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1 <br /> DEL-MONT CONSULTANTS , INC. <br /> ENGINEERING SURVEYING <br /> _ 125 Colorado Ave. • Montrose,CO 81401 • (970)249-2251 v (970)249-2342 FAx <br /> www.del-mont.coni . service0de!-mont.com <br /> GEOTECHNICAL STABILITY EXHIBIT—Horsefly Rock Source <br /> This site is for mining sandstone. The average thickness is 6 feet with a maximum of 8 feet <br /> thick. This is on a slope so the downhill/fill side will daylight to natural ground and the <br /> uphill/cut slope will be 3:1. There are no buildings or any structures outside the permit area that <br /> could be affected by excavation. Sufficient buffers will be maintained to neighboring property <br /> lines. <br /> From Table 2.5 in the SME Mining Reference Handbook, backfill is best classified as loose <br /> sand. This material has an internal friction angle of 28-34 degrees and no cohesion. This is <br /> worst case since the material will most likely be a mixture of sand and gravel which has an <br /> internal friction angle of 45-48 degrees and no cohesion. <br /> The Factor of Safety(FOS) for sides of the pit with a(3H:1 V) 18.4 degree slope in loose sand <br /> material with an assumed internal angle of friction of 28 degrees can be approximated by <br /> evaluating the internal angle of friction as follows: <br /> Tpngent of Internal Angle of Friction <br /> FOS= Tangent of Actual Angle of Failure Surface <br /> FOS =Tan28°/Tan 18.4° = 1.60 <br /> For a gravel/sand mixture which is mostly likely what will be present the FOS would be: <br /> FOS =Tan 45°/Tan 18.4°=3.00 <br /> The minimum safety factor in the permanent case is 1.5,therefore the 3:1 slopes will be safe as <br /> the final condition for a worst case scenario of sand only. <br /> The nearest identified fault is more than 2 miles to the east. This permit is in an area of low <br /> seismic activity. <br /> O L/., <br /> V�•ap�`S�999;FD <br /> Nicholas Barrett <br /> PE 42106 PE 6 r <br /> 38 37 <br /> . ANDP <br />