Damage to seawalls caused by high waves still occurs even today, and one of the patterns of
damage is failure involving ground. It is important to clarify the failure mechanism and to assess
the stability appropriately, and effective countermeasures are urgently needed for seawalls with
insufficient strength. The ground failure focused on in this study is due to instability caused by
ground saturation and lowering of the water level in front of the seawall during backwash. A series
of centrifuge model tests were conducted to reproduce the seawall failure, modelling both wave
propagation and ground deformation, to confirm the failure mechanism and to evaluate the effects
of various conditions. The seawall failure was reproduced by finite element analysis (FEA) of the
ground, using finite difference fluid analysis to calculate the water pressure input to the ground
deformation analysis. In the FEAs, the failure mechanism was verified from the viewpoint of
stresses by understanding the stresses and pore water pressure conditions in the ground. Through
these investigations, it was clarified that the saturation of the ground and the pore water pressure
conditions during backwash caused ground instability. Furthermore, the ground stability in the
seawall was evaluated using circular slip analyses, and the accuracy of the calculations was
confirmed. Finally, methods to increase the ground stability in the seawall were investigated. The
discussions based on the centrifuge model tests, FEAs, and circular slip analyses showed that
measures to prevent scour and loosening of the ground ahead of the seawall, the installation of sheet
piles or cement treated soil ahead of the seawall, the laying of a crushed stone layer below the
armoring panel, and the prevention of saturation will increase the stability of the ground.
Key Words: Wave, ground, seawall, centrifuge model test, fluid analysis, finite element analysis