It is said on the lateral spread induced by liquefaction that firstly, a quay structure such as a caisson displaces toward the sea, and then the back fill ground displaces laterally following the quay movement. Lateral displacement of a caisson is to be calculated if the "forces" acting on the caisson is known, whatever properties the back fill subsoil has. From such viewpoint, a method to calculate the lateral displacement of a quay structure is proposed. The continuity equation and the momentum equation within a time interval from t=0 to t are obtained and numerically integrated. The result is represented as a relation between re-sedimentation time of the once liquefied ground and caisson's displacement. The elapsed time necessary for the pore water pressure to dissipate in the back fill influences largely the caisson displacement. The water depth in front has large restricting effect on the motion of the caisson. The observed lateral displacements at the 1995 Hyogo-ken Nanbu Earthquake are classified into three types of diminishing characteristics with distance from the quay and well simulated by proposed calculation, where the dominant factor is not the rigidity of the liquefied layer, but the tensile strength of the non liquefied layer.
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