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When the abrasion test in the Los Angeles machine (AASHO Test T 96) is used, the stone should have a percentage loss of not more than 40 atter 500 revolutions. When the treezill8 and thawing test (AASHO Test T 103 . for ledge rock, Procedure A) is used, the stone should have a loss not exceeding 10 percent after 12 cycles at freezing and thawing. The limits given here for the tests should be checked by testing local rock that has given good service when used as riprap under similar environ- mental conditions. 3.1.1 - Gradation of Stone Many specifications for riprap contain no provisions for controlling gradation of the riprap other than a requirement that at least a given percentage of the stones should be heavier than a stated weight. Some specifications require the larger stone to be relatively unHorm in size. Failure to require well-graded stone may result in a blanket with large voids that will allow the embankment or the filter material to be with- drawn through the riprap by the action of the water. The superiority of a dense mass of well-graded angular stone over a mass of large stone with correspondingly large voids has been demonstrated by observation of completed installations and by tests at the Corps at Engineers Waterways Experiment Station (,2). The tests were made to deter- mine the size of rock needed to protect the slopes of overflow dikes on the Arkansas River. Two gradations, "A" and "A1" (tigure 8), failed under the same conditions although gradation "A" had maximum pieces 36 inches in equivalent diameter (2,300 pounds) as opposed to maximum pieces 24 inches in equivalent diueter (700 pounds) for the gradation "A1". The 50 per- cent stone size of each gradation was 16 inches (200 poundS). Two other gradations, "B" and "c" (figure 8), failed under the same conditions. Gradation "c" had maximum pieces 24 inches (700 pounds) in equivalent diameter and gradation "B" had maximum pieces 16 inches (200 pounds) in equivalent diameter. However, 75 percent ot each of these gradations consisted of stone 10 inches (50 pounds) in equivalent diameter or smaller. In the model tests, the large pieces were dislodged by undercutting resulting frlllll the removal of the smaller pieces. Murphy and Grace (,2) concluded that pieces of stone larger than those which represented same critical size (the 60 to 65 percent size in these tests) do not increase the effectiveness at the particular gradation. In figure 8 both gradation curves "A1" (24") and "B" (16") were found to be highly satisfactory by the Waterways Experiment Station. Curves approximately parallel to these curves and passing through the theoretical size (tigure 2) at the 50 percent point should make an acceptable gradation. Unless the quantity of riprap used at a particular location is large, it might prove undesirable to specify separate gradatiOns to fit conditions at each site. Several classes of riprap might be defined in the specifications and a suitable class selected for conditions at a particular site. 11-20 . . . , . .