Mechanisms of settlement of a rubble mound breakwater on a soft soil in tidal flats

Abstract

Because mudflats are compressible, substantial settlement will occur during construction of a rubble mound breakwater at such a tidal flat site. The use of geosynthetics as reinforcement is common during breakwater construction. Although numerical methods based on continuum mechanics have been used to model the behavior of embankment systems, they cannot provide insight into settlement because they cannot approximate the individual movement of the particles. Rubble mound breakwaters consist of rockfill, which is naturally discontinuous. A full-scale case study of a rubble mound breakwater constructed on soft soil is presented for which the effect of the pattern, size and number of layers of the geogrid on performance was monitored. To evaluate the control mechanisms of settlement of rubble mound breakwaters, a series of numerical simulations using the discrete element method were done to evaluate settlement caused by submersion of the particles into the seabed. The results showed that the submersion of the individual particles of rockfill into the seabed is the main mechanism of settlement in an unreinforced breakwater. This value is higher than for classic settlement (elastic or consolidation). However a reinforced geogrid acts as a separator and prepares a continuous base for the rockfill to prevent individual movement.