Engineering Rock Mechanics

258 Excavation principles

Table 15.1 Rock cutting devices and associated parameters

Cutter type Rock strength Applied forces

drag picks e70MPa F" F,

(conical or f&?4/aded)

discs 70-275MPa F" ' F,

(mounted singly or multiply)

button bits 275-41 5MPa F" F,

(mounted on discs or cylinder)

water jets all strengths not applicable

I

types of device used for rock excavation, including the use of flame

cutters, but we are only considering the conventional and widely used

devices here.

The action of all four devices is shown in Fig. 15.19. The top left-hand

diagram illustrates the cutting action of a drag pick. As can be seen

from the diagram, the cutting mode is likely to be a complex mixture

of tensile, shear and compressive modes of failure. We noted the basic

Mode I, Mode I1 and Mode I11 types of failure, together with their binary

combinations, in Fig. 14.13. The actual failure mode will be far more

complex than this, and it is questionable whether any directly practical

modelling of the process can currently be undertaken. There are many

extra factors such as the vibration of the cutting head, the stiffness of the

cutting tools and holders, and the irregular nature of the cut face (which

is comparable to the dimension of the cutting edge of the tools).

In Fig. 15.20, we illustrate the tangential (i.e. cutting) forces recorded by

a dynamometer, in which a drag pick was mounted, over a cutting distance

of about 250 mm. It is tempting to assume that there will be a direct

correlation between the peaks in this graph and the liberation of individ-

ual chips during the cutting process. However, deeper investigation reveals

that this is not the case; nor do any of the suggested failure criteria or

proposed cutting mechanisms adequately model the variation of forces

illustrated.

Because of the complex nature of the cutting process, with all the

attendent micro-structural effects and external complicating factors, the

pragmatic approach has been to use familiar strength parameters, e.g.

compressive strength (as demonstrated in Table 15.1), to empirically predict

cutting rates. Since most geomechanical rock properties are inter-related,

this approach has achieved some success. Current research is exploring the

possibility of modelling the cutting process as a chaotic system.