will resist the expected wave action. Rubble-mound jetties illustrated 

 in Figures 6-64 and 6-65 are adaptable to any depth of water and most 

 foundation conditions. Rubble-mound structures are used extensively. 

 Chief advantages are: settlement of the structure results in readjust- 

 ment of component stones, and increased stability, rather than in failure 

 of the structure, damage is easily repaired, and rubble absorbs rather 

 than reflects wave action. Chief disadvantages are: the large quantity 

 of material required, the high first cost if satisfactory material is not 

 locally available, and the wave energy propagated through the structure 

 if the core is not high and impermeable. 



Where rock armor units in adequate quantities or size are not eco- 

 nomically available, concrete armor units are used. Section 7.376, 

 Concrete Armor Units, discusses the shapes that have been tested and are 

 available. Figure 6-64 illustrates the use of Quadripod armor units on 

 the rubble-mound jetty at Santa Cruz, California. Figure 6-65 illustrates 

 the use of the more recently developed Dolos armor unit where 42- and 43- 

 ton doles were used to rehabilitate the seaward end of the Humboldt Bay 

 jetties against 40-foot breaking waves. (Magoon and Shimizu, 1971). 



6.712 Sheet-Pile Jetties . Timber, steel and concrete sheet-piles have 

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

 piles are used for jetties in various ways. These include: a single row 

 of piling with or without pile buttresses, a single row of sheet-piles 

 arranged so that the row of piles acts 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 which are 

 modifications of the double-wall type. An example of a cellular steel 

 sheet-pile jetty is shown in Figure 6-66. 



Cellular steel sheet-pile structures require little maintenance and 

 are suitable for construction in depths to 40 feet 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 stone 

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

 filling, because holes that corrode through the web have to be big before 

 the stone will leach out. Corrosion is the principal disadvantage of steel 

 in sea water. Sand and water action abrade corroded metal and leave fresh 

 steel exposed. The life of piles in this environment may not exceed 10 

 years. However, if corrosion is left undisturbed, piles may last more 

 than 35 years. Plastic protective coatings and electrical cathodic pro- 

 tection have effectively extended the life of steel sheet-piles. 



6.8 BREAKWATERS- -SHORE-CONNECTED 



6.81 TYPES 



In exposed locations, breakwaters are generally some variation of a 

 rubble-mound structure. In less severe exposures, both cellular steel 



6-88 



