Design and
20 analysis of
u n der g ro u n d
excavations
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, u 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.
- t Design against structural ly-controlled
instability
20.1.7 Background-pragmatic design, orientation and
size effectsIn 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 under-
ground observations. One can make useful decisions on excavation
location, orientation and support without detailed mechanical analysis-
and one can 'design with the rock'.
In the left-hand diagram of Fig. 20.1, the roof area is likely to be unstable
and one option for improving stability would, if viable, be to ensure that
the excavation were located in the massive rock. Similarly, in the right-hand