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May 28, 2009 Page 7 <br />Finally, at the very bottom, the weakest layer of the entire formation Q Floor mudstone was <br />assigned. This layer also formed the bottom of the G-Pit floor. <br />The phreatic surfaces assigned to the model were initially generated with the aid of data <br />obtained from two piezometers installed within the K-Pit. Preliminary phreatic surfaces assigned <br />to the models were projections of the most recent recorded piezometric levels at these two <br />piezometers within the K-Pit and their horizontal extrapolations beyond the K-Pit. After further <br />consideration, AAI believes that although the field piezometric data may represent in-pit <br />subsurface-seepage conditions accurately, they may not be sound enough to predict hydrologic <br />conditions underneath the toe buttress. AAI believes that long-term seepage under the buttress <br />will be controlled by the interface such as the one between the strata I1-I2 interburden sandstone <br />and Q Floor mudstone, which have high differential permeability. Also, short-term infiltration <br />due to rainfall or snowmelt could cause saturation zones within the overburden layer. Hence, <br />two phreatic surfaces were developed to simulate these two worst-case scenarios. In the first <br />case, the phreatic surface followed the top of the mudstone floor in the numerical models, whose <br />top surface was raised up to the elevation of the L Seam horizon. In the second case, the phreatic <br />surface was made to follow the top of the overburden layer. These two assumptions regarding <br />the phreatic surface were agreed upon by TMI staff as the most likely seepage conditions in the <br />long term. <br />2.2 Methodology <br />FLAC/Slope is a specialized version of FLAC designed specifically to perform factor-of- <br />safety (FS) calculations for slope-stability analysis. It provides a full solution of the coupled <br />stress/displacement, equilibrium and constitutive equations. Given a set of properties, the system <br />being modeled is determined to be stable or unstable. By automatically performing a series of <br />simulations while changing the strength properties (i.e., shear strength reduction technique), the <br />FS can be computed to correspond to the point of stability, and the critical failure (slip) surface <br />can be located. <br />2.3 Material Properties <br />' Properties of all the geologic layers, except for the spoil, simulated in the models were <br />the same as material properties calibrated during the G-Pit landslide analysis. The material <br />properties are presented in Table 1. The Mohr-Coulomb constitutive model was assigned to all <br />layers. The bedding of the Q Seam floor assumed strength values of zero cohesion and tensile <br />strength, and a 15° friction angle. All the rock layers, including the overburden, were assigned a <br />porosity value of 5%. <br />' Preliminary models were assigned with spoil properties (i.e., Spoil 1) based on <br />previously-established empirical relationships for similar spoils4 and in situ moisture-density test <br />results.s Subsequently, the cohesion value of the spoils were lowered, as the toe buttress will be <br />constructed in lifts of 25 to 50 feet thicknesses in the absence of real compactive effort other than <br />from the movement of heavy earth-moving equipment. The lower-strength (i.e., cohesion = <br />200 pounds per square foot [psfJ) spoil material was referred to as Spoil 2 and represented the <br />worst-case scenario with regards to spoil strength. <br />Agapito Associates, Inc.