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Supplemental Geotechnical Engineering Report Terracon <br />Proposed Slurry Cut-off Walls @ St. Vrain bevelopment <br />Weld County, Colorado <br />Terracon Project No. 25045300 <br />SUBSURFACE CONDITIONS <br />Geology: Surficial geologic conditions at the site, as mapped by the U.S. Geological Survey <br />(USGS) ('Colton, 1978), consist of Eolium of Upper Holocene to Bull Lake Glaciation Age. <br />This material generally consists of light brown to reddish brown to olive gray deposits of <br />windblown clay, silt, sand and granules. These materials, as mapped in this area, are <br />generally on the order of about 15 feet in thickness. <br />Bedrock underlying the surface units consists of the Denver and Arapahoe Formations of <br />Paleocene and Upper Cretaceous Age. These formations within this area have been <br />reported to include sandstone, claystone and siltstone, interbedded within pebble <br />conglomerate. The thickness of these units has been reported to be on the order of 300 to <br />900 feet. <br />Due to the relatively flat nature of the site, geologic hazards at the site are anticipated to be <br />tow. Seismic activity in the area is anticipated to be low, and the property should be <br />relatively stable from a structural standpoint. With proper site grading around the proposed <br />structures, erosional problems at the site should be reduced. We assume that scour <br />measures associated with the St. Vrain River will be addressed by others. Based upon <br />review of the subsidence hazard maps for the area as mapped by the Colorado Geological <br />Survey (ZAmuedo and Ivey, 1974) and our experience in the surrounding general area, the <br />potential for subsidence hazards associated with old mines and/or faults could exist. <br />However, a detailed evaluation of the subsidence potential is beyond the scope of this <br />study. <br />Mapping completed by the Colorado Geological Survey (3Hart, 1972), indicates the site is <br />located in an area of "Windblown Sand or Silt". This material has been described as having <br />low swell potential and may be subject to severe settlement or collapse when water is <br />allowed to saturate the deposits. Bedrock with higher swell potential may be locally less <br />than 10 feet below the ground surface. <br />Soil and Bedrock Conditions: As presented on the Logs of Borings from the previous and <br />current studies, surface soils to depths of 5 to 30 feet consisted predominantly of sands with <br />varying amounts of clay, silt, gravel and cobbles, and gravels with varying amounts of clay, <br />silt, sand, and cobbles. Some layers of clay with varying amounts of sand, silt and gravel <br />'Colton, Roger B., 1978, Geologic Map of the Boulder Fort Collins-Greeley Area, Colorado, United States <br />Geological Survey, Map I-855-G. <br />Ivey, J.B., 1974, Subsidence Hazard Map for the Boulder-Weld Coal Field, Boulder and Weld Counties, <br />Coloretlo, Colorado Geological Survey, Map 7361-6. <br />'Hart, Stephen 5., 1972, Potentially Swelling Soil and Rock in the Boulder•For[ Collins-Greeley Area, Front <br />Range-Urban Corridor Colorado, Colorado Geological Survey, Sheet 1 of 4. <br />4 <br />