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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />The results of in-situ permeability tests indicate that the <br />penetrated bedrock layers have permeabi1 ities which are generally <br />relatively high near the bedrock surface and which decrease in an <br />irregular manner with increasing depth. The bedrock permeabilities <br />also appear to decrease more rapidly with depth with increasing <br />distance from the walls of Gurley Canyon. Packer test results <br />indicate that measured permeabilities vary from as much as 2,000 <br />ft/yr (2 x 10-3 cm/se~) near the surface to 30D ft/yr (3 x 10-4 <br />cm/sec) at depths of 50 to 60 ft below the top of rock. The highly <br />fractured sandstone layers, particularly near the bedrock surface <br />display the highest measured permeabilities in many cases. <br /> <br />The results of the surface geophysical exploration program <br />indicate that the interlayered sandstones and shales beneath the <br />dam have a slight dip to the east which is evidenced by alternating <br />bands of high and low resistivity terrain when travelling from one <br />end of the dam to the other. The zones of relatively high <br />resistivity, generally interpreted to be outcrops of fractured <br />sandstone layers, are shown as shaded or stippled areas on Figure <br />B. These zones correlate reasonably well with areas of observed <br />foundation seepage. One zone also correlates well with the area of <br />observed historical sinkhole activity. <br /> <br />Based on the visual observations of bedrock outcrops and the <br />results of the subsurface exploration and geophysical exploration <br />programs, we believe that the historical sinkhole activity, <br />including the sinkholes observed over the last decade and the nine <br />sinkholes observed in 19B6, were probably caused by piping of <br />surface and/or embankment soils into and through the heavily <br />fractured sandstone layers composing the right valley wall of <br />Gurley Reservo"ir. Most of the sinkholes apparently terminated in <br />the outlet tunnel. The paths of the remainder of the sinkholes <br />appear to be confined to foundation and/or embankment materials <br />although the path configurations have not been deduced accurately <br />from the field exploration programs. The sandstone strata in this <br />area appear to be more openly jointed than elsewhere in the <br /> <br />-16- <br />