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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 />and seismic refraction surveys. Multi-level standpipe piezometers were <br />installed in each bore hole upon completion. The objective was to <br />characterize the embankment dam, the foundation of the dam and spillway; <br />investigate sources of potential construction material, and to gather samples <br />for laboratory testing. The work was performed during the months of August <br />and September, 1986, directly by, or under the supervision of MKE personnel; <br /> <br />Investigation of Twin Lakes Dam and Reservoir included aerial photography to <br />develop new topography for the project site. From thi s topography, updated <br />elevation versus area and capacity relations were computed. At the current <br />spillway crest elevation of 9295, the reservoir stores approximately 230 <br />acre-feet. Storage at this elevation was previously believed to be 278 acre- <br />feet, based on the original elevation-capacity table. An issue of concern to <br />the City has been that Twin Lakes Dam and Reservoir, in its current <br />configuration, is not capable of storing the entire decreed capacity of 460 <br />acre-feet. To store this volume, the spillway crest would need to be raised <br />eight feet, to elevation 9303. <br /> <br />2.1.2 Geologic Mapping <br /> <br />A surficial geologic map of the Twin Lakes Reservoir is included in Figure <br />V.l. The dam foundation, abutments, and northern, southern; and eastern <br />shores of the reservoi r area are composed of outcroppi ng Precambri an igneous <br />and metamorphic rock of the Idaho Springs Formation. The formation is <br />composed of gneiss, mica schist, and granite with secondary pegmatite <br />stri ngers parall el to fol i ati on. Schi stocity and fol i ati on stri ke N700E and <br />the dip is wavy but subvertical. The rock is weathered and oxidized at the <br />surface to an orange-brown color, although sound outcrops were mapped along <br />the northern side of the reservoir. The primary jOint set is parallel to <br />foli ati on, and a second set stri kes N450W, and di ps 900. The western end <br />of the reservoir area, including the spillway and ridge just west of the <br />reservoi r, are formed by gl aci al ti 11 of Pl ei stocene age overlyi ng bedrock. <br />The till consists of silty, sandy gravel with large subrounded erratics to <br />five feet in diameter. <br /> <br />-46- <br />