10.2 Slope failure mechanism

Figure 1 shows a number of methods to analyse slope monitoring results that may help to identify the mechanism of slope failure. This information can be useful in applying the appropriate method of stability analysis and in the design of stabilization measures. The combined displacement and velocity plot as shown in figure 1(a) depicts that the acceleration in the slope stopped after day 5. This change in behavior is clearly evident on the velocity plot, whereas the velocity is constant after day 5. In comparison, on the movement plot, the change in gradient is not so obvious. This slope movement would be typical of regressive type instability. Figure 1(b) shows the magnitude and dips of movement vectors for survey stations on the crest, mid-height and toe of the slide. The dip angles approximately equal the dip of the underlying failure surface, indicating that a circular failure is taking place in which the sliding surface is steep near the crest and near-horizontal at the base. This information also shows the location of the toe of the slide, which may not be essentially coincide with the toe of the slope. Figure 1(c)  shows a movement vector for a typical toppling failure in which the stations located on the overturning beds at the crest may move upwards by small amount, while there is little movement below the crest. Figure 1(d) shows contours of slope velocity plotted along a plane of the pit show both the extent of the slide, and the area(s) of most rapid movement.


Figure 1: Analysis of slope monitoring results (a) Regressive movement, (b) Movement vectors showing circular failure mechanism, (c) Movement vectors showing toppling failure mechanism and (d) Slope velocity contours showing extent of slope movement (Wyllie and Munn, 1979).