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1.OSLOPE STABILITY ANALYSIS <br />A slope stability analysis was performed for 21 separate proposed gravel pits. <br />The purpose of the analysis is to evaluate safe setback distances for pit <br />excavations from permanent man-made structures and hydrologic features. <br />1.1. METHODOLOGY <br />The digital model SB-Slope developed by Von Gunten Engineering Software, Inc. <br />was used to predict a safety factor for a given pit wall configuration. Soils layers <br />for a given pit cell were entered into Autodesk Land Desktop Development 3 and <br />point files for line geometry were extracted to text files. Phreatic surfaces were <br />predicted utilizing the Theis solution for 100 ft distance from the pit dewatering <br />trench and the Dupuit-Forchheimer approximation was used to predict the <br />drawdown curve closer than 100ft from the dewatering trench. Phreatic surfaces <br />near streams were approximated based on a typical stream recharge cross <br />section. <br />1.2. DATA INPUT <br />Model input consisted of soil layer geometry described above and soil physical <br />strength parameters. Soil strength parameters were taken from a memo from <br />Alan Sorenson of the Division of Minerals and Geology and Design of Small ' <br />Dams. The soils input data are summarized in table 1. Graphical and numeric <br />input data is given in Appendix I for each mining Phase and Cell. <br />Table 1 -SOILS DATA <br />Stratigraphic Layer Density Cohesion (psf) Phi (~) <br /> (pcf) <br />Silt and Clay -ML 90 4 34 <br />(moist) <br />Gravel (moist) GW- 130 0 35' <br />unsat. <br />Gravel (saturated). 137 0 35 <br />GW-sat. <br />Weathered Cfaystone 142 0 14 <br />(saturated) <br />Intact Claystone (dry) 150 500 25 <br />