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lunuarv 15, 2008 <br />4.2 K-Pit T & S Spoil Pile Model <br />4.2.1 .-Issionhrions and 1.imilulions <br />I'oge 17 <br />Simplifying assumptions about conditions are required to reduce the complexity of the <br />analvsis 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 31) model Acre used and an <br />elastoplastic Mohr-Coulomb material behavior was assumed. The Q-Floor mudstonc <br />laver 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 (ai,:,7,) was assumed constant and proportional to Poisson's effect <br />( v,1 -l'). <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 ofthe 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 />phrcatic surface was computed from straight lines between specified groundAater <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 ;Vfelhothd g - <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. F,ach 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 flaw. The main <br />limitation of the explicit tornnrlation for the equations of motion is that small time increments <br />and damping are required for quasi-static equilibrium analvsis. 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.