Mitigation of Seismic-Induced Damage in Rubble Mound Breakwaters using Innovative Countermeasures

Breakwaters are the critical coastal structures that protect the coastal and port areas from the devastating effects of sea waves, typhoons, and even tsunamis. Among them, Rubble Mound (RM) breakwaters are widely used due to their adaptability. However, following past major earthquakes, such as the 1995 Kobe (Japan), 1999 Kocaeli (Turkey), 2004 Indian Ocean, 2011 Great East Japan, and 2023 Kahramanmaraş (Turkey), it has been found that the stability of these structures depends not only on sea wave actions but also on seismic motions and the underlying seabed soils. As they are directly laid on marine sediments, they are vulnerable to seismic and liquefaction-induced damage. The deformation of seabed soils and breakwater components often results in settlement, lateral spreading, crest lowering, and, in severe cases, structural collapse. Such failures can significantly reduce protection against seismic and tsunami-induced wave overtopping. In seismically active coastal nations, they can greatly magnify the impact of subsequent tsunamis, turning localised failures into large-scale coastal disasters and associated socio-economic losses. Despite their global prevalence and critical role in coastal risk reduction, very limited research exists on countermeasures to enhance the seismic resilience of RM breakwaters. Thus, this study addresses this gap by experimentally investigating seismic failure mechanisms and developing innovative reinforcing techniques using 1g physical models, and then validating them through numerical modelling.

In the study, a prototype breakwater is scaled to a model scale. The conventional model consists of an RM breakwater resting on two layers of seabed (upper loose sand over lower dense sand) modelled and tested on a shake table under a sequence of earthquakes, including foreshocks (0.1g & 0.2g) and the mainshock (0.4g), in the form of sinusoidal waves at their base. Due to the liquefaction of the foundation soils beneath the breakwater and the maximum acceleration amplitude amplification at the base of the breakwater, the conventional model deformed and collapsed below mean sea level during the mainshock. To mitigate these effects, a reinforced configuration was developed, incorporating geogrid layers within the breakwater system and the interconnected geobags infilled with recycled concrete aggregates (RCA), as a replacement for conventional concrete armour units on the slope. RCAs are construction and demolition wastes; utilising them in place of concrete promotes the circular economy goals. Additionally, sheet piles were used in the seabed foundation to reduce lateral deformation during earthquakes. The effectiveness of the proposed countermeasure was assessed using parameters such as liquefaction potential, settlement, lateral displacement, and deformation patterns. Compared with the conventional model, the settlement of the reinforced breakwater was reduced by 62.7%, and lateral displacement was decreased by 60.3% during the mainshock. The model also effectively mitigated the potential for liquefaction by reducing excess pore pressure ratios and amplification ratios. The deformation patterns for the conventional and reinforced models are depicted in Figs. 1 and 2, respectively. Thus, the result demonstrated that the proposed technique significantly enhances seismic resilience and mitigation solutions for coastal infrastructure in earthquake and tsunami-prone regions.