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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 />sheet pile wall system is being considered in order to create a stable pedestrian/vehicular <br />pathway between the main peninsula and the island to the south of the swim beach lake. <br />The sheet piles should be constructed such that they intersect the main slurry wall alignment <br />in order to create a relatively impermeable barrier between the swim lake and the larger lake <br />to the south. The barrier will be imperative if differential water levels are needed to be <br />maintained between the swim beach and the larger lake body in order to prevent piping of <br />the embankment materials, which could result in destabilization of the embankments. For <br />preliminary planning purposes, sheet piles should be embedded to a depth of approximately <br />1.2 times the height of the exposed section to maintain lateral integrity of the sheet piles. If <br />significant lateral loading is imposed on the sheet piling, deeper embedment may be <br />required. The anticipated location of the sheet-piling is shown on the attached Preliminary <br />Slurry Wall Alignment Map, Figure 3, at the end of this report. <br />We understand that the water levels within the larger lakes and swim beach area will be <br />maintained at relatively constant levels. Additionally, motorized boat traffic will not be <br />allowed within the lake areas. Therefore, we anticipate that wave action will be relatively <br />limited. However, at a minimum, we recommend that rip-rap armoring be installed within 3 <br />to 4 feet of the mean high water and low water mark elevation for erosion protection of the <br />slopes within the larger lakes, since these slopes will be critical to maintain the integrity of <br />the slurry wall system and any embankments supporting structures. Rip rap would likely be <br />undesirable in the swim beach area and as previously outlined, will require relatively flat <br />slopes. Depending upon the relative flatness of the slopes in the swim beach area, erosion <br />control may not be necessary. Other alternatives for erosion protection such as geotextiles <br />and/or vegetative cover can be considered in areas where rip rap is undesirable but we <br />should be given the opportunity to review these to assess adequacy. We can provide typical <br />cross-sections for embankments at a later time as the alignment and design progresses. <br />GENERAL COMMENTS <br />It is imperative that Terracon be retained to review the final design plans and specifications <br />so comments can be made regarding interpretation and implementation of our geotechnical <br />recommendations in the design and specifications. Terracon should also be retained to <br />provide testing and observation during the excavation, grading, and construction phases of <br />the project. <br />The analysis and recommendations presented in this report are based upon the data <br />obtained from the borings performed at the indicated locations and from other information <br />discussed in this report. This report does not reflect variations that may occur between <br />borings, across the site, or due to the modifying effects of weather. The nature and extent <br />of such variations may not become evident until during or after construction. If variations <br />12 <br />