Over 250 caisson type quay walls were damaged in the Kobe Port during the 1995 Hyogoken-Nanbu Earthquake. The typical damage was observed, seaward horizontal displacements of the caisson walls were around 5m at maximum and 3m on the average and around 1m settled, the caisson walls also suffered tilting. To investigate the mechanism of damage, we carried out geotechnical investigations and field measurements including in situ freezing sampling and effective stress analysis. The results of effective stress analysis suggested that the decrease in shear resistance due to excess pore water pressure increase in the foundation soil under the caissons and in the backfill soil increased the deformation of the caisson walls.