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Using an assumed specific gravity of 2.87, a void ratio of 1.0, W = 33 and <br />data presented by Wilkins for specific surface, Leps calculated the values <br />shown in Table 4.3. <br />TABLE 4.3 <br />Rock Size Coefficients <br />Rock Size m m0.5 Wm0.5 <br />(in.) (in.) (in.l/2) (in/sec) <br />3/4 0.09 0.30 10 <br />2 0.24 0.49 16 <br />6 0.75 0.87 28 <br />8 0.96 0.98 32 <br />24 3.11 1.76 58 <br />48 6.43 2.54 84 <br />Leps reports that for free surface flow to exist in rockfill, i can never <br />be greater than 1.0, or piping will occur. For a gradient of 0.5, using <br />Eq. 4:7 and Table 4.3, the estimated void velocities for 3/4-inch gravel <br />and 24-inch rock are 0.6 and 3.3 feet per second, respectively. <br />Equation 4.1 was developed to predict flow through a uniform sized rock. <br />Spoil and associated underdrains will likely never meet this assumption. <br />Leps (1913) recognized this shortcoming and proposed that calculations be <br />based upon the median rock size (D50) which can reasonably be considered <br />as dominant for flow situations. This assumption is based upon less than <br />30, and preferably less than 10 percent minus 1-inch material. If more <br />than 30 percent minus 1-inch particles are present, Leps proposed treating <br />the material as a soil using Oarcy's equation. <br />The quantity of flow which may pass through rock-like spoil ~or a subdrain <br />system with less than 30 percent minus 1-inch particles may be estimated <br />as: <br />Q = A Vv n (Eq. 4.8) <br />-138- <br />