1. Armor Layer . 



Stone used in the armor layer should be hard and durable. Experience is 

 the best guide in choosing a durable stone. Whenever possible, stone which 

 has proven to be- satisfactory on earlier, similar projects should be used. 

 Persons familiar with local quarries can often provide information on stone 

 quality. Esmiol's (1968) study of rock used to protect the upstream slope of 

 earth dams concluded that granite or granitic-type rock is the best for riprap 

 and that the best means to evaluate durability before use are by a specific 

 gravity test, an absorption test, and a petrographic analysis. A recent sur- 

 vey of riprap stone quality by M.L. Giles (Research Hydraulic Engineer, U.S. 

 Army Engineer District, Kansas City, personal communication, 1979) indicates 

 that there are, at present, no foolproof tests which can give assurance of 

 rock durability, but that the specific gravity test is the single, most re- 

 liable method. 



Thomsen, Wohlt, and Harrison (1972) found that the gradation of stone used 

 in riprap had little influence on stability when the median weight, W50, was 

 used to characterize the stone size. Following Thomsen, Wohlt, and Harrison 

 (1972), this report uses W50 to characterize stone size. Their laboratory 

 tests of riprap stability included both narrow and wide stone gradations but 

 only a few tests were conducted with a gradation ratio, Wgs/W^, greater 

 than 8.0 (W35 is the weight of an armor stone where 85 percent of the total 

 weight of the gradation is contributed by stones of lesser weight; Wj 5 is 

 the corresponding weight for the 15-percentile stone) . Prototype-scale riprap 

 stability tests conducted by Ahrens (1975) used the stone gradation specified 

 in EM 1110-2-2300 (U.S. Army Corps of Engineers, 1971) and referred to as the 

 "EM" gradation. Portions of EM 1110-2-2300 have been superseded by ETL 1110- 

 2-222 (U.S. Army Corps of Engineers, 1978). The EM gradation specifications 

 for the maximum and minimum stone weights are 



Wmax = 4W 5 



Wmin = O.125W 50 



Ahrens established the following approximate empirical relations for the EM 

 gradation: 



W = 0.75W 50 



£§* « 4.9 

 W15 

 and 



W 15 = O.4W 50 (1) 



where W is the average weight of the riprap armor stone. Fully mixed, wide 

 gradations are probably as stable to wave attack as narrow gradations with the 

 same W50; however, gradations where the ratio Wqs/W^ exceeds 8.0 are not 

 recommended due to the shortage of data on their performance. The advantages 

 of a wide gradation over a narrow gradation are that a larger percentage of the 

 quarry-run stone can be used and that the filter layer-size criteria can be met 



