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Mr. Jim Mattern March 19, 2007 Page 48 As mentioned above, the influence of recent rains on increasing pore water pressures in the rock was important to the loss of stability. It is not known whether the water table as represented in the model is conservative or unconservative. It might be conservative because the entire slope is assumed fully saturated and undrained, but hydrostatically parallel to the ground surface. It might be unconservative because it assumes no head communication from higher conductivity beds above the weak mudstone seam providing pore pressures exceeding the ground elevation. To properly simulate this behavior a coupled hydro-mechanical model would be required. The roll of the box cut was to provide the required degree-of-freedom for large-scale kinematic block movement. Results indicate that the block was incapable of moving until forces from the block resulted in floor buckling due to excessive rock pressures in the floor. Complex buckling behavior was not simulated; rather, shear strength of the floor was reduced. There was no evidence of floor buckling in the dip pit prior to slope failure. However, the K-Seam coal exposed at the south end of the box cut did have an appreciable roll which might have contributed to the ease of buckling failure. It is probable that slope instability would have occurred eventually as mining attained the depth of the L-Seam thereby exposing the weak mudstone seam in the highwall face. SUMMARY Specific conclusions that can be drawn from these analyses are summarized by the following points: • The weak mudstone seam above main L-Seam is most likely the slip plane primarily because it is the weakest layer in the geologic sequence. • The model indicates that the main block slides to the north-northeast with the primary direction of movement down the 8° dip of the beds and, thus, the weak mudstone seam requires a low friction of less than 8° and no cohesion. • The main par[ of the landslide block is likely undisturbed above the slide plane, including H, I2, and K coal seams, because of the uniformity of the bedding across the site. It likely slid en mass. • Stresses on the sliding plane are not predicted to vary significantly across the area, suggesting global instability is inevitable when failure occurs. • The final failure mechanism that released the main block was likely tensile failure of the upper layers at the southern limit of the box cut. • The continuum-based numerical model is not capable of predicting large displacements without significant modifications. • Rock mass properties have been calibrated to Gs-6 Cut failure. To match observed failures, the low strength properties for weak mudstone seam and high pore pressures are required along bedding. Agapito Associates, Inc.