2014-11-14_PERMIT FILE - M2014043 (3)

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ALBERT FREI AND SONS, INC. Page 1 HATCHERY PIT Permit # M- 2014 -043 GEOTECHNICAL STABILITY EXHIBIT - Dry slopes This information is presented to address the plan to mine within 200 feet of the structures listed in Exhibit S around the north, east and south side of the mine. The mine depth closest to any structure where no waiver agreement has been obtained is 42 feet. As mining will occur within a slurry wall lined area that will be dewatered first and then mined dry the slope stabil- ity can be evaluated as a dry condition. The closest structure for which there is no damage agreement is the communications line that is approximately 10 feet east of the permit line and 4 feet below the surface, between the fence and Monaco St. In this area the setback from the permit /affected lands line to the mining dig line is 25 feet, so the distance from the outer most dig line to the structure will be 35 feet. The attached sketch is a cross section showing the setbacks and slopes we described below the colors noted in the text correspond with the slope lines shown on the sketch. The following information is the analysis data used to calculate the safety factor for the different conditions or proposed setback distance discussed in the following text. From previous studies it has been determined that an appropriate friction angle for Alluvial gravels along the upper South Platte river is 35° and is adequate when non - site - specific information is available. This number falls into the parameters listed in Rock Slopes: Design, Excavation and Stabilization, Publication No. FEWA -TS- 89 -045, Table 1 - Typical Soil and Rock Properties for Sand and Gravel, uniform grain size. The material in the mine wall, will be in- place dry sand and gravel, is assumed to be cohesionless and having an friction angle (D of 35° which is 1.43:1 (brown line) The formula I used for the factor of safety (FS) calculation is FS = Tan (D _ Tane, where (D is the friction angle of the sand and gravel and e is the slope angle from the toe of the cut to the closest structure. The working face will be near vertical as it progresses across the mine, but we have used 1A:1 slope ratio for the calcu- lations to establish the distance from the mining toe to the closest structure. This would be 10 feet from structure to permit line, 25 feet from permit line to dig line and 21 feet from top of dig line (TOD) to the toe of slope (TOS) for a total of 56 feet. Since the line is 4 feet below the surface the resulting slope angle (e) is 34.2° (black line) and the FS = 1.03. It is right on the limit of being safe if the slope failed to it natural angle of repose.