2005-03-14_REVISION - M2000016 (4)

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February, 2005 1.0 SLOPE STABILITY ANALYSIS A slope stability analysis was performed fortwenty-one (21) distinct proposed gravel pits at the Ft. Lupton Pit site in Weld County Colorado. The purpose of the analysis is to evaluate safe setback distances for pit excavations from permanent man-made structures and hydrologic features at the proposed site. 1.1 METHODOLOGY SB-Slope, v. 3.0 (Von Gunten Engineering Software, Inc.) is the slope stability application utilized for this study. The program uses the "Simplified Bishop Method of Slices' based on the limiting equilibrium circular failure surface method as developed by A.W. Bishop (1955). The program was used to predict the location on the soil surface above the proposed excavation where slope failure is imminent for the given soil conditions, i.e., where: r(available shear strength)/T~(required shear strength) =Factor of Safety (FS) = 1.0. Soil layers and thickness for a given pit cell were determined with the aid of soil borings and average soil thickness provided by Lafarge NA. Phreatic surfaces were predicted utilizing the soil boring information as well as the Theis solution for points greater than 100-feet distance from the pit dewatering trench and the Dupuit-Forchheimer approximation to predict the drawdown curve at points less than 100-feet from the dewatering trench. Phreatic surfaces near streams were approximated based on a typical stream recharge cross section. 1.2 DATA INPUT Model input data consisted of the soil stratigraphy, described above, and soil strength parameters. The stratigraphy data was obtained by analyzing historic soil borings performed by Lafarge, NA and their contractors. Soil boring logs in tabular format are provided in Appendix IV. When a cell did not have a specific water feature present such as a stream or irrigation ditch, the boring data was averaged for all of the borings within cell. When a stream or irrigation ditch was present in the vicinity of the cell excavation, the borings nearest the water feature were analyzed to determine the stratigraphy and water table depth that would be most representative of existing soils and depths. Soil strength parameters utilized for the analysis are conservative assumptions based on a memo from Alan Sorenson of the Colorado Division of Minerals and Geology and corroborated using information provided in Design of Small Dams. Soil strength testing procedures were not employed to specifically identify input soil parameters for the proposed site. The soils input data are summarized in Table 1 below. Graphical and numeric input data is given in the appendix for each mining phase and cell. Fort Lupton Pit Slope Stability Study, Page 1 of 5