In “The 2011 off the Pacific coast of Tohoku Earthquake”, many breakwaters in the coastal areas suffered extensive damage under tsunami overflow. It has been needed to develop the methods of reinforcing the breakwater, tenaciously.
　The widening work is one of existing reinforcement methods of tenacious breakwaters. But, it also has large cross section in port inside area. Furthermore, Scouring due to the overflow decreases the bearing capacity of the rubble mound and it makes the breakwater easy to fall down. On the other hand, one of the reinforcing methods to resist tsunami, installing a steel wall behind the breakwater, was proposed by Oikawa, et al. (2014). In the series of loading tests, the model caisson on the dry sandy ground wad loaded horizontally. Applying the combination of steel wall and filling between the wall and the caisson improved the horizontal resistance of the caisson effectively. And, this improving method changes the failure mode of the caisson from sliding into ground bearing failure even in scouring cross section.
　This paper deals with the stability of the breakwater with Steel Pipe Piles by the hydraulic model tests and the analyses. Two breakwater cross sections (‘O’ and ‘K’ port) were used in the tests to clarify the failure mechanism of breakwater with the combination of the wall (composed of steel pipe piles) and the filling, called “Steel Pipe Method” here, under tsunami overflow. In addition, CADMAS-SURF/3D was used in the analyses to examine external force against the breakwater while the caisson falls down.
　The summary is as follows.
1. The experimental results show that “Steel Pipe Method” improves stability of breakwater under tsunami overflow.
2. The main failure modes of the breakwater with “Steel Pipe Method” under tsunami overflow are ‘overturning of steel pipes’ and ‘overturning of caisson’.
3. The stability of the breakwater with “Steel Pipe Method” under tsunami overflow is checked, considering external force of Tsunami against the breakwater, reaction force of the steel pipes against the caisson through the filling, and ground reaction force against the steel pipes.