TABLE OF CONTENTS Chapters 19 and 20 of this book, on instability around underground excavations, are direct analogues of Chapters 17 and 18, which were concerned 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.
Structurally-controlled instability means that blocks formed by discontinuities 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.
The minimum requirement to define a discrete block is four non-parallel planes, which give rise to a tetrahedral block. In terms of the analysis of instability around an excavation, such a block can be formed by three discontinuity 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...
