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<br />OD2251 <br /> <br />Durango Pumoing Plant - Stage] <br /> <br />1.08 SWITCHYARD GEOLOGY <br /> <br />~ <br /> <br />A. <br /> <br />No subsurface explorations were perfonned in the switchyard area. Adjacent drill holes, <br />including DH- 108, indicate bedrock ofthe Point Lookout Sandstone underlies a thin <br />cover of alluvium, terrace gravels, and fill in this area. Some excavation and fill was <br />done in this area during UMTRA cleanup after the drilling ofDH-108. Data from <br />adjacent drill holes indicates that the alluvial materials and fill range from approximately <br />6 to IO feet in thickness overlying the top of bedrock. The bedrock surface within the <br />switchyard generally slopes toward the east, ranging from slightly above elevation 6475 <br />along the southwestern perimeter, flattening out toward the northeast where it averages <br />approximate elevation 6465. <br /> <br />B. The Point Lookout Sandstone consists of interbedded siltstone and sandstone in the <br />vicinity of the switchyard. The siltstone is occasionally sandy, gray to dark gray, and <br />laminated to very thinly bedded. It is moderately soft and is slightly weathered to about 5 <br />feet below bedrock surface then slightly weathered to fresh. The sandstone interbeds are <br />quartzose, fine grained, light gray, moderately hard, and laminated to moderately bedded. <br /> <br />C. Groundwater slopes to the northeast throughout the switchyard area, ranging from about <br />elevation 6455 on the northwestern side to about elevation 6465 on the southeastern edge, <br />averaging about elevation 6460. With the switchyard being placed entirely up-gradient <br />of Fault pol, no problems with groundwater quality are anticipated. A groundwater <br />contour map using 2001 data is included as drawing _ (69-529-10). <br /> <br />D. Surficial deposits and intensely weathered bedrock can be excavated by common means. <br />The slightly or moderately weathered bedrock will likely require drilling and shooting. <br /> <br />1.09 GEOLOGIC CONSIDERATIONS <br /> <br />A. General excavation for the pumping plant, settling basin, and inlet channel will be <br />performed by ripper-equipped dozers in the alluvium and shallow weathered bedrock. <br />The weathered bedrock averages about 3 feet below the rock surface. Drilling and <br />blasting will be required in fresher, more competent bedrock. The pumping plant <br />subgrade will be at approximate elevation 641O, requiring about 50 to 60 vertical feet of <br />blasting. Permanent cutslopes in surficial deposits will be stable at approximately 2: 1 <br />above the groundwater table. Alluvial cutslopes occurring below the groundwater table <br />are not anticipated. Some raveling ofloose rounded cobbles and boulders in the terrace <br />deposits should be expected. Removal of large boulders may cause instability of <br />overlying material as excavation nears the base of the terrace deposits. Bedrock <br />cutslopes will be stable at about 0.25: I with proper shooting techniques. Benching at the <br />top of rock surface may be required to prevent raveling of the surficial materials into the <br />excavation. Temporary slopes in surficial deposits may be cut at 1.5: I and will be <br />backfilled following construction. <br /> <br />B. Joint sets identified near the site which average about NI2W, 75 degrees NE, and NnW, <br />75 degrees NE, in combination with bedding of about N63E, 9 degrees SE, may form <br />wedge failures on northeast facing cutslopes which are cut steeper than about 0.5: I. Most <br />of these wedges are expected to be small due to the laminated to thinly bedded nature of <br /> <br />Geology <br />00320.19 <br />