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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 />1 <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />2.1.3 Subsurface Investigation <br /> <br />Twin Lakes Dam and Reservoir were investigated by three rotary bore holes; 11 <br />test pi ts, and 1600 1 i near feet of sei smi c refracti on survey 1 i nes, the <br />1 ocati ons of whi ch are shown in pl an on Fi gure V.l. Interpreti ve geology <br />sections are included on the same figure. Logs of the bore holes and test <br />pits, and seismic refraction work sheets are included in Appendix B. The bore <br />holes were drilled with a 6-inch diameter hollow stem auger utilizing a truck <br />mounted CME-55 rotary dri 11. Bedrock was core drill ed with an NX-di ameter <br />di amend bi t usi ng the wi re 1 i ne techni que. Standard penetrati on tests (SPT) <br />were performed and disturbed soil samples were obtained at five-foot intervals <br />uSing a split spoon sample within the hollow stem. Relatively undisturbed <br />soil samples were periodically retrieved using Shelby tubes. Bore hole <br />permeability tests were performed at selected elevations within the embankment <br />usi ng the falli ng head and constant head techni ques. Water pressure tests <br />were performed in the bedrock utilizing a single packer and applying pressure <br />in five ascending and descending stages. Test pits were excavated with a Cat <br />225 track-mounted backhoe. The machine was equipped with a three-foot wide <br />bucket and had an effective reach of 16 feet. <br /> <br />2.1.3.1 Dam <br /> <br />Bore holes TL-l, TL-2, and TL-3 were drilled from the crest of the dam through <br />the embankment and into bedrock. The borings showed the dam to be reasonably <br />homogeneous, but of low density, primarily composed of a silty, sandy gravel <br />derived from weathered bedrock with blocks of weathered rock to eight inches <br />in diameter. The soil was rich in mica and ranged in color from tan to <br />orange-brown. Embankment permeability was calculated to be approximately 6.6 <br />x 10-5 cm/sec., using borehole techniques. The embankment dam was found to <br />be in di rect contact with the bedrock at a maximum depth of 35 feet in bore <br />hole TL-l, The rock recovered was moderately weathered to sound, gray gniess <br />of medium texture. composed of about 40% biotite, with a distinct wavy <br />foliation and bands of cream colored pegmatite to several feet in thickness. <br />Occasionally, the pegmatite was fragmented around the larger feldspar crystals <br />and oxidized orange. Jointing was moderate with the principal joints being <br /> <br />-47- <br />