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• 0 0 <br /> Orchard Mesa Agg <br /> 11/94 <br /> EXHIBIT "Rule 6. 5" <br /> GEOTECHNICAL STABILITY EXHIBIT <br /> The proposed gravel pit, once all resources have been removed and <br /> reclamation has been completed, will be approximately 20 feet <br /> deep with 3 horizontal to 1 vertical side slopes. The material <br /> comprising the sides of the excavation is sandy gravel with <br /> cobbles up to 12 inches. The reclamation process will involve <br /> regrading the excavated surface to a uniform appearance. <br /> Stockpiled topsoil, consisting of silty clay, will then be placed <br /> Gfi �N on top of the exposed gravel. Approximately 4 inches of topsoil <br /> 6� will be placed on the bottom of the excavation and 8 to 12 inches <br /> Y placed on the slopes. It is estimated that the bottom of the <br /> j excavation will have approximately 12 inches of gravel over <br /> weathered shale bedrock. <br /> The stability of the slopes was determined using programmed <br /> sequences developed by the University of Texas. The program, <br /> entitled UTEXAS2, utilizes soil profiles along with respective <br /> unit weights and strength characteristics to determine the <br /> critical potential shear surface and its respective safety factor <br /> against movement. Spencers procedure was used to determine the <br /> Factor of Safety. The automatic search feature of the program <br /> was used to identify the critical shear surface. Only circular <br /> potential shear surfaces were considered. <br /> There were no lab tests conducted on the material encountered at <br /> this site, therefore conservative strength values were assigned <br /> to each material type. The shale underlying the gravel has a <br /> significant influence on the stability of the gravel slope. <br /> Although no strength tests were performed on the shale at the <br /> site, past experience on other projects in the Mancos Shale have <br /> shown a wide range of strength values with cohesion varying from <br /> 0 to 6000 pounds per square foot and friction angles varying from <br /> 12 to 55 degrees depending on the moisture conditions, extent of <br /> weathering, composition of the shale and strain history. The <br /> lowest strength values are found in areas where montmorillonitic <br /> clayey shale seams exist. In those areas, strain history is one <br /> of the most influential factors in determining strength. For <br /> example, the peak friction angle of a bentonitic clay shale might <br /> typically be 25 to 30 degrees, whereas the friction angle for <br /> the same shale could be 12 degrees or lower if the shale had <br /> experienced sufficient shear strain to establish a fully <br /> developed shear surface. The strength of the weakest types of <br /> shale could therefore range from a low friction angle of 12 <br /> X-1 <br /> Orchard Mesa Agg <br />