Massive concrete caps have been used with cover layers of precast 

 concrete armor units. The cap provides a rigid backup to the top row of 

 armor units at the crest. Instead of a concrete cap, solid or permeable 

 parapets have been used. (See Figure 6-68.) The breakwater at Pria, 

 Terceria, Azores was repaired using large quarrystone to support the 

 primary tetrapod armor units instead of the concrete cap on the crest. 

 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. 



To evaluate the need for a massive concrete cap for increasing struc- 

 tural stability against overtopping, consideration should be given to the 

 cost of including a cap versus the cost of increasing dimensions to pre- 

 vent overtopping and for construction and maintenance purposes. For a 

 structure of concrete armor units subject to overtopping, a massive con- 

 crete cap is not necessary for structural stability when the difference 

 in elevation between the crest and the limit of wave runup on the projec- 

 ted slope above the structure is less than 15 percent of the total wave 

 runup, unless a substantial saving would result from the use of concrete. 

 For this purpose, an all-rubble structure is preferable. Maintenance 

 costs for an adequately designed rubble structure are likely to be lower 

 than for any alternative composite type structure. 



Experience indicates that concrete placed in the voids on the struc- 

 ture 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 mainte- 

 nance costs. For a roadway, a concrete cap can usually be justified if 

 frequent maintenance of armored slopes is anticipated. 



Co 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^ — (7-108) 



where r is the total layer thickness in feet, n is the number of 

 layers of quarrystone or concrete armor units comprising the cover layer, 

 W is the weight of individual armor units in pounds, and w^ is the 

 unit weight in pounds per cubic foot. 



N, = A „ k^ (l - jfj (^j (7-109) 



where N^ is the required number of individual armor units for a given 

 surface area, A is surface area in square feet, k* is the layer co- 

 efficient, and P is the average porosity of the cover layer in percent. 



7-197 



