This rise to and fall from peak conditions over many hours, sometimes days, is 

 typical for severe storms in most areas of the world. The peak condition is 

 typically applied in extremal analyses, but the duration of conditions above a 

 threshold related to the stability of a proposed structure is also important. 

 Simulation of many hours (or many thousands of waves) is performed as standard 

 practice for breakwater stability tests by a number of prominent laboratories 

 (Owen and Allsop 1984 and Van der Meer and Pilarcyzk 1984). The effect of 

 duration on breakwater stability is discussed by Graveson, Jensen, and 

 Sorensen (1980), Jensen (1984) and Bruun (1985). 

 Foundation considerations . 



30. The weight of a rubble-mound breakwater and the hydraulic effects 

 it causes near its foundation are potential factors which can lead to a struc- 

 tural failure. Investigation of gravity related stability problems, such as 

 slip failure of the foundation or excessive (possibly differential) settle- 

 ment, requires the attention of a geotechnical specialist. Hydraulic problems 

 such as scour at the toe must be addressed in the earliest stages of design. 

 The suitability of a natural foundation and the possibilities for preventive 

 measures can ultimately determine the feasibility of constructing an entire 

 breakwater. Excavation of poor foundation materials and replacement with fill 

 or artificial improvement of the strength of natural materials can amount to a 

 substantial fraction of the project cost. The need to place filter materials 

 or other scour protection along a breakwater can also substantially constrain 

 the geometry of the armor and underlayers. Seismic stability analyses in 

 areas subject to earthquakes should be performed. All of these geotechnical 

 considerations require extensive field data consisting of numerous borings 



supplemented by acoustic surveys and penetrometer tests. 



Primary armor 



31. During the past 40 years many lengthy journal articles, textbook 

 chapters, and conference papers have been written on the subject of armor de- 

 sign for rubble-mound breakwaters. A discussion of the entire multitude of 

 practical considerations applicable to armor design would be beyond the scope 

 of this report. A comprehensive review is available by Baird and Hall (1984) 

 in which many of the most important factors in armor design are discussed. 

 Rubble-mound breakwaters have a tendency to be designed from the top down be- 

 cause the exigencies of design and construction of those portions exposed to 

 direct wave attack tend to constrain all other features. The stability 



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