Failure of a breakwater founded on a thick normally consolidated clay layer

Two breakwaters, 890 m and 670 m long were constructed on 55 m and 35 m thick normally consolidated clay layers in Busan, Korea. The design incorporated 12 m and 17 m deep trenches excavated in the clay and backfilled with sand. The two were similar in type and size, and their construction started simultaneously at adjacent sites. The east breakwater was successfully completed, but the west breakwater failed during the final stage of its construction. Vertical and horizontal displacements and pore pressure had been measured at a section for each breakwater throughout the construction process. Using the results obtained from the instrumentation, the behaviours of the two breakwaters are compared. The original breakwater design is re-evaluated with design parameters obtained from an additional investigation prior to the construction. It is revealed that the safety factor for arc sliding did not provide enough safety, even when the drainage effect due to consolidation was taken into account. Possible methods tracing its stability during construction are also discussed with currently available methods. A numerical analysis is performed to compare the monitored results and to evaluate the different behaviours of the breakwaters. Finally, possible causes of the failure of the breakwater are presented.