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Januwj, 15, 2008 Page 29 <br />4.2.5 Bounthn-v Comlition.5 <br />The vertical boundaries of the model assumed zero-displacement condition simulating <br />symmetry to the west over the hill crest and to the east across the G-Strike spoils. The ground <br />surface was free to move as needed and the model base assumed a zero-displacement condition. <br />The initial vertical stresses were gravitational and horizontal stresses assumed lithostatic loading <br />proportional to vertical stresses for Poisson's effect (1'/ 1-r). <br />4.3 Analysis Sequence and Cases Analyzed <br />four parameters were varied in assessing the overall spoil stability: slope angle. fill <br />height, groundwater level, and the bedding plane strength of the Q-Floor mudstone layer. Cases <br />analyzed were based on the combination of these factors. Fill heights and groundwater <br />elevations were relative to the slope toe on the Q-Floor mudstone after reining. The assumed <br />geometry liar this case is shown in Figure 17. <br />ollmim alai Tall, <br />?.?! Floor ?6ulel"u? - - - - <br />tipodh <br />Go, 1. HMI rn - <br />y Flom Mu4i,wite <br />Figure 17. FLAG Model (Toss Section Simulation of G-Pit Spoil Pile with a Slope of <br />37.6" (1.•1 I:1 V) Illustrating Material Groups and Groundwater Table <br />Four equilibrium states were computed for each spoil slope to determine the stability of' <br />the slope for a given groundwater condition. First, the spoil pile was incrementally raised under <br />unsaturated conditions checking stability to obtain the maximum stable spoil height. Next, the <br />model was rerun but with groundwater introduced to examine the effect on stability. Residual <br />strength was then applied to the Q-Floor mudstone bedding plane to simulate long-term <br />conditions and the stability was checked. The final step was to estimate the safety factor of the <br />highest stable slope. <br />The factor ofsatety was estimated by comparing, calibrated spoil strength to the strength <br />at the limit of equilibrium. The limit of equilibrium was determined by gradually reducing the <br />spoil strength until the model did not come to an equilibrium state.. Failure state of the model <br />was assessed using several criteria (including maximum out-of-balance farce. maximum <br />velocity, shear strain increment, plastic indicators). The safety factor was computed using <br />[:yuation 8. <br />Agapito Associates. Inc.