Behavior of a Caisson Subjected to a Horizontal Breaking Wave Force

Abstract

In coastal waters, caissons are used as breakwaters. Under the action of breaking waves, some caissons are initiated into motion. This study is concerned with the following questions: For a caisson subjected to a breaking wave force, what are (1) the possible modes of response and (2) the conditions under which a caisson would be initiated into these modes? These questions have not been studied before. The actual response of a caisson that takes place in each mode is not addressed in this study. A caisson is modeled as a free-standing rigid body placed on a frictional base subjected to a concentrated horizontal force of short duration applied on the seaward side face of the caisson. The conditions for initiation of each mode of motion are derived using the three equations of motion of a plane rigid body. The results are given in explicit analytical forms and presented graphically. For simplicity, the effects of water surrounding the caisson are not included. Knowing the magnitude and location of the force, coefficient of friction, and aspect ratio of the body, the modes of motion of the body may be identified easily from the graphs. It is shown that under the action of a force, a body may undergo various types of motion and its behavior is quite different, depending on whether the line of action of the force is applied above or below the center of mass of the body.