GENERAL36864 - Laserfiche WebLink

Home Browse Search

Mr. Jim Mattern March 19, 2007 Page 46 N50°E, Dip = 89°N, 0.37 ff) in Figure 34. These figures, when considering the aerial photo in Figure 32 and observed surface cracking, suggest that the shape of the southern limits of the landslide may be controlled by opening of existing fractures. It is not likely that these fractures played a significant role in initiating the landslide because bedding planes aze significantly weaker than these fractures. It is interesting to examine the orientation of principal stresses. At the time the stress measurements were made,' the measurements did not indicate any unusual stress levels and were consistent with a gravitational stress field whose principal orientations correlated joint set orientations in the massive sandstone beds. The model results seem to suggest that the orientation of principal stresses are also significantly influenced by the geometry of the hillside since to the southwest the bedding dip increases, which gives the major principal stress (6i) as N37°W, which happens to parallel the primazy fracture set. The results confum that the influence of strikeline mining contributed to instability of the slope. However, it should be noted that it was the mine's experience at the time of G-Strike mining that strikeline mining could be successfully performed until the September 2005 highwall failure. Evidence for this is that no major highwall stability problems have occurred in the current Z-Strike Pit, even in the presence of potential instability associated with the Z-Pit burn azea. It is possible that a much smaller landslide could have been initiated due to shallow mining of H-Seam. The material above the H-Seam is naturally unstable, as evidenced by aerial photos ofthe shallow historic escarpments. The model results indicate the layers above H-Seam remained stable, but this might be influenced by assumed properties. The model results indicate that the spoils buttressing the failed G-Strike highwall were insufficient to resist movement of the entire hillside. The purpose of the buttressing was to locally stabilize the highwall failure that occurred in G-Strike during September 2005. The results indicate that the slope was stable after the 2005 G-Strike highwall failure and prior to G-Dip Pit mining. The model results indicate that dip mining contributed to instability of the slope. Although mining in G-Dip Pit was not extensive, mining had progressed to the I2-Seam on the north and to K-Seam on the south leaving a thin floor `beam' above the weak mudstone seam. To get the slope to fail in the model floor buckling had to be simulated providing a degree of freedom for the landslide block to move. It was the mines experience that dip mining perpendicular to the plunge of the beds provides a more stable highwall than in strikeline mining, but global hillside failure was not anticipated and had not been experienced by the mine in the past. Displacements computed in the model were not large enough to reproduce the magnitude of displacements or rotation of the main slide block. This is because the model was set up to primarily focus on predicting instability and not matching the distance the block slid. A slip- interface would have been required in the model, but it would have been necessary to know the location of the slip plane prior to the analysis which might bias the model results. Agapito Associates, Inc. (2005), "Determination of Secondary Principal Horizontal Stresses at Trapper Mine Using the Downhole Overcoring Method," prepazed for Trapper Mining, Inc., December. Agapito Associates, Inc.