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Supplemental Geotechnical Engineering Report <br />Proposed Slurry Cut-off Walls @ St. Vrain Development <br />Weld County, Colorado <br />Terracon Project No. 25045300 <br />Terracon <br />The slurry cut-off walls should be "keyed" or "socketed" into the relatively impermeable <br />competent/intact claystone or sandstone bedrock to reduce groundwater seepage. <br />Competent bedrock was encountered at variable depths along the initially proposed <br />perimeter of the slurry cut-off wall that may likely result in different base elevations for the <br />walls. The claystone bedrock materials encountered were relatively intact to moderately <br />fractured structurally. Lenses of sandstone bedrock were either encountered beneath <br />and/or above the claystone bedrock materials in some of the borings and/or to the full depth <br />of exploration in some of the borings. In general, the packer permeability testing indicated <br />that the sandstone was relatively impermeable in most areas. However, very hard and <br />strongly cemented sandstone bedrock materials were encountered at depths of about 9 to <br />19-1 /2 in Boring Nos. 45, 47, and 48 where auger refusal was experienced at depths of <br />about 13 to 20 feet below existing site grade. The supplemental coring and packer <br />permeability testing in this area indicated that the upper 5 to 10 feet of the sandstone layer <br />was moderately to severely fractured. "Keying" or "rocketing" into this type of sandstone <br />bedrock could result in excessive seepage losses. Care should be taken during <br />construction to excavate through this layer for the slurry cut-off wall key way. Deeper key <br />ways for the slurry cut-off walls will likely be required in Phis area. In addition, the upper 1 to <br />2 feet of the claystone exhibited moderate weathering/fracturing and shout not be <br />considered competent/intact bedrock. <br />Following is a summary of the approximate boring elevations, approximate depth to <br />bedrock, approximate bedrock elevation, the anticipated elevation of the bottom of the slurry <br />wall, and the approximate thickness of bedrock excavation. <br />Table No, 2 <br /> <br />Boring <br />No. Approximate <br />goring Elevation <br />(feet) Approximate Top of <br />Bedrock Elevation <br />(feet) Approximate Bottom <br />of Slurry Wail <br />Elevatlon (feet) Approximate <br />thickness of <br />bedrock excavation <br />(feet) <br />Packert 4827 4823 4810 13 <br />Packert 4832 4810 4803 7 <br />Packer3 4822 4815 4807 8 <br />Packer4 4822 4799 4792 7 <br />1 4838 4820 4810 10 <br />2 4830 4810 4,803 7 <br />3 4829 4809 4602 7 <br />4 4829 4813 4806 7 <br />5 4829 4809 4802 7 <br />6 4828 4808 4801 7 <br />7 4827 4807 4600 7 <br />7 <br />