plants instead of stones. The low levels of protection provided by plants in the early phase after planting requires a process-based understanding of soil-wave-interaction. One process that is considered essential is liquefaction where the soil undergoes a phase-change from solid-like to fluid-like behaviour which could reduce the safety of the system. The aim of this publication is to analyse the results of column experiments on wave-induced soil liquefaction and to develop a numerical model which is able to describe the entire process from the pre-liquefaction phase to the following reconsolidation in order to support the analysis of liquefaction experiments. Numerical simulations of the column experiments were done using a fully coupled hydro-mechanical model implemented in the open-source software FEniCS. A permeability model derived from granular rheology allows the simulation of dilute as well as dense suspensions and sedimented soil skeletons. The results of the simulations show a good agreement with the experimental data. Theoretical limits in the liquefied state are captured without the common modelling segmentation into pre-and post-liquefaction phase. Due to the modular structure of the implementation, the constitutive setting can be adjusted to incorporate more complex formulations in order to study the influence of wall friction and non-linearity in soil behaviour.

The silt seabed can undergo liquefaction under wave action, resulting in the liquefied silt seabed exhibiting nonNewtonian fluid characteristics and fluctuating in phase with the overlying waves. The fluctuation of the liquefied silt seabed can impose periodic forces on the buried pipelines, posing a significant threat to their safety. This study achieves the measurement of the non-Newtonian fluid rheological properties of wave-induced liquefied silt, through the improvement of the falling-ball method. The improved falling-ball method enables in situ measurement of the rheological properties of liquefied silt in fluctuation state. This method is applied in two wave flume experiments to investigate the effects of wave intensity and the liquefaction process on the rheological properties of liquefied silt. Building on this foundation, a computational fluid dynamics (CFD) numerical model is developed to simulate the wave-liquefied silt interaction, utilizing the rheological properties of the liquefied silt obtained from experimental measurement. The model is used to evaluate the fluctuation velocity of the liquefied silt under field conditions and its forces acting on buried pipelines. The research findings provide foundational data for more accurate simulations of the movement of wave-induced liquefied silt and its effects on structures.

This paper aims to investigate the wave-induced evolution of small-strain stiffness and its effects on seismic wave propagation. To this end, an advanced numerical framework based on the dynamic porous media theory was developed, in which the Iwan multi-surface constitutive model was adopted to model the soil behavior during cyclic loading. Moreover, the numerical framework integrates key parameters such as ocean wave characteristics and depth-dependence seabed conditions to model the intricate interactions between waves and the seabed. Following model verification via analytical solutions and previous experimental data, comprehensive parameter studies are conducted, from which the effects of different wave conditions and seabed properties on the dynamic response of the seabed were obtained, revealing the wave-induced small- strain stiffness spatial and temporal variation. Subsequently, simulations of geophysical monitoring instants are conducted, assessing the impact of evolving small-strain stiffness on seismic wave propagation. The findings highlight the implications of stiffness changes on seismic wave propagation characteristics. The study provides valuable insights into the challenges and opportunities associated with interpreting geophysical data in dynamic submarine environments, offering implications for subsurface characterization and monitoring applications.

Residual liquefaction, a significant issue in marine engineering, results from accumulated pore-water pressure in the seabed due to cyclic shear stresses, which compromises soil stability. This study aims to investigate residual liquefaction around gravity-based marine structures by means of a 2D numerical model. The model employs a two-step procedure: First, the stresses in the soil domain are determined via solving Biot equations, and subsequently the generation and diffusion of accumulated pore pressure in the soil is simulated by means of a pressure diffusion equation with a source term. The model was first validated against analytical solution for pore pressure buildup in the seabed under progressive waves, and against experimental data for residual liquefaction around a buried submarine pipeline. The results showed that the model can satisfactorily capture pore pressure buildup and residual liquefaction in the seabed around structures. Once validated, the model was utilized to model the pore-water pressure buildup and residual liquefaction potential around a caisson breakwater under the action of standing waves and the wave-induced rocking motion of the caisson, separately and in combination. Spatial distribution of liquefaction potential was determined in the seabed soil around the caisson with and without a bedding layer on the seabed. The model results revealed the critical role of the bedding layer in reducing liquefaction susceptibility under standing waves and rocking motion, and highlighted that the rocking motion alone poses a significant risk of inducing residual liquefaction in the seabed around the caisson.

Large offshore wind turbines (OWTs) may encounter extreme misaligned wind and wave conditions throughout their lifetime, which could trigger side-to-side resonance of the OWTs and thus substantial reduction in fatigue life. This study aims to (1) understand the dynamic response of fixed-bottom OWTs under misaligned wind and wave conditions, and to (2) propose an active torque control algorithm for dynamic loading mitigation. For these purposes, an integrated aeroelastic model, coupled with an advanced soil-monopile interaction (i.e., p-y+M-theta model), is built in OpenFAST for the DTU 10MW OWT supported by a monopile in soft clay. The numerical results show that under misaligned wind and wave conditions, where the wave peak period is likely to approach the tower natural period, the dynamic loading along the side-to-side direction dominates the fatigue design of the OWT. To mitigate the side-to-side dynamic loading, an active torque control algorithm is designed with feedback from measured side-to-side tower vibration to enhance damping, as well as feedforward from measured incoming wave height to counteract external force. Through the use of the feedback-feedforward active torque controller, the side-toside dynamic loading of the 10 MW OWT is significantly reduced, with the fatigue life extended from 19 to 39 years.

Wave-induced submarine slope instability and its subsequent submarine landslide pose a huge threat to the coastal communities and offshore infrastructure. This study conducted a wave flume experiment to understand the effect of low-permeability layer on the excess pore water pressure response and the instability of the layered submarine clayey slope under wave actions. The experiment captured the whole process of soil progressive liquefaction and instability of submarine clayey slope. When the wave propagates from the toe to the crest of the slope, the wave shoaling results in the soil at the slope crest above the low-permeability layer liquefy and then slide down due to the wave oscillation and scouring. The low-permeability soil layer leads to a delay in the accumulation of excess pore water pressure. However, once the excess pore water pressure is accumulated, this layer restrains the dissipation of excess pore water pressure resulting in significant liquefaction potential of the soils below this layer. Due to the capping effect of the low-permeability soil layer, there was no significant sliding of the soil mass below it. Our findings might provide an implication and guiding significance for offshore site selection and the coastal engineering safety.