concrete armor units Is that they usually have a higher stability coefficient 

 value and thus permit the use of steeper structure side slopes or a lighter 

 weight of armor unit. This advantage has particular value when quarrystone of 

 the required size is not available. 



Table 7-10 lists the concrete armor units that have been cited in 

 literature and shows where and when the unit was developed. One of the 

 earlier nonblock concrete armor units was the tetvapod, developed and patented 

 in 1950 by Neyrpic, Inc., of France. The tetrapod is an unreinforced concrete 

 shape with four truncated conical legs projecting radially from a center point 

 (see Fig. 7-108). 



Figure 7-109 provides volume, weight, dimensions, number of units per 1000 

 square feet, and thickness of layers of the tetrapod unit. The quadripod 

 (Fig. 7-108) was developed and tested by the United States in 1959; details 

 are shown in Figure 7-110. 



In 1958, R. Q. Palmer, United States, developed and patented the tvihav. 



This concrete shape consists of three cylinders connected by three radial arms 



(see Fig. 7-108). Figure 7-111 provides details on the volume, dimensions, 

 and thickness of layers of tribars. 



The dolos armor unit, developed in 1963 by E. M. Merrifield, Republic of 

 South Africa (Merrifield and Zwamborn, 1966), is illustrated in Figure 

 7-108. This concrete unit closely resembles a ship anchor or an "H" with 

 one vertical perpendicular to the other. Detailed dimensions are shown in 

 Figure 7-112. 



The toskane is similar to the dolos, but the shapes at the ends of the 

 central shank are triangular heads rather than straight flukes. The tri- 

 angular heads are purported to be more resistant to breakage than the dolos 

 flukes. A round hole may be placed through each head to increase porosity. 

 Dimensions are shown in Figure 7-113. 



As noted in Table 7-8, various other shapes have been tested by the Corps 

 of Engineers. Details of the modified aube and hexapod are shown in Figures 

 7-114 and 7-115, respectively. 



As noted, the tetrapod, quadripod, and tribar are patented, but the U.S. 

 patents on these units have expired. Patents on these units may still be in 

 force in other countries, however; payment of royalties to the holder of the 

 patent for the use of such a unit is required. Since other units in Table 

 7-10 may be patented, in the U.S. or elsewhere, the status of patents should 

 be reviewed before they are used. 



Unlike quarrystone, concrete armor units have a history of breakage 

 problems. If a unit breaks, its weight is reduced; if enough units break, the 

 stability of an armor layer is reduced. For dolosse, for instance, model 

 tests by Markel and Davidson (1984a) have demonstrated that random breakage of 

 up to 15 percent or up to 5 broken units in a cluster will have little effect 

 on stability. Breakage exceeding these limits may lead to catastrophic 

 failure of the armor layer. 



7-215 



