Therefore, as design wave heights increase, it becomes more economical to 

 use the more efficient concrete shaped structures such as tetrapods, 

 tribars, and dolos (see Section V). 



(2) Fill Material for Caissons . Due to its density and generally- 

 low cost, stone fill material is frequently used to ballast caissons and 

 sheet-pile cells. Rockfill should be well graded and free of loam and 

 organic material in order to have the highest density and minimize settle- 

 ment or, in the case of a perforated caisson, minimize the loss of material 

 due to currents or wave action. 



(3) Toe Protection . One of the major causes of failure, or struc- 

 tural damage, of breakwaters has been the undercutting of the toe of the 

 structures. When waves impinge on these structures, they not only exert 

 large impact forces on the armor stone, or face of a vertical structure, 

 but they may also impose strong uplift forces on the lower armor stone and 

 toe stone. Thus, the armor stone must be carried to sufficient depths to 

 resist these forces. An additional problem is the turbulence created in 

 these depths, particularly in the case of waves breaking directly on the 

 structure. This can create scour of the sandy bottom and result in under- 

 mining the toe stone resulting in general collapse of the armor layer and 

 exposure of the smaller stone of the underlayers. This can be controlled 

 either by carrying the armor and bedding layers to sufficient depth or the 

 toe section can be overbuilt in anticipation of the quarrystone settling 

 into scour holes . The design of such toe protection is dependent upon wave 

 height and relative depths of the toe protection as compared to the depth 

 of the natural bottom (U.S. Army, Corps of Engineers, CERC, 1977). The 

 same care must be taken as in the design of the rubble structure. 



b. Shore-Connected Structures . 



(1) Breakwaters, Jetties, and Groins . 



(a) General . The primary difference between a breakwater and 

 a jetty or a groin is that the jetty or groin must have a sand- tight core 

 in order to prevent the passage of littoral materials or currents through 

 the structure, whereas breakwaters may be designed to be either permeable 

 or impermeable. All these structures may be subjected to very large 

 breaking or nonbreaking waves. Overtopping of breakwaters and groins in 

 the breaker or uprush zone may be acceptable, but overtopping of jetties 

 must be restricted to prevent passage of sand into navigation channels. 

 Breakwaters connected to shore would otherwise be designed to use stone in 

 the same manner as offshore breakwaters. 



(b) Jetties . Jetties are usually constructed from the shore- 

 line through the breaker zone seaward to 12- to 18-meter (40 to 60 foot) 

 depths. They are generally perpendicular to the shoreline. However, due 

 to perhaps as much as a 30 skew, or because of variable wave directions, 

 their alinement may vary from to 90 from the direction of wave travel. 

 Because of variable depths, different parts of the structure may be exposed 

 to unbroken, breaking, or broken waves. Thus with careful design, the 

 elevation, total cross section, and size of armor rock can be varied to 

 produce an economical structure. As jetties are used to define harbor or 

 river access to the sea they may be subjected to major tidal or river 



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