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Mr. Jim Mattern March 19, 2007 Page 6 G-Pit Model Geometry The simulated region covers an area 4,900 ft west to east by 6,200 ft south to north. Model coordinates are consistent with the Colorado State Plane Coordinates referenced to (1,430,000E, 400,000N) in feet. The boundaries of the model extend from 4,900 to 9,800 ft in the east-west direction (X-coordinates) and 0 to 6,200 ft in the north-south direction (Y-coordinates) as shown in Figure 3. The model depth goes from the irregular ground surface down to an elevation of 6,750 ft mean sea level (MSL). The dip of the beds in the eastern portion of G-Pit is known to be about 8° towazds due north. The dip of the beds increases to 10°-12° in the south and southwest portion of the G-Pit and are oriented more northeasterly toward the N25°E direction. The topography and geology was interpolated from six north-south sections located at 4,900, 5,700, 6,830, 7,170, 8,700, and 9,800 ft west to east, respectively. These sections aze indicated in Figure 1 by the magenta lines. A linear interpolation algorithm was used for grid point interpolation between sections. The resulting mesh consisted of 408,079 8-noded, 5-tetrahedral zones defined in the model (28% more were deleted because they were above the interpolated topography). G-Pit Model Material Core logging identified over 46 distinct geologic strata, of which some have similar rock types, conditions, and properties and were combined. A total of 23 material layers were simulated in the model. The ubiquitous-joint constitutive models are applied to each rock layer. The ubiquitous-joint model is an anisotropic elasto-plasticity model that includes weak planes of specific orientation embedded in aMohr-Coulomb solid. The mechanical deformation and strength properties for both the in situ rock mass and bedding planes are provided in Table 1 for each rock type. In situ rock mass properties are based on averages from laboratory tests5 adjusted using the widely-accepted Hoek-Brown empirical methodb involving Geologic Strength Index (GSI) and Rock Mass Rating (RMR) from core logs. The assumed orientations of bedding planes are listed in Table 2. Spoils are assumed a medium compacted soil material with low modulus and low strength whose properties are given in Table I. The lithology is based primarily on Borehole OS-Gl-CCR (Figure 4a) and the six geologic sections of major coal seams. The layers as implemented in the model are shown in Figure 4b. G-Pit Model Boundary Conditions Model boundaries respond as if there is material beyond the limits of the model. The four vertical sides (i.e., east, west, north, and south sides) aze roller boundaries with azero- displacement condition, which means the geometry outside the model mirrors the geometry inside. This is accurate because mining is generally not near the boundaries and their location has been selected as a line of geometric symmetry. The ground surface was unrestrained and the s Agapito Associate, Inc. (2006), "Trapper Mining Inc.-Rock Mechanics Core Testing Results," prepazed for Trapper Mining, Inc., February 6. e Hoek E. and E. T. Brown (2000), "Empirical Strength Criterion for Rock Masses," Journal of Geotechnical Engineering Div., ASCE 106(GT9), 1,011,035. Agapito Associates, Inc.