Stability analysis of a rubble mound breakwater resting on the weak seabed by 2D finite element modelling-Paper

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The study evaluates the stability of a rubble-mound breakwater built over a seabed containing a thin sand layer, thick soft clay, and deeper medium-stiff clay. Because ground improvement is difficult in marine conditions, the design uses staged construction and waiting periods to preload the seabed, allowing consolidation to increase soil strength.<br /><br />Finite-element analyses in PLAXIS modelled the soil and breakwater materials using appropriate constitutive models. The breakwater was built progressively to a final height of 10.5 m, with stability and consolidation assessed at each stage. Three construction schedules, lasting 760, 780, and 800 days, were compared.<br /><br />The analysis showed that settlement concentrated mainly in the 23 m thick silty-clay layer, reaching approximately 700 mm by the end of construction. The final factor of safety was 1.8 for all schedules. However, the factor of safety declined as the structure grew, reaching 1.2 at an intermediate stage before rising as consolidation improved soil strength. The critical failure mechanism was a deep-seated base failure through soft clay beneath the breakwater.<br /><br />The 760-day schedule was identified as the most efficient option. The findings support staged construction over roughly two years as a way to improve soft seabed strength through preloading without special ground treatment. The design must accommodate anticipated settlement, and precautions are recommended for partially completed sections during cyclones or monsoon conditions.

Keywords

breakwater stability

staged construction

soft clay seabed

consolidation

finite element analysis

PLAXIS

preloading

settlement

factor of safety

marine geotechnical engineering