Underground
79 excavation
instability
mechanisms
Chapters 19 and 20 of this book, on instability around underground
excavations, are direct analogues of Chapters 17 and 18, which were con-
cerned with surface excavation instability. In this chapter, the underground
instability mechanisms are presented. In Chapter 20, the design and
analysis of underground excavations are discussed in the context of these
mechanisms.
In Section 16.2, the distinction between structurally-controlled and stress-
controlled instability mechanisms is explained. Accordingly, this chapter
has been devoted to these two primary instability modes, considering also
composite instability modes and the effect of time and weathering on
excavation stability.
19.1 Structurally-controlled instability
mechanismsStructurally-controlled instability means that blocks formed by discon-
tinuities either fall or slide from the excavation periphery as a result of an
applied force (usually the force due to gravity) or stresses induced by the
process of excavation. Hence, we include the kinematic feasibility of
tetrahedral blocks and, later, instability in stratified rock.
7 9.7.7 Kinematic feasibility analysis
The minimum requirement to define a discrete block is four non-parallel
planes, which gve rise to a tetrahedral block. In terms of the analysis of
instability around an excavation, such a block can be formed by three dis-
continuity planes and one plane representing the excavation periphery. Because
we are limiting the analysis to the simplest case-that of tetrahedral
blocks-the blocks themselves may be identified as spherical triangles on
the hemispherical projection. This is because the plane of the projection
represents a plane which is one face of the tetrahedral block and the
remaining three planes which form block faces are given by the great circles
representing the discontinuities. Thus, a study of instability mechanisms