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GEOTECHNICAL STABILITY EXHIBIT <br />The mining plan calls for mining the perimeter of the excavation at a slope of 2H:1 V except for a <br />15' slope length of 3H:1 V along the waterline, where the slope will be 3H:1 V. In addition, the <br />minimum horizontal buffer from the edge of excavation to the edge of the pernut boundary or any <br />structure is 35 feet. See attached drawing sheet in this section. <br />The initial pit excavation was started in 1988 and an area of approximately 2.5 acres was mined <br />from 198$ to 1990. This excavation revealed a gravel material from the Uncompaghre River <br />terrace that was cobbly in nature with very little fines. Some historical cementation of fines did <br />permit an excavation of a near vertical slope at the east end of the initial excavation which was <br />approximately 25 feet high. This was done by excavator from the surface and from a bench just <br />above water level. No pit pumping was allowed at this pit so the water was maintained at <br />approximately 5 feet of the surface. The top 1' of surface material was topsoil from the original <br />imgated field and overburden thickness averages 6 feet thickness over the area to be mined. No <br />sloughing or slope failures of any kind have been seen in this slope from 1990 to 2007. Very little <br />reject fines was produced from processing the material in the Western Gravel plant on the other <br />side of the River. Due to these chazacteristics, it is safe and also conservative to ignore any strength <br />produced by historical cementation and assume that the material's slope strength is cohesionless <br />and is based entirely on the internal angle of friction of the material. <br />The angle of repose of loose material of this type is approximately 31 degrees to 36 degrees. The in <br />place bank material will have an angle of repose of approximately 34 to 38 or more degrees. The <br />table enclosed in this section shows the various friction angles for various types of compacted (in <br />place) gravels. This table is from Yang Huang's Stability Analysis of Earth Slopes. The material on <br />site would be classified as a GP type gravel, since it is clean, with little fines. It is assumed that the <br />intemal angle of friction of this material is 37 degrees. <br />North R-34 Pit 1/01 <br />52 <br />