As one of the tsunami prevention methods, caisson-type breakwaters have been reinforced by building an embankment over the foundation mound. The resistance mechanism of the composite ground of embankment and mound has not yet been clarified, and no methods have been established to assess the stability. Seepage force induced by the water-level difference between the front and back of the breakwaters could cause the lifting up of mound stones around the corners of a caisson, which is known as boiling. The possibility of the boiling is also not understood. Furthermore,
it is considered that the seepage force will reduce the bearing capacity of the mound, but this has not yet been quantitatively investigated. The present study aimed to demonstrate them by using centrifuge model tests, finite element analyses, circular slip analyses, and seepage flow analyses. The centrifuge model tests showed that the embankment behind a caisson enhanced the resistance force, and the load-displacement relation was found to be characteristic in terms of requiring large displacement until deriving the peak strength. Circular slip analyses based on the model test results had high calculation accuracy, and could be selected as the method for assessing stability, provided that the shear strength parameters used in calculations are appropriately chosen. In addition, seepage flow analyses were able to reproduce the field of seepage force by considering nonlinearity. Furthermore, it was clarified that the size of the mound stones affected the possibility of boiling. The centrifuge model tests and finite element analyses demonstrated that seepage force decreased the bearing
capacity, and that the reduction rate was at most around 10～16% in the tested breakwater (water-level difference: 8.3～9.3 m).