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Mr. Jim Mattern March 19, 2007 Page 27 parameter to observed fracturing. Displacements are used for this. Figure 18 shows a comparison of model surface displacements for G-Strike pit failure to mapped surface cracking from September 2005 failure. It appears that surface cracking occurred somewhere between 0.12-0.14 ft surface displacement. This is not unreasonable given the data collected from surface benchmark movements. Where this prediction breaks down is when displacements become large and the entire hill begins to move, which can be seen in Figure 19. Nonetheless, the model can be used to predict failure by using this limiting 0.12-0.14 ft surface displacement as failure criteria. To summarize the calibration analysis, high pore pressures and very low bedding plane strengths were required on the weak mudstone seam in the model in order to match the observed failure in the Gs-6 strike highwall face. These would not have been expected to be present given available information on water levels in observation wells and laboratory testing. G-DIP PIT PRE-MINING CONDITION From the calibrated model we are now interested in what it can tell us about the condition of the slope prior to excavation of the G-Dip Pit box cut. By examining the failed zones on only the weak mudstone seam, shown in Figure 20, it can be seen that there is failure disturbance predicted nearly 3,000 ft updip of the Gs-6 highwall. Yet the I2-K sandstone interbed layer above the weak mudstone seam (~60 ft deep) remains in an essentially unfailed elastic condition, as indicated in Figure 21. Note that in this figure, the tensile failed zones (green and orange colors) are where the historic landslides have been identified from aerial photographs. These areas are predicted by the model to the southeast and southwest from this figure. As a result of the weak mudstone seam being disturbed a long distance updip, measurable displacements occur updip as well. Figure 22 indicates that displacements greater than about 0.15 ft were isolated to the immediate Gs-6 highwall face area (same state as Figure 13). However, small displacements extend 3,000 ft updip. Even at OS-GI-CCR core hole location where the TDR was installed 1,220 ft updip of the Gs-6 highwall failure, the predicted displacement is 0.84 inches (0.07 ft) prior to dip mining. To summarize the pre- dip pit mining conditions, by assuming high pore pressures and low strengths on the weak mudstone seam in the model, the predicted disturbance on the weak mudstone seam extends a significant distance updip of the Gs-6 highwall. However, even under these extreme conditions the model predicted the overall slope to be stable before the Gd-2 dip box cut was made. G-DIP BOX CUT PIT CONDITIONS The model was then extended to simulate dip mining to the east of the G-Pit area. Assuming high pore pressures, low strengths on the weak mudstone seam, and partial mining in the box cut (to I2-Seam in north and K-Seam in south) predicted displacements significantly Agapito Associates, Inc.