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Mr. Jim Mattern <br />March 19, 2007 <br />Page 2 <br />discusses the practical implications of the results regarding failure conditions and mechanisms of <br />the landslide. <br />Ci-PIT NUMERICAL MODEL <br />A mine-scale numerical model of the G-Pit area of the mine was developed. The general <br />mine plan is shown in Figure 1. At the time of the slide, mining was occurring in G-Dip Pit cut <br />#2-3, Z-Strike Pit cut #21, and further to the west in F-Dip Pit. Previous mining had occurred <br />north of Z-Strike Pit cuts #1-2Q, north of G-Strike Pit cut #4-6 (abandoned after the <br />September 2005 highwall failure), and minor shallow mining in A-Dip Pit. The schematic cross <br />section through main landslide block, shown in Figure 2, illustrates what the slope looked like <br />before and after the slide. The letters in the figure represent sequence of coal seams. This figure <br />represents the assumed failure mechanism hypothesis that the model will attempt to reproduce. <br />The objective of the model was to (1) back-analyze the G-Strike Pit highwall that failed <br />in September 2005 by calibrating rock mass properties to the failure, and (2) examine possible <br />failure mechanisms leading to the recent landslide. The model has been developed utilizing the <br />three-dimensional (3D) fmite-difference code, FLAC3D.3 Average rock mass material <br />properties are used in the analysis based on laboratory testing and field characterization of <br />conditions. The purpose of the analysis is to allow failure mechanisms to occur given realistic <br />conditions. However, due to the complexity of conditions, some assumptions made in the model <br />were iteratively modified to delineate local failure and global failure propagation. <br />G-Pit Model Assumptions <br />Assumptions about conditions are required to reduce the complexity of the analysis to a <br />manageable magnitude. Simplifying assumptions have been made in the analysis, the most <br />important of which are as follows: <br />1. The model geometry is simplified. The geometry is generated with data from six north- <br />south sections from Trapper Mining Inc.'s (TMI) geologic model.4 The model <br />discretization is relatively coarse: 70 ft x 70 ft laterally. This might have the effect of <br />reducing displacements, but does not significantly influence the mode of failure, which is <br />of primary interest in this analysis. <br />2. Rock mass behavior is assumed inelastic deformations with superimposed ubiquitous <br />joint behavior parallel to bedding. This model superposes one set of fractures on a Mohr- <br />Coulomb model. Only bedding planes are considered; non-linear deformations of <br />vertical fractures, especially fracture opening, are not accounted for. The low tensile <br />s FLAC3D Version 3.0 (2005), Fast Lagrangian Analysis of Continua in 3 Dimensions, "User's Guide," Itasca <br />Consulting Group, Inc., Minneapolis, MN. <br />Trapper Mining, Inc. (2005), (No Name), AutoCAD file of borehole contours or each major stratigraphic layer, <br />[Borehole-contours. dwg]. <br />Agapito Associates, Inc. <br />