of a stable cap (Mettam, 1976; Jensen, 1983). Hydraulic model tests by Carver 

 and Davidson (1976; 1983) have investigated the stability of caps with high 

 parapet walls proposed for Corps structures. 



To evaluate the need for a massive concrete cap to increase structural 

 stability against overtopping, consideration should be given to the cost of 

 including a cap versus the cost of increasing dimensions (a) to prevent 

 overtopping and (b) for construction and maintenance purposes. A massive 

 concrete cap is not necessary for the structural stability of a structure 

 composed of concrete armor units when the difference in elevation between the 

 crest and the limit of wave runup on the projected slope above the structure 

 is less than 15 percent of the total wave runup. For this purpose, an all- 

 rubble structure is preferable, and a concrete cap should be used only if 

 substantial savings would result. Maintenance costs for an adequately 

 designed rubble structure are likely to be lower than for any alternative 

 composite-type structure. 



The cost of a concrete cap should also be compared to the cost of covering 

 the crest with flexible, permeable concrete armor units, perhaps larger than 

 those used on the slopes, or large quarrystone armor. Bottin, Chatham, and 

 Carver (1976) conducted model tests on an overtopped breakwater with dolos 

 armor on the seaward slope, but with large quarrystone on the crest. The 

 breakwater at Pria, Terceria, Azores, was repaired using large quarrystone 

 instead of a concrete cap on the crest to support the primary tetrapod armor 

 units. Two rows of large armor stones were placed along the shoreward side of 

 the crest to stabilize the top row of tetrapods. An inspection in March 1970 

 indicated that this placement has performed satisfactorily even though the 

 structure has been subjected to wave overtopping. 



Hydraulic model tests are recommended to determine the most stable and 

 economical crest designs for major structures. 



Experience indicates that concrete placed in the voids on the structure 

 slopes has little structural value. By reducing slope roughness and surface 

 porosity, the concrete increases wave runup. The effective life of the 

 concrete is short, because the bond between concrete and stone is quickly 

 broken by structure settlement. Such filling increases maintenance costs. 

 For a roadway, a concrete cap can usually be justified if frequent maintenance 

 of armored slopes is anticipated. A smooth surface is required for wheeled 

 vehicles; tracked equipment can be used on ribbed caps. 



(3) Thickness of Armor Layer and Underlayers and Number of Armor 

 Units . The thickness of the cover and underlayers and the number of armor 

 units required can be determined from the following formulas: 



r = n k / ::-\^/^ (7-121) 



A 



ikj 



where r is the average layer thickness in meters (or feet), n is the 

 number of quarrystone or concrete armor units in thickness comprising the 

 cover layer, W is the mass of individual armor units in kilograms (or weight 

 in pounds), and w^ is the mass density in kilograms per cubic meter (or unit 

 weight in pounds per cubic foot). The placing density is given by 



7-236 



