The stability of sheet pile quay walls on a thick clay deposit against pseudo-static and dynamic loading was studied through two series of centrifuge model tests. In the tests, a layer of over consolidated kaolin clay was prepared over a layer of dense Toyoura sand in a rectangular container. The model quay wall was set to the
bottom of the sand layer. An area in the sea-side adjacent to the quay wall was improved with normal Portland cement-treated Kawasaki clay. At 50 g centrifugal acceleration, the clay deposit was consolidated and a cyclic horizontal line load of about 0 to 70 kN/m was applied to the quay wall in the case of pseudo-static loading series and four to five stages of shaking were applied to the quay wall in dynamic　loading series. The width of the improved area was varied and its performance was compared with that of a quay wall embedded in unimproved ground. Results of the study indicated that the improved area provided significant resistance against static and dynamic loading. The ground improve-ment can reduce 30% to 60% bending moment during the dynamic loading. The horizontal deflection of the quay wall decreases rapidly with the increase in area of the CDM until it reaches a certain limit. In addition, a numerical model to estimate the mechanical behavior of the sheet pile quay wall is presented. The numerical model show good agreement with the centrifuge test results of both series.