New Design Method on Bearing Capacity of Rubble Mound of Composite Breakwater with Consideration of Deformation

The bearing capacity for composite breakwater's mound is considered as a problem of foundation stability. Expected displacement due to mound failure under eccentric inclined load by repeating wave forces is estimated. The displacement is calculated by the use of angular motion equation along virtual failure arc, similarly to Newmark's method in earthquake engineering. A computation program, which takes into account various conditions for individual breakwater, was developed. It has been found that the relationship between the expected displacement and the design safety factor of bearing capacity varies with the ratio of breadth to height for the caisson box. If an allowable expected deformation in service life is introduced in breakwater design, it is prospective to reduce caisson breadth especially in the cases of larger breadth height ratios. The program was used in back analysis of case of breakwater failure that occurred in Tomakomai west port. The settlements of caissons, ranging from 7 cm to 45 cm, were observed after a series of typhoons. By using the high wave record, and accounting for the effect of wave force increase due to incompletely armored wave-breaking blocks, the calculated settlements and horizontal displacements were in agreement with the range of deformations observed in spite of considerable scatter.