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January 15, 2008 <br />4.2 K-Pit T & S Spoil Pile Model <br />4.2.1 Assumptions and Limitations <br />Page 27 <br />Simplifying assumptions about conditions are required to reduce the complexity of the <br />analysis to a manageable magnitude. The key assumptions made included the following. <br />The model geometry was created by developing vertical cross section through the 3D <br />model of the G-Pit Landslide. The average dip of the bedding along this projection is <br />13°. The true dip of the bedding in the K-Pit area ranges 8°-25°, but the 13° is <br />representative of the average across the area. Safety factors of the spoils are inversely <br />proportional to the bedding dip angle, so the steeper the bedding dip, the less stable the <br />spoil pile will be. <br />• The calibrated rock mass properties from the G-Pit landslide 3D model were used and an <br />elastoplastic Mohr-Coulomb material behavior was assumed. The Q-Floor mudstone <br />layer included the effect of weak bedding (i.e., ubiquitous joint behavior). <br />• In situ stresses increased with depth assuming gravitational weight of each lithologic <br />layer. The stress ratio (ah:Q,,) was assumed constant and proportional to Poisson's effect <br />(v/l -v). <br />• The elastoplastic spoil behavior assumed a strength loss with continued inelastic strain <br />(strain-softening behavior). Only the friction angle of the spoil was reduced since the <br />spoils are assumed to have negligible cohesive and tensile strength. Spoil properties were <br />taken from the results of the spoil calibration analysis. <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%. <br />• It is assumed that mining is to the Q-Seam and spoils would be piled on the weak <br />Q-Floor mudstone. The worst design condition is when spoils are placed directly on the <br />Q-Floor without buttressing the toe. <br />4.2.2 Methodology <br />The FLAC program employs an explicit finite difference solution method. The rock is <br />represented by discretized elements, adjusted by the topography and bedding. Each element <br />behaves according to the constitutive relationship in response to applied boundary restraints. <br />The grid moves and deforms with the material. FLAC uses an explicit Lagrangian solution <br />scheme with a mixed-discretization technique to ensure accurate inelastic flow. The main <br />limitation of the explicit formulation for the equations of motion is that small time increments <br />and damping are required for quasi-static equilibrium analysis. This limitation is overcome by <br />using inertia scaling and auto-damping and ensures that the failure mode is not influenced and <br />the model comes to an equilibrium state. <br />Agapito Associates, Inc.