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<br />2) The plunge of the lines of intersection exceeds the angle of friction. <br />' The trends of the lines of intersection that can daylight onto the slope have been <br />identified with arrows on the stereonets for each of the wall orientations (Figures 11 - <br />' 16). Based on the results discussed in Section 4, the limiting resistance to sliding is <br />drawn as a dashed circle on the stereonets having a radius equivalent to a friction angle <br />' of 33°. Markland's two criteria for sliding are satisfied when the intersection of the <br />discontinuities plot on the stereonets within the hatched regions that are bounded by the <br />' friction circle and the great circle representing the plane of the slope face. Further <br />evaluation of the stereonet plots can then identify the specific discontinuity surfaces <br />' where sliding may occur. <br />Planar failure is a special case of wedge failure. If Markland's criteria is satisfied and <br />the dip direction of either of the surfaces forming the intersection falls between the dip <br />' direction of the slope face and the trend of the line of intersection, then sliding will occur <br />on that plane rather than along the line of intersection. The following four conditions, <br />' defined by Hoek & Bray (1977), must be met in order for true planar failure to.occur: <br />' 1) The plane on which sliding occurs must strike parallel or nearly parallel to the slope <br />face. <br />2) The failure plane must "daylight" in the slope face (i.e. the dip of the plane must be <br />smaller than the dip of the slope face). <br />3) The dip of the failure plane must be greater than the angle of friction of the plane. <br />' 4) Release surfaces, which provide negligible resistance to sliding, must be present in <br />the rock mass to define the lateral boundaries of the slide. <br />As indicated earlier, the data for each set cluster around the representative value that is <br />plotted on the stereonets. To illustrate the possible variation in discontinuity dip angles <br />' and directions, and how this can affect the interpretations and conclusions that can be <br />made, multiple foliation discontinuity dip angles have been drawn for F-1 on each <br />' stereonet. The alternate foliation dip angles for F-1 correspond to one standard <br />deviation above and below the mean dip angle of 36° (i.e., 46° and 26°). <br />1 <br />-14-