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the plane of the slope face. Further evaluation of the stereonet plots can then <br />identify the specific discontinuity surfaces where sliding may occur. <br />Planar failure is a special case of wedge failure. If Markland's criteria is satisfied <br />and the dip direction of either of the surfaces forming the intersection falls <br />' between the dip direction of the slope face and the trend of the line of <br />intersection, then sliding will occur on that plane rather than along the line of <br />' intersection. The following four conditions, defined by Hoek & Bray (1977), must <br />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 <br />slope face. <br />2) The failure plane must "daylight" in the slope face (i.e. the dip of the plane <br />must be 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 <br />plane. <br />' 4) Release surfaces, which provide negligible resistance to sliding, must be <br />present in 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 <br />that is plotted on the stereonets. To illustrate the possible variation in <br />discontinuity dip angles and directions, and how this can affect the <br />' interpretations and conclusions that can be made, multiple foliation discontinuity <br />dip angles have been drawn for F-1 on each stereonet. The alternate foliation <br />' dip angles for F-1 correspond to one standard deviation above and below the <br />mean dip angle of 36° (i.e., 46° and 26°). <br />6.2.1 East Wall <br />Historically, the east wall has had no major instability issues. The stereonet plot <br />for the east wall was derived from the full dataset (Figure 10) shows multiple <br />' plunges of lines of intersection less than the angle of the slope face, resulting in <br />the interpretation that the potential for sliding exists along these intersections. <br />-18-