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As expected, the permeability tests indicate the fill as a whole is relatively free <br />draining. For comparison, the following permeability values are representative of typical <br />materials: <br />Clay and fine silts less than 10 feet per year <br />Coarse silt and fine sand 40 to 2,000 feet per year <br />Medium to coarse sand 3,000 to 400,000 feet per year <br />Fine gravel more than 500,000 feet per year <br />Tests at very shallow depths show lower permeabilities because of compaction by <br />truck traffic. At intermediate depths permeabilities are very high, and at greater depths <br />are somewhat less because of consolidation due to the weight of the overlying fill. <br />Permeabilities near the base of the fill are so high that meaningful values cannot be <br />obtained by field tests. <br />Natural sorting of materials in the dumping process results in the largest and <br />most competent rock fragments being placed in the base of the fill resting on the pervious <br />clinker and alluvium of the valley floor, as well as at the base of each lift. The average <br />porosity (void volume) of the fill mass is probably in the range of 30 to 40 percent and <br />much greater than these values in the lower portion of each lift. Assuming that all the <br />annual runoff of Streeter Gulch percolates into the embankment, is impounded therein, <br />and exits at an essentially uniform rate as groundwater discharge, an annual inflow of <br />100 acre feet is equivalent to a discharge rate of about 60 gallons per minute, or less <br />than 0.2 gpm per foot width of the canyon in its narrowest reach. It is apparent that it <br />would be physically impossible to saturate the mass of the embankment, a consideration <br />that is important to an analysis of its stability. <br />Stability of the Embankment <br />The nature of the materials forming the spoil fill, the manner in which they are <br />being placed, the hydrology of the area, and the manner in which surface runoff will <br />be handled eliminate any possible concern regarding the mass stability of the completed <br />embankment. As noted earlier, the natural angle of the repose of material being placed <br />in the embankment is in the order of 38° (equivalent to a slope of approximately 1 vertical <br />6 <br />