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May 28, 2009 <br />3.0 ANALYSIS RESULTS <br />Page 9 <br />Numerical models of three different sections (i.e., East, Mid and West) of the toe buttress <br />were developed to investigate long-term FS values against global failure. For all three sections, <br />potential ground-water conditions and spoil properties were varied. The results are presented <br />below. <br />3.1 Mid-Section Model <br />The first set of Mid-section model runs included the entire K-Pit and the toe buttress. As <br />mentioned earlier, the phreatic surface was assigned based on recently obtained piezometric <br />levels from the K-Pit that was extrapolated horizontally through the toe buttress. Spoil 1 <br />material properties were used for the in-pit backfill, as well as the spoil in the toe buttress. <br />Analysis results of the model indicated that the spoil fill within the toe buttress was weaker than <br />the spoil backfill within the K-Pit and failure surfaces were most likely to develop within the toe <br />buttress. With the elevated phreatic surface, the FS value against global failure was determined <br />to be 2.48 (Figure 3a). In the next stage of model runs, the model was limited to only the toe <br />buttress and the southern end of the model was truncated at the northern edge of K-Pit highwall. <br />However, the spoil properties and phreatic surface elevations were kept the same. The analysis <br />results indicated no significant change in FS value (= 2.46) against slope failure, as compared to <br />' the previous case (Figure 3b). This indicated that in-pit spoil geometry has little bearing on the <br />structural stability of the toe-buttress structure, at the present configuration. <br />In the next stage of model runs, the lower spoil-strength parameters were used (Spoil 2) <br />with the toe buttress-only model and with the same elevated phreatic surface as used before. The <br />lower strength of the spoils in the toe buttress was reflected in the lower than optimum FS value <br />' obtained from the analysis of this model, i.e., a value of 1.16 (Figure 3c). However, AAI does <br />not believe that the existence of such a seepage condition is feasible within the toe buttress. <br />Hence, AAI developed two alternative seepage conditions, which have the highest potential to <br />occur over the long term. In both the cases, the phreatic surface was expected to be controlled by <br />the geologic formations underlying the toe-buttress fill. <br />' In the first case, the phreatic surface followed the top of the mudstone floor and in the <br />second, the phreatic surface followed the top of the overburden layer. In the models developed <br />using the modified groundwater conditions, the toe buttress was assigned the material properties <br />1 of Spoil 2. Analysis results of the models with these two indicated identical FS values (= 1.86) <br />against slope failure (Figures 3d and e). This FS value should be adequate for long-term stability <br />of the toe buttress. An additional observation made from the numerical analysis results was that <br />' the failure surfaces in all the cases examined were confined within the spoil buttress, indicating <br />foundation failure or movement issues may be disregarded. <br />3.2 East-Section Model <br />The preliminary model of the East section was developed similarly to the Mid section, <br />and it included the K-Pit backfill and an elevated phreatic surface projected from field <br />piezometric levels. The toe buttress in this model was assigned properties of Spoil 1. As <br />observed in the case of the Mid-section models, the most likely failure surface developed within <br />the toe buttress and FS value against global slope failure within the toe buttress was determined <br />to be 3.06 (Figure 4a). From the observation made during the simulation of Mid-section models, <br />Agapito Associates, Inc.