Building a breakwater with prefabricated caissons on soft clay

This paper presents a case history of the construction of an offshore breakwater on soft clay using prefabricated, semicircular-shaped concrete caissons. The project was located near Shanghai Port, China. The breakwaters were designed as gravity-retaining structures, and used for deepening of the navigation channels along the Yangtze estuary. Some sections of the breakwaters were installed on a thick layer of soft deposit. During the construction, the caissons in one section failed under a heavy storm. The causes of failure were investigated. The design of the breakwater and the soil improvement works are described in this paper. Surcharge preloading and prefabricated vertical drains were adopted to improve the soft soils below the caissons. Some other measures, such as the use of anti-sliding mats for the base of the caissons to increase the base friction, and the use of geotextile with sand-filled geotextile tubes and geotextile and concrete block composites to prevent scour, were also adopted. These measures were proven to be effective in maintaining the stability of the breakwaters against subsequent heavy storms.