Currently, state-of-the-art design in coastal structures separately investigates hydraulic design and verification of geotechnical stability.
Phenomena such as seismic and storm wave-induced liquefaction require detailed understanding of/modelling of interactions of hydraulic and geotechnical processes at or within the structure, and in the foundation soil. But these processes must presently be modelled separately as no models are suitably coupled.
A more comprehensive approach was proposed, coupling hydrodynamic and geotechnical models, in order to assess the stability of the ensemble structure-foundation soil. Computational fluid dynamics (CFD) or physical models can be used to provide loads on structures due to wave action, which are then used as inputs for the geotechnical analysis performed with the numerical model SWANDYNE. This is able to solve a time stepwise soil consolidation analysis and then evaluate the foundation geotechnical response.
Physical model tests were completed on a typical multi-layered rubble mound breakwater to investigate pore pressure distributions within porous mounds under wave action. Results were then used to validate previous experimental results and numerical codes, initially OTTP-1D (based on depth averaged non-linear shallow water) and ANSYS CFX (Computational Fluid-Dynamics model), and (later) OpenFOAM.
Results from the experimental tests were also used as boundary conditions for the FEM geotechnical model SWANDYNE to assess the ability of that model to predict wave-induced liquefaction in cohesion-less foundation soils underneath caisson and rubble mound breakwaters. Additional work then explored the ability of the open-source CFD code OpenFOAM to propagate waves within porous media and to predict pore pressure distributions under wave action.
Results are generally encouraging. Overall, predictions of pore pressure extracted from these CFD models show relatively good agreement with results extrapolated from pressure gauges during laboratory tests. In addition, use of OpenFOAM has significantly accelerated the simulations, running more than 100 waves in only a few hours for a 2D flume case.
The project has demonstrated the possibility of an integrated approach for both the hydraulic design and analysis of geotechnical stability of rubble mound breakwaters, coupling CFD with FEM models. At this stage however, the procedure requires two steps – where the outputs of the CFD model are converted and then used as input for the FEM geotechnical model. Future developments may link the two models more robustly in a single integrated model where equations solving the wave-induced pore flow and the relative stress induced in the soil are solved simultaneously. This would significantly increase the level of confidence in the design procedure and refine current techniques for marine structures on liquefiable soils.
||Scott Dunn, Clemente Cantelmo, William Allsop|
|Keywords||Pore pressure, breakwater, rubble mound, CFD, FEM, liquefaction|
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