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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 upper part of the Dakota Formation in this area typically <br />consists of interlayered beds of relatively flat lying sandstone, <br />shale, siltstone and coal. The sandstone, which is both strong and <br />brittle relative to the other rock types, is the dominant cliff <br />forming rock in this area and is commonly highly fractured. <br />Sandstones are exposed adjacent to the dam in both the upstream and <br />downstream directions and form prominent shelves on the sides of <br />Gurley Canyon downstream of the dam. The dominant jointing or <br />fracture patterns in the sandstone are typically nearly vertical <br />and nearly horizontal. The near vertical fractures typically <br />display the following orientations: Strike N450W to N750W and <br />Strike N50oE. The dip on the northwest trending is typically <br />vertical to very nearly vertical with a slight inclination to the <br />southwest. The dip on the northwest trending fractures varies as <br />much as 300 from the vertical. The fracture spacings and fracture <br />widths noted in surface exposures in the dam vicinity typically <br />range from 2 inches to' 12 inches and 0.01 inches to 1 inch, <br />respectively, based on our observations. The shale beds which also <br />form the upper part of the Dakota Formation are also exposed <br />adjacent to the dam. The old borrow area located just downstream of <br />the far right abutment of the dam which is believed to have the <br />source of much of the embankment fill during the 1947 raise, may <br />have been developed in one of the softened shale layers. <br /> <br />Rock cores from the dam foundation obtained during the <br />subsurface exploration program contained layers of sandstone, <br />shale, siltstone, claystone and coal. The rock cores were generally <br />highly fractured and produced relatively low values of percent <br />recovery and Rock Quality Designation (RQD). The sandstone strata <br />showed the greatest degree of fracturing in nearly all of the <br />cores. Fracture densities in the sandstone strata near the outlet <br />tunnel have been estimated by SRK to be approximately 2.1 percent <br />of the rock mass based on the visual evaluation of rock cores from <br />boreho1 es BH-l, BH-2 and BH-B. Many of the fractures conta i ned <br />various degrees of limonite or iron oxide staining, indicative <br />generally of the presence of water at some time in the fractures. <br /> <br />-15- <br />