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3 <br />and Riverwash (Ro). The SCS describes these soils in Exhibits (14 - 17).. In <br />general this site surface soils are 10 feet of silty clay and 20 feet of river cobble and <br />smaller over the Mancos Shale. <br />Slope Evaluation <br />Given the data from soil logs and soil properties (described herein), the limits of <br />mining were calculated in conformance with generally accepted engineering <br />practices. The depth of mining is deepest at the north end of the site near D Road. <br />These deeper depths of mining necessarily require greater setbacks of the <br />excavation in order to protect the home, utilities, and D Road itself. Exhibit 18 <br />shows the graphical interpretation of the limits of excavation in this area. This <br />interpretation is based on borehole #5. The soil fog for borehole #5 shows that <br />there is 16' of overburden and 22 '/i of gravel on top of the Mancos Shale. <br />Calculations indicate that the angle of slope failure in the mined gravel is <br />approximately 48 degrees. If this angle is drawn through the surface of the ground, <br />the limit of failure is approximately 35' from the toe of the excavated slope. This <br />distance is approximately the same as the length of failure calculated when <br />considering a circular failure mode in the drained silty clay overburden. Given this <br />information and in order to simplify the calculation for the distance of failure, tfiis <br />report uses the 48 degree slope as the basis for estimating the distance of failure. <br />This estimate is valid given the soil properties and thicknesses as described herein. <br />The setbacks required from the toe of the mined excavated slope must consider <br />an appropriate factor of safety. Rigorous methods may be used to calculate the <br />factor of safety. These rigorous methods generally arrive at a minimum value of <br />approximately 1.5. This report uses a factor of safety of 2.0 near critical structures <br />and 1.0 to 1.5 elsewhere. <br />Slope Setbacks Defined <br />The setbacks for this project are based on importance of adjoining permanent man- <br />made structures and the depth of excavation. It is assumed in this study that the <br />gravel resource will be mined to the top of the Mancos Shale. Exhibit 19 illustrates <br />that the setback, as described herein, is measured from the toe of the cut slope. <br />Figure A of Exhibit 19 shows that additional gravel resource may be claimed if the <br />gravel is cut at a 48 degree slope angle. The slopes shown herein are based on <br />preserving permanent man-made structures and not oriented towards safety of <br />mining personel. In order to assure their safety, all mining slopes should conform <br />to OSHA standards. Also these setbacks are based on typical properties of the soil. <br />The setbacks should be verified on an ongoing basis as the resource is mined in <br />order to best protect the adjoining sites permanent man-made structures. <br />The setback, as determined from borehole #5, for the northern border, adjacent to <br />D Road and next to the existing home, is 70 feet. This setback should be measured <br />from the Right of Way (ROW). Exhibit 20 shows that this setback is met. The visual <br />berm that borders D Raod is 70' wide (Exhibit 21) and the limits of excavation are <br />setback an additional 30'. Therefore it appears that the present plan shows <br />sufficient setback to preserve permanent man-made structures located along D <br />