2010-05-07_PERMIT FILE - C2009087 (48)

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-13- • and lower in the cut; and the gray shale which occurs near the top of the cut and near the footwall. Our analysis is presented in Appendix B (Figs. B -1 through B -3) and indicates that the brown sandstone and gray shale will be kinematically stable at cuts no steeper than 75 degrees. The gray sandstone is indicated to be stable at about 90 degrees. Figure 6 presents a comparison between indicated stable slopes and slope angles which existed at the time of our field investigation. The observed angles are very near those indicated to be stable for formations below the brown sandstone. The brown sandstone and overlying interbedded shale and coal exist at angles 10 or more degrees flatter than those indicated to be stable. At this time it is not known where the formations were originally cut at the existing slopes, or if the slopes are the result of weathering and slaking • or rockfall. Flattening as a result of rockfall does not appear to be likely since there was little rock material at the base of the highwall. There was, however, evidence of slaking, resulting in some talus along the base of the highwall. Most of the material appeared to be similar to the upper inte- bedded coal and dark gray shale. Extensive slaking of the shales has apparently taken place over the 3± year exposure. Marklin's method indicates that the entry wall will be "stable" or safe at cuts no steeper than 75 degrees, but does not directly yield a factor of safety against mass failure when friction between blocks is neglected. The assumed failure mechanism is along joints which daylight on the cut face. The size of the failure which can result using Marklin's method is dependent upon the continuity of the failure plane or joints. The joint patterns we measured indicate fairly good continuity of major and minor joint orientations n U