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<br />EM 1110-2-1601 <br />. 1 July 70 <br /> <br />"Y <br /> <br />, <br /> <br />resistant to movement. The stone should have sharp, clean edges at the <br />{ tersections of relatively flat faces. Cobbles with rounded edges are less <br />\,,,- <br />resistant to movement, although the drag force on a rounded stone is less <br />than on sharp-edged, cubical stones. As the internal friction angle of a <br />graded mass of cobbles is less than an equal mass of cubical stones, the <br />cobble mass is more likely to be eroded by channel flow. The following shape <br />limitations should be specified for riprap obtained from quarry operations: <br />. (1) The stone shall be predominantly angular in shape. <br />(2) Not more than 25 percent of the stones reasonably well distributed <br />throughout the gradation shall have a length more than 2.5 times the breadth <br />or thickness. <br /> <br />(3) No stone shall have a length exceeding 3.0 times its breadth or <br />thickness. <br />These limitations apply only to the stone within the riprap gradation and not <br />to any qua.rry spalls and waste that may be allowed. When a high percentage <br />of quarry wastes or spalls is allowed or quarry run stone is used that does <br />not meet the above-listed limitations, an increase in riprap thiclmess should <br />b.\!.provided as prescribed in i below. <br />f <br />. d. Stone Size and Weil1;ht. The ability of riprap revetment to resist <br />erosion is related. to the size and weight of stones. Design criteria may be <br />expressed in terms of the stone weight, W 50' where the numerical subscript <br />denotes the percentage of the total weight of the graded material, including <br />quarry wastes and spalls, that contains stones of les s weight. The relation <br />between size and weight of stone may be expressed by <br /> <br />D s = U~ r/3 <br /> <br />where <br /> <br />D = equivalent-volume spherical stone diameter, ft <br />s <br />W = ston~ weight, lb <br /> <br />y = saturated surface dry (SSDl specific weight of stone, pc! <br />s <br /> <br />, <br /> <br />14d <br /> <br />38 <br /> <br />"-",&/ <br /> <br /> <br />., <br />