LOADINGS ON RUBBLE-MOUND BREAKWATERS DUE TO EARTHQUAKES

Abstract

It might reasonably be asked, why study the reaction of a rubblemound breakwater to earthquake loadings? After all, it is essentially a pile of rubble before an earthquake and is probably only a lower or otherwise deformed pile of rubble afterwards. Repairs are simply a matter of adding more stones. Even if damage occurred, the breakwater might still offer partial protection. This is precisely why, to date, breakwaters are mainly designed for wave loadings. There is practically no documented literature'^-' concerning breakwater design for earthquake loading! However, the oceans are now being tapped as possible locations for industrial installations such as offshore deep water ports, refineries, and power plants. Such facilities must be adequately protected since failures might result in heavy financial losses and cause severe environmental repercussions. Therefore, breakwaters which might be used for this protection can no longer be treated as structures whose failure would only be of secondary consequence. It is thus reasonable to ask whether breakwaters to serve these purposes should also be designed against earthquake loading, since even partial failures might not be acceptable. If they should, then what type of design problems can one expect to encounter and how should one handle them? The present work is aimed at exploring these problems through laboratory experiments. At present, there is no existing breakwater that has been designed on the basis of earthquake loading. Likewise, observations of breakwaters after continuous earthquakes are also scarce. Okamoto'2' reports crumbling of riprap, uneven settlement and loosening or tilting of the upper parts of Japanese breakwaters that have undergone earthquakes. After the large Kanto earthquake of 1927, for instance, breakwaters at the ports of Yokosuka and Yokohama approximately 50 kilometers from the epicenter developed irregularities over their entire lengths. In no cases, however, did any of the breakwaters topple or overturn. During the great Alaska earthquake of 1964, breakwaters at Kodiak City and Seldovia, Kenai Peninsula were badly damaged. The damage to the Kodiak City breakwaters was well documented'3'with photographs and surveys taken after the earthquake. The damage was attributed to both earthquake motion and the tsunamis which swept the area.