Abstract
This chapter presents the underground excavation instability mechanisms. It explains the distinction between structurally-controlled and stress-controlled instability mechanisms. It devotes to these two primary instability modes and also considers the composite instability modes, as well as 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 (the force due to gravity) or stresses induced by the process of excavation. Therefore it includes the kinematic feasibility of tetrahedral blocks primarily and, later, instability in stratified rock. The stresses around elliptical openings can be treated in an analogous way to present for circular openings. There is much greater utility associated with the solution for elliptical openings than circular openings, because these can provide a first approximation to a wide range of engineering geometries, especially openings with high width/height ratios (e.g. mine stopes and civil engineering caverns). The chapter also recognizes the three main modes to analyze the instability around underground openings.
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