Wave-Induced Strength Weakening of Soft Clay Below a Prefabricated Caisson Dike

A case history of constructing offshore guide dikes on soft clay is presented in this paper. The guide dikes were constructed near the Shanghai Port, China, for deepening of navigation channel along the Yangtze Estuary. The guide dikes were designed as gravity retaining structures using prefabricated, semi-circular shaped concrete caissons. Some sections of the guide dikes 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, associated with the FE analysis. 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 guide dikes against subsequent heavy storms