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6.2 Wedge Stability Analysis <br />As presented in earlier annual reports, the method of stereographic analysis <br />developed by Markland (1972) was used for this study. Markland's test is <br />designed to identify the possibility of a wedge failure in which sliding takes place <br />along the line of intersection of two discontinuities (Hoek & Bray, 1977). <br />According to Markland's test, sliding can occur when: <br />1 } The plunge of the lines of intersection is less than the dip of the slope face. <br />2) The plunge of the lines of intersection exceeds the angle of friction. <br />The trends of the lines of intersection that daylight onto the slope have been <br />identified with arrows on the stereonets for each of the wall orientations (Figures <br />10-12 &1416). The friction angle is drawn as a circle on the stereonets at 33°, <br />and has been modified from previous annual reports based on the results of the <br />laboratory testing conducted for the 2003 geotechnical investigation (L&A, 2003). <br />Markland's two criteria for potential sliding are satisfied when the intersection of <br />the discontinuities plot on the stereonets within the hatched regions that are <br />bounded by the friction circle and the plane of the slope face. Further evaluation <br />of the stereonet plots can then identify the specific discontinuity surfaces where <br />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 />-14- <br />