The effects of submerged berms on the stability of rubble mound breakwaters

Berms deployed at the toe of conventional rubble mound breakwaters may be useful for various purposes, i.e. to reduce wave loads acting on the breakwater armor elements, to reduce wave overtopping, to limit momentary liquefaction occurrences under the structure and to protect from the scouring the breakwater toe. In the first part, the Thesis is aimed to provide a new design criterion for the armor layer of conventional breakwaters with submerged berms marked by small thickness compared to water depth. Indeed, past research studies focused on the influence of relatively high berms on the stability of the armor layer. The design of the berm itself was not tackled herein. The effects of submerged berms on the incident waves transformation have been evaluated by means of a numerical model, validated by using experimental data. Then, a parametric correction factor of the incident significant wave height at the toe of the structure is provided and included in well-established design criteria. The experimental comparison confirmed the reliability of the proposed method by highlighting the importance to use design criteria within their validity ranges, in order to avoid an unsafe dimensioning of the armor elements. In the second part, the Thesis is aimed to evaluate how submerged berms configuration influence the seabed soil response and momentary liquefaction occurrences around and beneath breakwaters foundation, under dynamic wave loading. The effects of submerged berms on the incident waves transformation have been evaluated by means of the aformentioned phase resolving numerical model for simulating nonhydrostatic, free-surface, rotational flows. The soil response to wave-induced seabed pressures has been evaluated by using an ad-hoc anisotropic poro-elastic soil solver, developed within the OpenFOAM framework. Once the evaluation of the seabed consolidation state, due to the presence of the breakwater, has been performed, the dynamic interaction among water waves, soil and structure has been analyzed by using a one-way coupling boundary condition. A parametric study has been carried out by varying the berm configuration (i.e. its height and its length), keeping constant the offshore regular wave conditions, the water depth and the elastic properties of the soil. Results indicate that the presence of submerged berms tends to mitigate the liquefaction probability if compared to straight sloped conventional breakwater without a berm. In addition, it appears that the momentary liquefaction phenomena are more influenced by changing the berm length rather than the berm height. In the last part, the Thesis is focused on the role of submerged berms on the random wave-induced momentary liquefaction. A numerical study has been carried out by selecting three ad-hoc domain, among those tested under regular wave condition. Indeed, the influence of random wave loading on the momentary liquefaction occurrences is investigated through a comparison with the corresponding representative regular wave results. Due to the wave randomness, the random wave-induced momentary liquefaction depths are larger than the corresponding representative regular ones. This means that the evaluation of the momentary liquefaction occurrences, under the action of representative regular waves, could underestimate the liquefaction depth, determining an unsafe design in the practice.