the seaward side and may be incorrect for leeside concrete or quarrystone 

 units (Merrifield, 1977; Lillevang, 1977). The presence of a concrete cap 

 will also affect overtopping forces on the lee side in ways that must be 

 quantified by modeling. When both side slopes receive similar wave action (as 

 with groins or jetties), both sides should be of similar design. 



(7) Secondary Cover Layer . If the armor units in the primary and 

 secondary cover layers are of the same material, the weight of armor units in 

 the secondary cover layer, between -1.5 H and -2.0 H, should be greater than 

 about one-half the weight of armor units in the primary cover layer. Below 

 -2.0 H, the weight requirements can be reduced to about W/15 for the same 

 slope condition (see Fig. 7-116). If the primary cover layer is of 

 quarrystone, the weights for the secondary quarrystone layers should be 

 ratioed from the weight of quarrystone that would be required for the primary 

 cover layer. The use of a single size of concrete armor for all cover layers- 

 -i.e., upgrading the secondary cover layer to the same size as the primary 

 cover layer — may prove to be economically advantageous when the structure is 

 located in shallow water (Fig. 7-117); in other words, with depth d ^ 1.5 H , 

 armor units in the primary cover layer should be extended down the entire 

 slope. 



The secondary cover layer (Fig. 7-116) from -1.5 H to the bottom should 

 be as thick as or thicker than the primary cover layer. For cover layers of 

 quarrystone, for example, and for the preceding ratios between the armor 

 weight W in the primary cover layer and the quarrystone weight in the 

 secondary cover layers, this means that if n = 2 for the primary cover layer 

 (two quarrys tones thick) then n = 2.5 for the secondary cover layer from 

 -H to -2.0 H and n = 5 for that part of the secondary cover layer below 

 -2.0 H . 



The interfaces between the secondary cover layers and the primary cover 

 layer are shown at the slope of l-on-1.5 in Figure 7-116. Steeper slopes for 

 the interfaces may contribute to the stability of the cover armor, but 

 material characteristics and site wave conditions during construction may 

 require using a flatter slope than that shown. 



(8) Underlayers . The first underlayer directly beneath the primary 

 armor units should have a minimum thickness of two quarrystones Cn = 2) (see 

 Figs. 7-116 and 7-117). For preliminary design these should weigh about one- 

 tenth the weight of the overlying armor units (W/10) if (a) the cover layer 

 and first underlayer are both quarrystone, or (b) the first underlayer is 

 quarrystone and the cover layer is concrete armor units with a stability 

 coefficient I^ < 12 (where 1^ is for units on a trunk exposed to 

 nonbreaking waves). When the cover layer is of armor units with IC > 12 , 

 such as dolosse, toskanes, and tribars (placed uniformly in a single layer), 

 the first underlayer quarrystone weight should be about W/5 or one-fifth the 

 weight of the overlying armor units. The larger size is recommended to 

 increase interlocking between the first underlayer and the armor units of 

 high 1^ . Carver and Davidson (1977) and Carver (1980) found, from hydraulic 

 model tests of quarrystone armor units and dolosse placed on a breakwater 

 trunk exposed to nonbreaking waves, that the underlayer stone size could range 

 from W/5 to W/20 , with little effect on stability, runup, or rundown. If 

 the underlayer stone proposed for a given structure is available in weights 

 from W/5 to W/20 , the structure should be model tested with a first 

 underlayer of the available stone before the design is made final. The tests 



7-239 



