Constructing breakwater with prefabricated caissons on soft clay

A case history of constructing offshore breakwater on soft clay is presented. The breakwater was constructed near the Shanghai Port, China, for deepening of navigation channel along the Yangtze Estuary. The breakwater elements were designed as gravity retaining structures using prefabricated, semi-circular shaped concrete caissons. Some sections of the breakwater were installed on a thick layer of soft soils. During the construction, the caissons in one section failed under a heavy storm. The causes of failure were investigated by running dynamic triaxial tests on undisturbed soil samples taken from the construction site. It was found that the dike failure was induced by the strength weakening of the soft soil layer below the foundation. The design of the guide dike and the soil improvement works are described in this paper. Surcharge preloading and prefabricated vertical drains was adopted to improve the soft soils below the caisson. The soil improvement measure was proven to be effective in maintaining the stability of the breakwater against subsequent heavy storms.