24 2 Excavation principles
75.7.3 Energy and the excavation process
There has been considerable discussion over the years on optimizing the
use of energy during excavation, traditionally expressed as the concept of
specific energy, i.e. the amount of energy required to remove a unit volume
of rock (J/m3). The design of excavation machines may well incorporate the
idea of minimizing specific energy in order to maximize energy savings and
increases in penetration rate, and to minimize destructive vibrations in the
machine.
Seminal work, conducted many years ago at the US Army Cold Regions
Laboratory, New Hampshire, involved a study of the specific energy values
associated with different forms of ice fragmentation. These ran the whole
gamut, from hand excavation, through conventional explosives, to machine
excavation and more exotic techniques such as thermal lances. Predictably,
perhaps, it was found that the more exotic forms of fragmentation were
associated with higher specific energes, and hand excavation was asso-
ciated with the lowest specific energy. This was because man could take
advantage of the pre-existing fractures, but all other forms of excavation
failed to exploit this opportunity.
In the history of mining, there have been few revolutionary advances in
technology. The first of these was the application of explosives to rock
excavation, with others being the ability to pump water from great depths
and the development of self-guided tunnel boring machines-which are
able to construct tunnels automatically to fine tolerances.
There are only two fundamental ways of inputting energy into the rock
for excavation: one is by blasting, the other is by mechanical means. The
two methods are illustrated in Fig. 15.3, where energy input is plotted
against time. The energy is either input in large quantities over very short
durations, or in smaller quantities essentially continuously. So, from purely
practical considerations, excavation has to utilize a cyclical method with
periodic blasting or a continuous method by machine. As far as the authors
are aware, no one has yet developed a technique for combining the meth-
ods, e.g. excavating by continuous blasting which takes place immediately
ahead of a tunnel boring machine.
Large magnitude, short duration
Energy , .I pulses associated with blastingBlasting >
Small magnitude,
essentialy continuous
input associated with
mechanized excavationTime
Figure 15.3 Energy input rates for blasting and mechanized excavation.