Some practical aspects: grouting and blasting 1 6 1which subsequently blocks the flow paths through the rock mass. Luckily,
during such a grouting process, the grout will follow the path of least
resistance, generally along the discontinuities with greatest aperture and
persistence-which are the very ones which conduct the most water. We
will wish to optimize such a grouting process, bearing in mind the types
of discontinuity array that may be present-as, for example, that
illustrated in Fig. 9.8.
Generally, such optimization will involve tailoring the location and
orientation of the injection boreholes, together with the grout type,
injection rates, pressures and volumes, to the discontinuity geometry. This
is essentially an empirical process, but obviously considerations such as
those described above are of great help in establishing the fundamental
design principles.
Blasting technology has also tended to be almost entirely empirically
based: the type of blasting round, the quantity of explosive and the
detonation procedures have been established by trial and error. A rock
which has no discontinuities has to be fractured by the blasting; in a rock
which is very heavily fractured, it may only be necessary to disaggregate
the rock mass without actually inducing any failure of intact rock. This
leads us to utilize the fact that, as the blasting causes firstly a 'stress wave'
to travel through the rock followed by a buildup of gas pressure within the
borehole, we ought to tailor the type of explosive to the discontinuity
geometry. By varying the explosive, the proportionate energy associated
with the stress wave and gas pressure can be varied according to whether
we are trying to break the rock with the stress wave or disaggregate it with
the gas pressure.
There is an elegant extrapolation of this concept whereby, via the
engineering, we can effectively create a large artificial discontinuity exactly
where it is required. This technique is known as pre-split blasting and will
be discussed in Chapter 15. The aim of the method is to create the final
excavation surface btfore bulk blasting the remaining rock this artificial
discontinuity then prevents stress wave damage of the intact rock and
disaggregation of the rock mass behind the final excavation surface. New
and innovative engineering techniques can be developed if the principles
of rock mechanics are known and understood. Here, within the pre-exist-
ing discontinuity pattern, an artificial discontinuity has been introduced
having a greater, and yet more beneficial, effect than any others. One can
imagine the extension of this concept to other subjects, such as controlled
drainage and controlled rockbursts.