Engineering Rock Mechanics

260 Excavation principles

energy cutter spacings

No interaction

at large spacings

I at optimal spacing

Distance between

cutters, d

Figure 15.21 Generic curve of specific energy versus cutter spacing.

Considering the other three cutting mechanisms illustrated in Fig. 15.19,
the same remarks and applications apply in practice. There has always been
debate about the failure mechanisms operating beneath freely rolling disc
cutters, and whether they should be mounted singly or in sets to take
advantage of interaction during the cutting process. In Fig. 15.21 we illus-
trate a generic curve of distance between the cutters on the horizontal axis
and specific energy (the energy required to remove a unit volume of rock)
on the vertical axis. When the distance between the cutters is low, con-
siderable overcutting or accessory grinding takes place; when the minimum
specific energy is reached the cutters are positioned such that optimal inter-
action occurs during the breakage process; when the distance between the
cutters is large, the cutters are acting independently and hence there is no
benefit gained from cutter interaction. One approach to the application of
rock mechanics in TBM design has been to minimize specific energy require-
ments. In particular, this has the effect of reducing vibrations and
increasing the life of the cutters and other machine components. However,
the construction process using mechanized excavation is a complete system,
and the optimization of the system (either in terms of advance rate or cost)
may or may not involve minimizing the specific energy of cutting.
The action of water jets, shown in the bottom right-hand diagram of
Fig. 15.19, is a specialized subject beyond the scope of this book and the
reader is referred to the publication by Hood et al. (1990) for further details.
High pressures, of the order of 70 MPa, are used. We also mentioned earlier
that combinations of the cutter types shown in Fig. 15.19 could be used.
This is especially so for water jets, because they can either be used for water
jet assisted mechanical cutting, mechanically assisted hydraulic cutting or
as an integral part of the overall cutting process, for example, in
dust and spark suppression. Very high specific energies are likely to be
associated with hydraulic cutting methods, but they do have the
advantage that there is no mechanical link between the rock and the cutting
machine.
More rock is excavated by large tracked machines (flywheel power up
to 0.5 MW) fitted with massive ripping tines (see Fig. 15.22) than by other
mechanical means. The main method by which the appropriate machine