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3-3 <br />2.2.2. The vertical permeability is estimated to be equal to the <br />horizontal permeability based on the rapid drop in piezometric head <br />between piezometers along the upstream dam crest and the piezometers <br />along the downstream dam crest (see Figure 5). The lack of apparent <br />flow in the blanket drain at the toe of the dam indicates that seepage <br />movement is not preferentially in the horizontal direction, further <br />indicating the apparent relationship of vertical and horizontal perme- <br />abilities as previously discussed. <br />Permeability of the upstream shell of the dam is estimated to be an <br />order of magnitude greater than the core material (50 x 10-5 cm/sec). <br />This provides consistency in the observed low head loss between the pond <br />4 and the piezometers along the upstream crest of the embankment. <br />1 3.2 DAM FOUNDATION <br />3.2.1 Materials <br />The foundation conditions for the model were established as (a) alluvial <br />soils from 4 to 36 feet thick as shown in Figure 3, overlying (b) <br />weathered and fractured bedrock varying from 7 to 17 feet, underlain by <br />(c) relatively intact bedrock. <br />The alluvial soils are very heterogeneous, ranging from silty and clayey <br />sands to clean sands and open work gravels. The key trench is not <br />excavated completely through the alluvial sail layer along the entire <br />dam axis. The clean sands and open work gravel zones in the alluvial <br />soil layer may be entirely natural or they may have been partially <br />developed by the seepage flows moving beneath the dam. <br />The variation in alluvial soil thickness and its irregularity as shown <br />in Figure 3 provides the potential for differential settlement. <br />Differential settlement in the foundation can result in tensile stress <br />In the overlying embankment. Sufficient settlement to cause tensile <br />