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sliding is drawn as a dashed circle on the stereonets having a radius equivalent <br />to a friction angle of 33°. Markland's two criteria for sliding are satisfied when the <br />intersection of the discontinuities plot on the stereonets within the hatched <br />regions that are bounded by the friction circle and the great circle representing <br />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 interpretations <br />and conclusions that can be made, multiple foliation discontinuity dip angles have <br />been drawn for F-1 on each stereonet. The alternate foliation dip angles for F-1 <br />correspond to one standard deviation above and below the mean dip angle of <br />36° (i.e., 46° and 26°). <br />-18- <br />