(2) Concrete Cap for Rubble-Mound Structures . Placed concrete has 

 been added to the cover layer of rubble-mound jetties and breakwaters. Such 

 use ranges from filling the interstices of stones in the cover layer, on the 

 crest, and as far down the slopes as wave action permits, to casting large 

 monolithic blocks of several hundred kilograms. This concrete may serve any 

 of four purposes: (a) to strengthen the crest, (b) to deflect overtopping 

 waves away from impacting directly on the lee side slope, (c) to increase the 

 crest height, and (d) to provide roadway access along the crest for construc- 

 tion or maintenance purposes. 



Massive concrete caps have been used with cover layers of precast concrete 

 armor units to replace armor units of questionable stability on an overtopped 

 crest and to provide a rigid backup to the top rows of armor units on the 

 slopes. To accomplish this dual purpose, the cap can be a slab with a solid 

 or permeable parapet (Czerniak and Collins, 1977; Jensen, 1983; and Fig. 6-64, 

 (see Ch. 6)), a slab over stone grouted to the bottom elevation of the armor 

 layer (Figs. 6-60 and 6-63, or a solid or permeable block (Lillevang, 1977, 

 Markle, 1982, and Fig. 6-65)). 



Concrete caps with solid vertical or sloped walls reflect waves out 

 through the upper rows of armor units, perhaps causing loss of those units. 

 Solid slabs and blocks can trap air beneath them, creating uplift forces 

 during heavy wave action that may crack or tip the cap (Magoon, Sloan, and 

 Foote, 1974). A permeable cap decreases both of these problems. A parapet 

 can be made permeable, and vertical vents can be placed through the slab or 

 block itself (Mettam, 1976). 



Lillevang (1977) designed a breakwater crest composed of a vented block 

 cap placed on an unchinked, ungrouted extension of the seaward slope's under- 

 layer, a permeable base reaching across the crest. Massive concrete caps must 

 be placed after a structure has settled or must be sufficiently flexible to 

 undergo settlement without breaking up (Magoon, Sloan, and Foote, 1974). 



Ribbed caps are a compromise between the solid block and a covering of 

 concrete armor units. The ribs are large, long, rectangular members of 

 reinforced concrete placed perpendicular to the axis of a structure in a 

 manner resembling railroad ties. The ribs are connected by reinforced 

 concrete braces, giving the cap the appearance of a railroad track running 

 along the structure crest. This cap serves to brace the upper units on the 

 slopes, yet is permeable in both the horizontal and vertical directions. 

 Ribbed caps have been used on Corps breakwaters at Maalea Harbor (Carver and 

 Markle, 1981a), at Kahului (Markle, 1982), on Maui, and at Pohoiki Bay, all in 

 the State of Hawaii. 



Waves overtopping a concrete cap can damage the leeside armor layer 

 (Magoon, Sloan, and Foote, 1974). The width of the cap and the shape of its 

 lee side can be designed to deflect overtopping waves away from the 

 structure's lee side (Czerniak and Collins, 1977; Lillevang, 1977; and Jensen, 

 1983). Ribbed caps help dissipate waves. 



High parapet walls have been added to caps to deflect overtopping seaward 

 and allow the lowering of the crest of the rubble mound itself. These walls 

 present the same reflection problems described above and complicate the design 



7-235 



