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May 28, 2009 <br />2.4 Boundary Conditions <br />Page 8 <br />The vertical boundaries of the model assumed a horizontal zero-displacement condition. <br />The ground surface was free to move as needed and the model base assumed a vertical zero- <br />displacement condition. <br />¦ Table 1. Geotechnical Properties of Materials in K-Pit and Toe Buttress FLAC/Slope <br /> Models <br /> Rock Mechanics Properties <br />Layer Stratum Dry Density Cohesion Tensile Strength Friction Angle <br />No. (pcf) (ksf) (ksf) (degrees) <br />1 Overburden 134.2 2.5 1.5 15.9 <br />2 11 Seam, coal 80.9 7.9 0.7 18.2 <br />3 I142 Interburden, sandstone 136.5 25.0 6.7 40.6 <br />4 Q Floor mudstone 134.1 0 0 15.0 <br />5 Spoilt 100.0 1.3 0 20.0 <br />6 Spoil2 100.0 0.2 0 20.0 <br />• The calibrated rock-mass properties from the G-Pit landslide FLAC31) model were used <br />and an elasto-plastic Mohr-Coulomb material behavior was assumed. The Q Floor <br />mudstone layer was assigned weak bedding-strength parameters instead of rock-mass <br />parameters. <br />• In-situ stresses increased with depth assuming the gravitational weight of each lithologic <br />layer. <br />• Groundwater seepage was not directly simulated; rather, an effective stress analysis was <br />performed with pore pressure based on depth below an assumed water table. The <br />phreatic surface was computed from straight lines between specified groundwater <br />elevation points. The rock-mass density was assumed saturated below the phreatic <br />surface based on a rock porosity of 5%. However, the spoil was assumed to have a <br />porosity of 25%. <br />• The three spoil toe-buttress section models were simulated with three different <br />groundwater conditions, i.e., elevated phreatic surface, as extrapolated from the K-Pit, <br />phreatic surface along top of the overburden layer and phreatic surface along the top of <br />mudstone floor layer. <br />• Summarized characteristics of the K-Pit toe buttress is presented in Table 2. <br />Table 2. K-Pit Toe Buttress Characteristics <br />Volume of Horse Gulch fill <br />Anticipated make-up <br />Maximum slope of fill <br />Placement method <br />4.7 million cubic yards <br />30% sandstone 70% shale <br />3H:1 V <br />100% truck <br />Intermediate bench/contour ditch spacing 100 feet <br />Maximum lift thickness 100 feet (nominal 50 feet) <br />Agapito Associates, Inc.