easier (discussed in the next subsection) ; the disadvantage is that the stone 

 may become segregated and some areas of the revetment can be unusually vulner- 

 able to wave attack. 



The thickness of the armor layer should be great enough to accommodate the 

 largest stone in the gradation. To do this, the thickness of the layer must 

 be slightly greater than a typical dimension of the largest stone. A typical 

 dimension may be computed using the cube root of the volume of the stone. For 

 the EM gradation, the typical dimension of the largest stone is 



(VWV /3 . (AW50V/ 3 =1 . 59 /W50Y 

 \ w r / \ w r / ' \w r / 



/3 



where wr is the unit weight of the stone in kilograms per cubic meter. The 

 recommended minimum armor layer thickness, r m i n , was set at twice the typical 

 dimension of the median stone, i.e., 



r mi n-2.o(^) I/3 «) 



Equation (2) provides sufficient thickness to accommodate the largest stone in 

 the EM gradation. EM 1110-2-2300 also recommends that r min be at least 0.30 

 meter (1 foot) . 



Flat and rod-shaped stones should not be used in the riprap armor grada- 

 tion. The lift and drag forces on flat stones and the drag forces on rod- 

 shaped stones are greater in proportion to their weight than the more desirable 

 angular and blocky shapes. Flat and rod-shaped stones may also require a 

 greater armor layer thickness to accommodate them and they do not key in well 

 with the other stones. Stones with a maximum dimension greater than three 

 times their minimum dimension are not recommended for the armor gradation. 



2. Underlayers . 



The stone used in the layer just beneath the armor layer (i.e., the filter 

 layer) should be large enough to prevent removal of stone through the voids in 

 the armor layer by wave action. To describe the required stone-size relation- 

 ship between the armor and filter, it is convenient to use the concept of a 

 typical stone dimension again. Let the typical stone dimension be given by 



W \!/3 

 D* =| — 



where the subscript x indicates the percent of the weight of the total grada- 

 tion contributed by stones of lesser weight. The proper size relationship 

 between the 15-percentile size of the armor and the 85-percentile size of the 

 filter is given by 



D 15 (armor) 

 D 85 (filter) 



< A.O (3) 



