Engineering Rock Mechanics

240 Excavation principles

openings, whereas the stopes (the openings left by mining ore) can be
allowed to collapse during mining, as governed by the design process.
There are different design considerations in civil and mining engineer-
ing, related to the operational life of the openings themselves. A
radioactive waste repository might have to operate satisfactorily for 5000
years, an undersea transport tunnel for 125 years, a mine shaft for 20 years,
a metal mine stope for^1 year and a longwall mining opening for^1 week.
These time spans relate to the discussion in Chapter^13 on the time-depen-
dent aspects of rock mechanics and the implementation of appropriate
models in rock engineering projects.
In order to remove part of a rock mass, it is necessary to introduce
additional fractures over and above those occurring in situ. Three critical
aspects of excavation are immediately introduced:

(a) the post-peak portion of the complete stress-strain curve must be

@) the in situ block size distribution must be changed to the required

(c) by what means should the required energy be introduced into the rock?

We will discuss each of these aspects in turn.

reached (cf. Fig. 6.1);

fragment size distribution; and

75.7.7 Attuining the post-peak portion of the complete

stress-strain curve

In Chapter 6, the complete stress-strain curve has a pre-peak portion of

the curve which is mainly, although not completely, associated with linearly

elastic behaviour. In this portion, there is little large-scale failure and little

dissipation of energy on load cycling. In order to provide the necessary large-

scale fragmentation, a part of the intact rock must be taken into the post-peak

portion of the complete stressstrain curve. Note that subsequently we wish

to remain in the pre-peak portion of the curve for rock stability. It follows

that an excavation boundary is an interface between two fundamentally

different engineering objectives and materials, as illustrated in Fig. 15.1.

Rock beyond excavation

periphery remains intact: peak

strength not reached ;

///////////LL////’///,/,

support objectives

&d ?,

v)

fragmented: taken into

Strain post-peak region

Figure 15.1 The complementary objectives of excavation and support, as related

to the complete stress-strain curve.