This paper describes the soil-structure interaction behavior of a new reinforcing technique for breakwater foundation against earthquake and tsunami induced damage. The technique involves the use of gabion on the top of rubble mound, reinforcing the foundation soil with steel sheet piles, and the use of sealing material between the sheet piles and breakwater. The effectiveness of the proposed technique and the soil structure interaction during earthquake and tsunami loading were evaluated through physical and numerical modeling. As part of the physical modeling for such soil-structure interaction problem, a series of 1g shaking table tests and centrifuge model tests were performed. In addition, a hydraulic model testing apparatus was developed, which can simulate the soil-structure interaction during tsunami overflow, the resulting seepage as well as the scouring. Numerical simulation was also performed for evaluating the effect of tsunami overflow. Soil-structure interaction behaviors were made clear through comparisons of conventional foundation and reinforced foundation models.