2006-03-17_REVISION - M1982015 - Laserfiche WebLink

Home Browse Search

Supplemental Geotechnicai Engineering Report Terracon Proposed Slurry Cut-off Walis @ St. Vrain Development Weld County, Colorado Terracon Project No. 25045300 approximately 42 acres and 52 acres, respectively and a smaller self-contained swim beach area (7.1 acres) to the north of the larger lakes. Several small islands and two peninsulas are also planned to be constructed within and around the perimeter of the lakes for amenity/aesthetic purposes and for support of planned structures including a clubhouse facility. A sheet pile wall or similar construction is being considered to accomplish water/groundwater separation of the swim beach area from the larger lake body and to provide access from the main land to the island feature adjacent to the swim beach. The proposed lake water levels vary in elevation from approximately 4,818 to 4,821 feet. It is our understanding that motorized boats will not be used within the lakes and that wave action within the lakes will be minimal. We assume that the proposed slurry cut-off walls will consist of a mixture of bentonite and preferably on-site soils. It is our understanding that the existing three lakes are the results of reclamation operations associated with previous gravel pit mining operations that were performed in the area. SITE EXPLORATION The scope of the services performed for this project included site reconnaissance by a field engineer and packer in-situ permeability testing of different bedrock strata at the site to characterize the permeability characteristics of the bedrock materials. As previously outlined, a previous geotechnical engineering report was prepared in which a subsurface exploration program to assess the depth and relative quality of the bedrock materials was performed. Field Exploration: During our previous study, a total of 48 test borings were drilled during the period between October 27 and November 19, 2004, to depths of about 5 to 40 feet below existing site grade at the approximate locations shown on the Boring Location Map, Figure 1 for the previous study. The borings were drilled along and/or near the proposed alignment of the slurry cut-off wall at a spacing of about 300 feet on-center. For this current study, a total of 4 addtional test borings (Packer 1, 2, 3 and 4) were drilled to depths of about 17 to 32 feet below existing site grade at the approximate locations shown on the Boring Location Map, Figure 1 for the purpose of in-situ permeability testing. The boring locations were selected in order to pertorm packer permeability testing of different bedrock materials at the site based on the preliminary information from the previously referenced study. All borings were advanced with struck-mounted drilling rig utilizing 4-inch diameter, solid and 6-inch diameter hollow-stem, continuous-flight auger. The borings were located in the field by pacing from property lines and/or existing site features. Elevations were taken at each boring location by interpolation from contours indicated on the provided Site Plan prepared by Carroll and Lange, Inc. The accuracy of 2