Engineering Rock Mechanics

Underground instability 341

Figure 19.3 Tunnel boring machine.

in surface blasting cannot be used so effectively underground because
of the influence of the rock stress. Although the use of tunnel boring
machines, as shown in Fig. 19.3, causes less additional disturbance
compared to blasting, they are susceptible to significant changes in rock
stiffness and strength. If the rock becomes too soft, there is insufficient
resistance for the gripper pads to hold to provide the forward thrust;
if the rock becomes too strong, the machine will not be able to cut
the rock adequately - and both these factors could occur at the same
time.
Thus we see that, for rock engineering underground, more information
may well be required about the rock conditions than for engineering at
the surface, but generally less is available.
Similarly, it is difficult to predict the support that will be required in
a rock tunnel without detailed information on the rock conditions. The
tunnel may need no support in some regions and a stiff, strong support
in other areas, as illustrated in Fig. 19.4. A ‘design as you go’ approach