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D04155GE <br />We obtained soil samples of the materials encountered in the test borings. The soil strength <br />parameters phi, (~) and cohesion (C), were assessed using direct shear strength test methods in <br />our laboratory. Based on the laboratory data and the observations of slope angles in the field we <br />used an angle of internal friction (~) of thirty-four (34) degrees and a cohesion of fifteen (15) <br />pounds per square foot in our analysis. The strength parameters of each type of soil tested were <br />used in conjunction with the slope profiles generated from site observations to assess the slope <br />stability. <br />Most, if not all, slope stability analysis methods involve estimation of the forces within a soil <br />mass which influence the stability of a slope. The forces may be grouped into two (2) general <br />categories, those that tend to drive the soil mass movement and those that resist soil mass-- <br />movement. The ratio of resisting forces to driving forces is commonly referred to as the slope <br />theoretical factor of safety. If the resisting forces are greater than the driving forces the theoretical <br />factor of safety is greater than one (1). If the resisting forces are equal to the driving forces, the <br />theoretical factor of safety is equal to one (1), minor changes in the soil moisture content or slope <br />conditions might cause movement. If the driving forces exceed the resisting forces the theoretical <br />factor of safety is less than one (l)and the slope is currently in a failure condition with periodic <br />movement. The engineering community generally considers a theoretical factor of safety greater <br />than 1.5 as stable and those ratios between 1.0 and 1.5 as marginally stable. The theoretical factor <br />of safety used in design is influenced by the confidence level of the data obtained on the sloped <br />areas. Depending on the importance of the areas adjacent to a slope and the confidence level of <br />the analysis used a theoretical factor of safety of less than 1.5 may be used as a basis for <br />assessment used where design of retaining structures or slope reinforcement is planned. <br />3.1 Infinite Slope Analysis <br />We performed an infinite slope analysis on the area above the gravel pit assuming that <br />approximately the upper ten (10) to fifteen (15) feet of the talus slope is relatively active and <br />assuming that this activity has developed a somewhat planar failure surface below and parallel to <br />the slope surface. The theoretical factor of safety obtained from our infinite slope method analysis <br />ranged from about 1.06 to 1.09. This analysis indicates that only a very slight change in the field <br />conditions will result in a theoretical factor of safety of less than 1.0 which means that failure of <br />the slope is imminent during times when the strength characteristics of the slope are influence, <br />such as during precipitation and/or runoff. This analysis is applicable to all of the slopes above <br />the gravel pit, above County Road 125, and in all of the talus slopes with similar slope angles in <br />the area. <br />3.2 Modified Bishops Method of Slices <br />We used computer modeling techniques and the XSTABL slope stability software to analyze the <br />stability of the slope on a cross section through the central portion of the gravel pit. The modified <br />Bishop's method of slices analyzed the forces within a soil mass assuming that the failure will <br />occur along asemi-circular arc. This analysis is presented below. <br />~TCI~EI t FXYT~1 ~~SiT~`TF~ES <br />CONSULTING GEOTECHNICAL ENGINEERS AND <br />-S- MATERIALS TESTING <br />