courses. Side slopes and armor unit sizes are designed so that the structure 

 will resist the expected wave action. Rubble-mound jetties (see Figs. 6-60 

 and 6-61), which are used extensively, are adaptable to any water depth and to 

 most foundation conditions. The chief advantages are as follows: structure 

 settling readjusts component stones which increases stability, damage is 

 repairable, and the rubble absorbs rather than reflects wave action. 

 The chief disadvantages are the large quantity of material required, the high 

 initial cost of satisfactory material if not locally available, and the wave 

 energy propagated through the structure if the core is not high and 

 impermeable. 



Where quarrystone armor units in adequate quantities or size are not 

 economical, concrete armor units are used. Chapter 7, Section III,7,f dis- 

 cusses the shapes that have been tested and are recommended for considera- 

 tion. Figure 6-60 illustrates the use of quadripod armor units on the rubble- 

 mound jetty at Santa Cruz, California. Figure 6-61 illustrates the use of the 

 more recently developed dolos armor unit where 38- and 39- metric ton (42- and 

 43- short ton) dolos were used to strengthen the seaward end of the Humboldt 

 Bay, California, jetties against 12-meter breaking waves (Magoon and Shimizu, 

 1971). 



b. Sheet-Pile Jetties. Timber, steel, and concrete sheet piles are used 

 for jetty construction where waves are not severe. Steel sheet piles are used 

 for various jetty formations which include the following: a single row of 

 piling with or without pile buttresses; a single row of sheet piles arranged 

 to function as a buttressed wall; double walls of sheet piles, held together 

 with tie rods, with the space between the walls filled with stone or sand 

 (usually separated into compartments by cross walls if sand is used) ; and 

 cellular-steel sheet-pile structures (see Fig. 6-62), which are modifications 

 of the double-wall type. 



Cellular-steel sheet-pile structures require little maintenance and are 

 suitable for construction in depths to 12 meters on all types of founda- 

 tions. Steel sheet-pile structures are economical and may be constructed 

 quickly, but are vulnerable to storm damage during construction. If coarse 

 aggregate is used to fill the structure, the life will be longer than with 

 sandfill because holes that corrode through the web have to become large 

 before the coarse aggregate will leach out. Corrosion is the principal 

 disadvantage of steel in seawater. Sand and water action abrade corroded 

 metal near the mudline and leave fresh steel exposed. The life of the piles 

 in this environment may not exceed 10 years. However, if corrosion is not 

 abraded, piles may last more than 35 years. Plastic protective coatings and 

 electrical cathodic protection have effectively extended the life of steel 

 sheet piles. However, new alloy steels are most effective if abrasion does 

 not deteriorate their protective layer. 



VIII. BREAKWATERS, SHORE- CONNECTED 



1. Types. 



Variations of rubble-mound designs are generally used as breakwaters in 

 exposed locations. In less exposed areas, both cellular-steel and concrete 

 caissons are used. Figures 6-63, 6-64, and 6-65 illustrate structural types 

 of shore-connected breakwaters used for harbor protection. 



6-88 



