Combined kinematic analysis of complete excavations 323NFigure 18.10 Example assessment for a slope of orientation 295"/75"-flexural
toppling instability.
Thus, to assess the kinematic feasibility for direct toppling instability, we
superimpose the specific overlay (in this case, Fig. 18.9(b)) onto a projection
of the poles for the rock mass discontinuity data (in this case, the data
shown in Fig. 18.1), with the result shown in Fig. 18.10.
It can be seen that the potential for flexural toppling is low, as the region
of instability coincides with the limit of the cluster associated with discon-
tinuity sets C (and B, bearing in mind that some of the discontinuities
associated with this set appear within the region of instability). As before
with the other instability mechanisms, however, we would wish to identify
the precise nature of this geometry in the field to ensure that, indeed, the
possibility of such an instability mechanism was low-e.g. are the relevant
discontinuities sufficiently persistent, or are they a minor impersistent set
with short trace lengths?
18.2 Combined kinematic analysis of complete
excavations
When considering a proposed surface excavation in a rock mass, the
kinematic feasibility of all of the four mechanisms described in Section 18.1
must be established, and for all potential slope orientations. In some
projects, the slope dip direction may be dictated by considerations other
than rock mechanics, e.g. a fixed highway route requiring cuttings. Even
the slope dip may be fixed, but the rock engineer will be able to make a
contribution to optimizing the stability of the slope. In other projects, such
as an open-pit mine or quarry, all slope dip directions may have to be