TABLE OF CONTENTS In Chapter 19, we concentrated on the specific instability mechanisms relating to underground excavations. These were instability due to block movement, instability due to stress effects, and circumstances where both occur. In this chapter, we consider these mechanisms within the overall context of the design and analysis of underground excavations. One does not know, a priori, which mechanisms are the ones that will be operating, and hence the ones to defend the excavation against.
There are two essential precursors to such design and analysis. The first is a mechanical understanding of the rock mass and the requirements of the project being undertaken; the second is the consideration of which mechanisms are relevant, including factors such as the presence of a nearby fault, which may or may not have been detected by site investigation. With these provisos, the subjects described in this chapter will provide guidance for the design and analysis of all underground excavations.
In this section, the foundation laid in Section 19.1 is used to consider design against structurally-controlled instability. Even without detailed analysis of the blocks formed by discontinuities, a great deal can be achieved by pragmatic design of roofs and other excavation boundaries. The procedure is the intelligent general use of the principles already described. In Fig. 20.1, there are two cases where instability in the roof is apparent from underground observations. One can make useful decisions on excavation location, orientation and...
