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SUPPLEMENTAL GEOTECHNICAL ENGINEERING REPORT PROPOSED SLURRY CUT-OFF WALLS @ ST. VRAIN DEVELOPMENT WELD COUNTY ROAD 30 (HIGHWAY 66) AND INTERSTATE 25 WELD COUNTY, COLORADO Terracon Project No. 25045300 November 9, 2005 INTRODUCTION This report contains the results of our supplemental geotechnical engineering exploration for the proposed slurry cut-off walls to be located within the southern portion of the St: Vrain development located near the southeast corner of Weld County Road 30 (Highway 66) and Interstate 25 in Weld County, Colorado. The purpose of these services is to provide information and geotechnical engineering recommendations relative to: • subsurface soil and bedrock conditions. • approximate depth and relative quality of the bedrock. • groundwater conditions. • in-situ bedrock permeability. • slurry cut-off wall design considerations. earthwork. drainage. The recommendations contained in this report are based upon the results of field and laboratory testing, engineering analyses, our experience with similar soil and bedrock conditions and slurry cut-off walls, and oUr understanding of the proposed project. Information from the previous preliminary geotechnical engineering report- for the overall development, Terracon Project No. 25045122 dated May 26, 2004 and the previous geotechnical engineering report for the slurry cut-off walls dated November 24, 2004, was reviewed prior to the preparation of this current study. PROPOSED CONSTRUCTION Based upon the information provided, we understand that slurry cut-off walls are planned to be constructed along the perimeter of three existing lakes located within the southwestern portion of the St. Vrain development. Slurry cut-off walls will be required for the planned lake improvements in order to isolate water/groundwater condtions from the adjacent St. Vrain River. The construction of the slurry walls will result in the creation of two larger lakes 1