will determine whether this economical material will support a stable primary 

 cover layer of the planned armor units when exposed to the site conditions. 



The second underlayer beneath the primary cover layer and upper secondary 

 cover layer (above -2.0 H) should have a minimum equivalent thickness of two 

 quarrystones; these should weight about one-twentieth the weight of the 

 immediately overlying quarrystones (1/20 x W/ 10 = W/200 for quarrystone and 

 some concrete primary armor units). 



The first underlayer beneath the lower secondary cover layer (below -2.0 

 H) , should also have a minimum of two thicknesses of quarrystone (see Fig. 

 7-116); these should weigh about one-twentieth of the immediately overlying 

 armor unit weight (1/20 x W/15 = W/300 for units of the same material). The 

 second underlayer for the secondary armor below -2.0 H can be as light as 

 W/6,000 , or equal to the core material size. 



Note in the "recommended" section of Figure 7-116 that when the primary 

 armor is quarrystone and/or concrete units with K^j < 12 , the first 

 underlayer and second (below -2.0 H) quarrystone sizes are W/10 to W/15. 

 If the primary armor is concrete armor units with ^d > 12 , the first 

 underlayer and secondary (below -2.0 H) quarrystone sizes are W/5 and W/10 . 



For a graded riprap cover layer, the minimum requirement for the under- 

 layers, if one or more are necessary, is 



15 (cover) — 85 (under) 

 where D,c f cover) ^^ ^^^ diameter exceeded by the coarsest 85 percent of the 

 riprap or underlayer on top and Dgr Cunder') ^^ ""^^ diameter exceeded by the 



coarsest 15 percent of the underlayer or soil below (Ahrens, 1981). For a 

 revetment, if the riprap and the underlying soil satisfy the size criterion, 

 no underlayer is necessary; otherwise, one or more are required. The size 

 criterion for riprap is more restrictive than the general filter criterion 

 given at the beginning of Section III,7,g, above, and repeated below. The 

 riprap criterion requires larger stone in the lower layer to prevent the 

 material from washing through the voids in the upper layer as its stones shift 

 under wave action. A more conservative underlayer than that required by the 

 minimum criterion may be constructed of stone with a 50 percent size of 

 about W50/20. This larger stone will produce a more permeable underlayer, 

 perhaps reducing runup, and may increase the interlocking between the cover 

 layer and underlayer; but its gradation must be checked against that of the 

 underlying soil in accordance with the criterion given above. 



The underlayers should be at least three 50 percent-size stones thick, but 

 not less than 0.23 meter (Ahrens, 1981). The thickness can be calculated 

 using equation (7-123) with a coefficient of 3 rather than 2. Note that, 

 since a revetment is placed directly on the soil or fill of the bank it 

 protects, a single underlayer also functions as a bedding layer or filter 

 blanket. 



(9) Filter Blanket or Bedding Layer . Foundation conditions for 

 marine structures require thorough evaluation. Wave action against a rubble 



7-240 



