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<br />United Compnaies, Inc. <br />February 15, 1994 <br />page 5 <br />Cohesionless soil slopes do not fail in rotational type failures <br />but in slough or wedge type failure where the angle of the cut <br />slope declines to near the angle of repose. A wedge type stability <br />analysis was run using a 1:1 slope to a 1.5:1 (horizontal to <br />vertical) slope. A safety factor of 1.26 was determined for these <br />parameters and with no seismic acceleration. The gravel pit <br />extension is in a seismic zone one which is a low potential <br />earthquake zone. Seismic acceleration is generally assumed to be <br />between 0.10 and 0.20. California uses a value of 0.25 where <br />earthquake potential is high and the intensity of the earthquakes <br />are large. With a seismic acceleration value of 0.10 the above <br />discussed wedge has a safety factor of 1.07. The safety factor is <br />above unity and is adequate for the temporary slope. Using <br />California's much more intense value of acceleration the factor of <br />safety would be 0.87 which is less than unity. This would indicate <br />some sloughing or wedge type failure would occur to decrease the <br />slope angle. A 2:1 slope was analyzed with the more intense <br />acceleration value (0.25) with a 1.08 safely factor determined. An <br />intense earthquake is not expected and has not occurred since <br />records have been kept of the area. <br />Numerous soil experts, including the Bureau of Reclamation in their <br />Earth Manual, recommend excavated slopes for this type operation to <br />be 1:1 on a standard basis. Some small sloughing is expected to <br />occur with time due to vibration or wetting from rainfall or snow <br />or frost heave. These same sources recommend 1.5:1 slopes for <br />permanent borrow pit and reservoir area back slopes. However, they <br />do recommend the slope be reduced to 3:1 where the public has <br />access. The gravel pit is being planned to be turned into a <br />reservoir and the permanent slopes are planned to be 3:1 as <br />recommended. <br /> <br />