2004-04-23_REVISION - M1985029 - Laserfiche WebLink

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GEOTECHNICAL STABILITY EXHIBIT The only buildings or structures that could be affected by any geotechnical stability issues at the mine are the Kirschvink residence and well, the CDOT owned South Platte River bridge for Highway 9 and the immediate portion of Highway 9 downstream due to the location of the Pond A dam within the permit azea immediately to the west. The slope stability of this dam was studied extensively in the previous permit evaluation, including site evaluation and slope stability modeling. The results, which are enclosed in Appendix C, show that the dam has a safety factor more than sufficient to protect all structures downstream. All reclaimed slopes within the mining areas will be restored to a minimum slope of 3H:1 V, which should have no problems with slope stability at such a mild angle. For the mining slopes of 2H:1 V, the following demonstration is presented: These mined slopes will be in alluvial gravel materials which were identified in all past test excavations over many decades. The existing gavel material has few fines (< 100 mesh) and the material does not get its strength from cohesion, but internal angle of friction. Partly rounded gravel of this type in place has an angle of repose of approximately 40 degrees (from Huang -Stability Analysis of Earth Slopes). For non-cohesive soils, this is also the internal angle of friction of the material. The overall slope of 2.OHa.OV is an angle of 26.6 degrees, therefore, the factor of safety for non-cohesive soils is defined as the ratio of the an lg e of repose of the material to the actual angle to be emoloved on the slope. In this case. this is 26.6/18.4 = 1.50. This is a very good safety factor and poses no potential for instability. Also, the factor of safety is completely independent of the presence of water, since it does not affect the stability ofnon-cohesive materials. Alma Placer 7/03 93