346 Underground excavation instabiliv mechanisms
Figure 19.8 Identification of kinematically admissible sliding blocks at a non-
overhanging inclined surface.on the perimeter of the spherical triangle lies beyond the friction circle
drawn below the horizontal (i.e. has a dip steeper than @), sliding is
kinematically feasible. In essence, the two cases are the same: it has just
been necessary to account comprehensively for the excavation surface
orientation.
For the case of a horizontal roof, the possible modes of sliding (parallel
to the line of maximum dip of a plane or parallel to the line of intersection
of two planes) can be identified from the projection geometry. The analo-
gous interpretation for a horizontal excavation surface has been discussed
in Section 19.1.1, and the same techniques are used in these cases.
Identification of stable blocks. The stable blocks will be those that do not satisfy
any of the instability criteria described above. In particular, in Fig. 19.9,
blocks are shown at both overhanging and non-overhanging surfaces
which are stable because the friction angle is not exceeded on any relevant
plane or block edge. The accompanying sketches show that the shape of
the blocks is such that they can be removed from the excavation surface,
but the orientation of the block faces relative to the friction angle is such
that sliding cannot occur.
The analysis refers only to the instability of a single block. We have not
studied the potential instability of a suite of blocks, nor whether the
instability of a single block will lead to one or more blocks also being
unstable. It is important to know, for support considerations, whether an
unstable block is an isolated event or whether it may trigger catastrophic
ravelling of the rock mass. In Chapter 20, the support requirements of
individual blocks are considered, but the analysis of a ravelling rock mass
is beyond the scope of this book. However, the principles and techniques
presented here do form the basis for an understanding of such an analysis.
19.2 Stress-controlled instability mechanisms
The instability mechanisms described in Section 19.1 are all driven by
forces, and in particular the force due to gravity. Such a force is known as