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Mr. Jim Mattern March 19, 2007 Page 33 increased, but the slope remained stable since the model came to equilibrium condition. The model was modified to try to simulate the observed buckling failure of the pit floor. This was done by reducing the rock shear strength in the pit floor down to the weak mudstone seam to zero cohesion and 5° of friction. The failure of the pit floor provides the degree of freedom for the landslide block to move and, thus, the model was no longer in equilibrium. A large portion of the hill slope became unstable. Under these conditions, the model predicts a significant increase in down hill displacements, as shown in Figure 23. This movement results in active tensile failure at the top of hill, as shown in Figure 24 for the I2-K Interburden sandstone layer. This is the same area where tensile cracking was observed in the field as illustrated on the photo in Figure 25. Figures 26 and 27 indicate active shear failure along bedding planes continues on the weak mudstone layer above main L-Seam all the way up the hill. The results in this figure also indicate the weak mudstone layer below Q-Seam floor fails, although the differential displacement is not significant and the ground above Q Floor to the L Roof is predicted stable. Stability is indicated in the model by differential velocities (even though the solution is quasi-static in time)-that is, velocities become an indicator of out of balance forces driving instability. Hence, high velocity areas are interpreted as unstable and low velocity areas are interpreted as stable. When assuming the best estimate properties for the weak mudstone seam the slope is predicted to be stable. This can be demonstrated by looking at the velocity contours predicted on H-I2 Interburden sandstone layer after Gd-2 box cut is mined, as shown in Figure 28. All velocities are less then 10.6 ft/second and the model is interpreted to be in stable equilibrium. For the case when low bedding strength is assumed for the weak mudstone seam, the velocities significantly increase over most of the slope. Figure 29 illustrates velocity contours predicted on H-I2 Interburden sandstone layer for this condition. The white region (un- contoured) indicates velocities greater than 105 ft/second, which are interpreted to be unstable. Also shown in this figure aze the predicted failed zones to the south and west of the high velocity region. The failed zones to the south and west would be the regions where vertical cracking occurs as the block moves downdip. However, the model is not capable of simulating the required large displacements. So, why does failure occur only when the weak mudstone seam and high water table are assumed, and why doesn't failure occur on the weak mudstone seam below the Q Seam? The Failure mechanism can be demonstrated on the Mohr-Coulomb diagram shown in Figure 30. We know from lab testing that there are two weak mudstone seam: (1) one above L-Seam and (2) one below Q-Seam. Their shear strength envelopes are illustrated by the straight lines in the figure where the red line is for the weaker seam above L-Seam and the green line is the seam below Q-Seam. Because these seams are at different depths below ground, their stress state, shown as the solid circles, will be different (red circle is for the weaker seam above L-Seam and the green circle is the seam below Q-Seam). The effect of pore water pressure is to shift the center of the Mohr circles towards the origin, represented by the dotted circles. The resulting stress state is referred to as the effective stress. Where the Mohr circle touches the strength Agapito Associates, Inc.