Additional points 263span generally decreases with depth from 10-30 m spans in the 50-400 m
depth range, reducing to 10-20 m spans in the 400-1000 m depth range.
Deeper caverns are possible, with rock stress being the main mechanical
constraint.
In this chapter, we have highlighted some of the key issues relating
to the excavation of rock. The questions have tended to be descriptive
and relatively simple in order to illustrate the principles. In engineering
circumstances, rock excavation is a complex rock fragmentation process,
highly dependent on the nature of the rock mass and the engineering
techniques utilized. It is also critical to assess the excavatability of a
rock mass and to tailor the excavation technique to the circumstances
and the engineering objective. For example, if a TBM is purchased and
subsequently found to be inadequate, a large amount of time and money
may be lost. The complexity of the process is such that a great deal of
work is still required in this field, involving a combination of theoretical,
empirical and pragmatic approaches, all of which are enhanced by a
thorough understanding of rock mechanics principles.