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SHANNON F~WILSON, INC. <br /> <br />The direct sheaz tests performed in conjunction with the current investigations indicated <br />friction angles of 9 to 24 degrees and cohesion values of 800 to 1800 psf. In genera], the friction <br />angles obtained from these direct shear tests are slightly lower than the friction angles <br />determined during previous studies, and the cohesion values aze similar to those obtained from <br />previous laboratory testing. <br />To determine the strength parameters for use in our stability analysis of the valley fill, we <br />disregarded the four tests with the highest friction angles (curves 1 through 4 on Figure 7) and <br />selected an average strength envelope based on the remaining strength tests. The average <br />strength parameters used in the stability analysis (curve 11 on Figure 7) is a friction angle of 20 <br />degrees and a cohesion 850 psf, Far our parametric analysis, the friction angle was varied from <br />15 to 25 degrees and the cohesion was varied from 300 to 1400 psf. The range of strength <br />parameters evaluated in the parametric analysis is shown as the shaded region on Figure 7. <br />Because of the size limitations of field sampling and laboratory testing equipment, the <br />strength tests were performed on the finer (sand, silt, and clay) portions of the overburden soil <br />and do not consider the inclusion of gravel to boulder-size rock fragments within the soil matrix. <br />In addition, the shaded range of strength parameters shown on Figure 7 encompasses the lower- <br />bound laboratory strength data. Therefore, it is our opinion that strength values used in our <br />analysis are reasonable. <br />6.2.3 Bedrock <br />The intact bedrock is very low to low strength (hard to very hard when considered as a <br />lithified soil) siltstone, claystone, and sandstone. Deep seated failures through the bedrock are <br />not considered likely below the base of valley fills. Bedrock properties of 40 degrees, cohesion <br />of 3000 psf and unit weight of 130 pcf were used for valley fills. <br />6.3 Groundwater <br />Our baseline stability analysis case did not include groundwater. For the parametric analysis of <br />the valley fill, the groundwater level was varied from the soil/spoil fill contact to 20 feet above <br />the base of the spoil fill to 40 feet above the base of the spoil fill. <br />• <br />23-I-01105-200-Rl.doc 23-I-01105-200 <br />