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Project 06606 <br />Page 2 of 2 <br />The soils encountered at this site have varying angles of repose. The soils that <br />will control the stability of the slope are anticipated to be the sand and gravel. <br />Based on our review of the boring logs and our experience nearby, we anticipate <br />that the gravel will be very sandy, with sand to gravelly sand lenses. <br />Based on our experience in this area and reference geotechnical engineering <br />literature ("Fundamentals of Geotechnical Analysis" by I. S. Dunn, L. R. <br />Anderson and F. W. Kiefer) an anticipated conservative angle of internal friction <br />for this material is approximately 26°. This is based on the assumption that <br />dewatering will keep the face of the pit moderately dry during the period of time <br />that the pit is active. This corresponds to a slope of just over 2:1. It has been our <br />experience that the risk of instability at that slope is low. However, due to the <br />potential layering of the site soils, the sandy nature of the gravel layers and lensic <br />nature of the soils at this site, we are recommending a maximum slope of the pit <br />walls of 3:1 (3 horizontal to 1 vertical). Therefore at a maximum assumed depth <br />of mining of 33 feet, a minimum setback of 99 feet from any man made structure <br />will afford adequate protection from the risk of slope instability. <br />Waterways present a somewhat increased risk of instability due to the continued <br />presence of a source of water and the fact that it is likely that the groundwater <br />table will slope upwards toward the elevation of the water surface elevation in <br />the waterway. Therefore, we recommend that there be an offset of a minimum of <br />100 feet from the South Platte River, where no mining takes place. From that <br />point, the 3:1 slope discussed above would be applicable. <br />Thank you for consulting with us on this phase of the project. If you have any <br />questions concerning this report, please do not hesitate to contact us. <br />Sincerely, <br />SCOTT, COX AND ASSOCIATES, INC. <br />M. Edwar assgow, PE <br />Professional Geologist