Engineering Rock Mechanics

324 Design and analysis of surface excavations

evaluated by an engineer who is expected to recommend the various slope
dip values to ensure stability. In this case, we adopt an approach whereby
all of the overlays in turn are rotated around the complete perimeter of the
projection, indicating the regions of kinematic feasibility associated with
the different mechanisms.
Because discontinuities occur in sets, the analysis leads to identification
of slope orientations which are kinematically infeasible and other orien-
tations which are kinematically feasible for the different mechanisms. The
orientations then have to be considered together. We usually find that there
are ranges of slope dip directions where steep (or even vertical) slopes are
safe and other ranges which are susceptible to one or more mechanisms
of instability. It must be remembered that the hemispherical projection
technique only utilizes orientations and not locations. It follows that if the rock
mass shows any degree of inhomogeneity, it may be necessary to consider
the rock mass in 'structurally homogeneous' domains, each of which is
analysed separately.
The results of this type of complete analysis-performed using the data
in Figure 18.1-are shown in Fig. 18.11, in which the steepest slope dips
preventing the development of the relevant instability mechanism are
tabulated, together with the net result which is the steepest safe slope. The
associated diagram shows a plan of how the crest of an excavation with a
circular floor would appear.

Slope crest - - - - - Slope toe

34s 000 01s

330 030

315 045

300

270 285

255 '--'

240

22s

210 1 50

195 180 165

Figure 18.11 Example assessment for a circular floored excavation-plane, wedge

and direct and flexural toppling instabilities.