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2. SUBSURFACE CONDITIONS <br />2.1 Lyman Henn Investigations <br />2.1.1 Test Borings and Rock Coring <br />Lyman Henn completed 18 test borings along or near the proposed slurry wall <br />alignment to characterize subsurface conditions. The test boring locations are shown in <br />plan view on Sheet C -002. A summary of test boring results is presented in Table I. <br />Test boring reports are included in Appendix A. The test boring program included 4- <br />1/4 -inch diameter hollow -stem auger drilling in the soils and bedrock, NX -core drilling <br />in bedrock in six of the boreholes, drive sampling of the soil and bedrock, and bedrock <br />in -situ permeability (packer) testing in six boreholes. <br />Test borings revealed alluvial soils overlying sedimentary bedrock. The alluvial soils <br />consist of a fine to coarse alluvium (overburden) extending from the ground surface to <br />depths ranging from just below ground surface to 15 ft below ground surface (bgs) and <br />coarse alluvium (sand and gravel) extending below the fine alluvium to the top of <br />bedrock. Overburden was encountered in all of the borings, as indicated on Table I. <br />The depth to the top of bedrock ranged from 35.4 to 70.0 ft bgs, with an average depth <br />of 60.2 ft bgs. Bedrock elevations ranged from 4585 ft to 4621.6 ft. All borings except <br />borings LH -5 through LH -7 and LH -15 have been surveyed by Survey Systems, Inc. of <br />Evergreen, CO. At the time of this report writing, the alignment has not been surveyed. <br />The alignment will be surveyed prior to start of construction of the slurry wall. <br />The overburden consists primarily of clayey sand, sandy lean clay, silty sand and silt. <br />The sand and gravel consisted of well graded sand, well graded sand with gravel, poorly <br />graded sand and poorly graded sand with gravel with localized areas of clayey sand, silt <br />and fat clay deposits. The sedimentary bedrock consisted of completely weathered, tan - <br />orange to fresh, blue gray shale of the Pierre Shale Formation. The extent of <br />weathering in the bedrock appears to occur within the upper 2 to 7.5 ft of the bedrock. <br />The bedrock rock quality designation (RQD) ranged from 0 to 97 percent. Low RQD is <br />an indicator of fractured or completely weathered rock. The fractures within the cored <br />bedrock generally were described as primarily horizontal, extremely close to <br />moderately close, tight to open, smooth to slightly rough, planar to stepped, with no <br />infill. <br />Bedrock generally consists of a laminated siltstone with small interbeds of claystone <br />and clayey siltstone. Weathered sections appear tan - orange in color and grades with <br />depth to blue gray slightly mottled bedrock. Some localized areas contained traces of <br />very fine grains of sand, especially near the surface of bedrock. <br />The hydraulic conductivity of the bedrock ranged from 8.4 x 1(1' centimeters per <br />second (cm/sec) to 1.2 x 10 cm/sec when measurable by water pressure testing in 11 <br />tests. The upper range of the aforementioned is representative of the weathered <br />stratum. The upper range hydraulic conductivity will be 1.4 x 10 if only the fresh <br />bedrock is considered For some intervals of the water pressure tests, the apparent <br />G:\PROJECPS \111025 Geisert Gravel Pit Slurry Wall \O \O1 \Geisert Slurry Wall Design Report.doc <br />2 <br />