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Mr. Jim Mattern March 19, 2007 Page 5 strength in mudstone layer above L-Seam does not fully account for large displacements of opening vertical fractures. Its influence to the model is not significant, but results in under-prediction of downdip displacements. 3. Material properties are assumed uniform for each layer and were averaged from over 2801ab tests from six core holes. In situ stress ratio (Qy:v„) was assumed constant and estimated assuming the horizontal stress is proportional to Poisson's effect (u/1-v) based on recent stress measurements in G-Pit. 4. The model does not directly account for time-dependent effects. The solution employs a quasi-static method for solving time-explicit equations of motion and force balance. Time-dependent properties aze only considered in the sense that equilibrium conditions are solved for using uniform peak strength and again using uniform residual strengths. 5. Water surface is assumed worst case as parallel to the ground and 10 ft below surface. The pit and spoils are assumed drained, but this assumption has minimal impact on the updip slope stability. Methodology Modeling was performed with the FLAC3D computer program. The program is a well- known 3D time-explicit, finite-difference program for engineering mechanics computation. FLAC3D is an industry accepted tool for solution of 3D geomechanics problems in geotechnical engineering. The program is capable of simulating inelastic behavior of 3D soil or rock structures (or of other materials) that undergoes plastic flow when elastic yield limits are reached. Materials are represented by polyhedral elements within a 3D grid that is adjusted by the user to fit the shape of the object to be modeled. Each element behaves according to a prescribed linear or nonlinear stress/strain law in response to applied forces or boundary restraints. The material can yield and flow, and the grid can deform by moving with the material. The time-explicit, Lagrangian calculation scheme and the mixed-discretization zoning technique used in the program ensure that plastic collapse and flow are accurately computed. Because no matrices are formed, large 3D calculations can be made without excessive computer memory requirements. The limitation of the time-explicit formulation is that small time increments and damping are required for quasi-static analyses, but these are overcome by automatic inertia scaling and damping that does not influence failure modes. The model has been set up based on several observations from the trip report. Most importantly, it is thought that the landslide occurred on a single plane or seam resulting in block movement (translation and rotation) kinematically governed by the geometry of bedding and mining. The cross section schematic m Figure 2 has a sliding plane mechanism. However, in the absence of field data, it is assumed that failure occurs on a thin seam. In this way, the model will determine the most likely seam without pre-specifying the failure seam or plane. Agapito Associates, Inc.