Engineering Rock Mechanics

3 1 6 Design and analysis of surface excavations

Figure 18.4@) shows the completed specific crescent-shaped overlay for
a slope dip of 75" and an angle of friction of 30". In the design process, it
will probably be the innermost boundary of the crescent which is the most
variable-i.e. how steep the slope can be without wedge instability
occurring.
The final step in assessing the kinematic feasibility for wedge instability
is to superimpose the specific overlay (in this case Fig. 18.4@)) onto a
projection representing all the intersection possibilities for the rock mass
discontinuity data. This is usually achieved by considering one represen-
tative plane from each discontinuity set and determining the set inter-
sections. A more accurate method would be to determine the intersections
resulting from all inter-set combinations of discontinuities and treat these
as a set of intersections. The result for this example, using the former
method, is shown in Fig. 18.5.
Once more, the advantages of the overlay technique are apparent.
First, there are only two lines of intersection along which wedges
are potentially unstable-these are formed by the intersection between
discontinuity set B and sets A and E. Again, the exact value of the angle
of friction (i.e. the position of the outermost boundary of the crescent)
is unimportant, but the slope angle itself is paramount. By reducing
the slope angle, and hence moving the innermost boundary of the
crescent away from the centre of the projection, wedge instability can be
minimized. Returning to the field, one can visually assess the nature of the
lines of intersection IAB and IBE to establish the shape and size of the
wedges.

N

Figure 18.5 Example assessment for a slope of orientation 295"/75"-wedge
instability.