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<br />10 <br /> <br />cobbles and pebbles, both for ~~e pervious portion of the embankment <br /> <br />and the aggregate for concrete. The presence of fine sand and silt <br /> <br />in this material may be a cause for concern if it is not free-draining. <br /> <br />COarse material is not available in the immediate vicinity of <br /> <br />the dam site. The use of more remote sources of coarse aggregate will <br /> <br />depend upon transportation costs and the physical characteristics of <br /> <br />the rock, specifically, how they would blend with the pervious materials. <br /> <br />Riprap will have to be shipped in from Rocky Mountain sources <br />where hard and durable granite is available. <br /> <br />ITEMS TO CONSIDER <br />1. A three-dimensional model should be made to determine the <br /> <br /> <br />beneficial effects of a cutoff wall acting simultaneously with pressure <br /> <br /> <br />relief wells. Good computer simulation modeling capability exists for <br /> <br /> <br />such studies at several places. <br /> <br />2. A model for studying the local transient effects of inflow <br /> <br />during flood stages would be most helpful in determining maximum <br /> <br />gradients and the duration of peak gradient conditions. <br /> <br />3. The Pierre Shale varies enormously in physical characteristics <br /> <br />from place to place at the dam site. It has a notorious reputation for <br /> <br />swelling under certain conditions and slaking when exposed to air and <br /> <br />water. Field tests at several localities, including pits or trenches, <br /> <br />would provide more data on which to base the design of various <br /> <br />structures. <br /> <br />4. A continuous seismic profile along the dam axis defining the <br />