Many earth structures and foundations were damaged in Kobe port area during the 1995 Great Hanshin earthquake. Damage to the pneumatic caisson foundations for the Kobe Ohashi bridge was of a particular interest. Though the pneumatic caissons were embedded to a depth of K.P.-31m, there were about 0.6 to 0.8m seaward displacement induced at the caissons. In order to identify the mechanism of deformation, a two-dimensional effective stress analysis is performed to take into account the soil-structural and water-structural interactions in this study. The constitutive model used is a multiple shear mechanism type defined in strain space and can take into account the effect of rotation of principal stress axis directions. The earthquake acceleration recorded at a depth of 83 meters on the Port Island during the 1995 Great Hanshin earthquake is used as input bedrock motion. The results of analysis show that the seaward residual displacements are 0.46 m at the top of a caisson and 0.48 m at the top of the other caisson, resulting in a total of 0.94m. The order of computed residual displacement is in good agreement with the
measured. The results indicate that the excess pore water pressure increase in the foundation significantly affected the performance of caissons. The effect of soil improvement is also discussed.