Deformation of Composite Breakwaters Due to Ground Shaking

Abstract

ABSTRACT This paper reports the deformation of composite breakwaters in the port of Kobe, due to the 1995 Hyogoken- Nambu earthquake. Two types of acoustical surveying technique were employed to characterize the underwater surface around breakwaters No. 6 South and No. 7. A multi-narrow beam sonar was used for geometrical measurements, and a side-scan sonar was used for revealing geological and dynamic features of the seabed. The cross section profiles obtained from surveying with a fan of multi-narrow beams are presented, showing a clear picture of the deformation that occurred in the rubble mound. Several circular depressions were found in the side-scan image showing the seabed about 10 m away from the toe of the rubble mound in breakwater No. 6 South. They appear to be sand boils. By comparing the side-scan images of breakwaters No. 6 South and No. 7 it can be seen that the surcharges over the sand surface should suppress the occurrence of liquefaction to some extent. The surcharge effect is discussed with emphasis on the stability of composite type breakwaters, referring to the results of shaking table tests conducted on the saturated sand deposits with model structures.