276 Stabilization principles
Curve 1 represents the ‘perfect’ excavation case, in which there is no
disturbance of the remaining rock and there is no deviation from CHILE
behaviour.
Curve 2 may occur in a machine-driven tunnel, in which there is a slight
disturbance to the remaining rock with the result of increased final
displacement.
Curve 3 could represent high-quality blasting, where more disturbance
is inevitable and the resulting displacements are increased yet further.
All three of these curves intersect the boundary displacement axis which
indicates that no support is required, providing the displacements are
tolerable.
Curve 4, representing the response following poor-quality blasting,
indicates that support is essential if stability of the excavation is to be
maintained. This is another example of interaction within rock engi-
neering-in this case, the direct link between excavation technique and
stabilization requirement.
If support is required, we can gain an indication of the efficacy of partic-
ular support systems by plotting the elastic behaviour of the support, the
available support line, on the same axes as the ground response curve. We
have plotted a soft support and a stiff support (considered as a radial
stiffness) together with ground response curves in Fig. 16.7. The points of
interest are where the available support lines intersect the ground
response curves: at these points, equilibrium has been achieved.
There are other aspects of practical significance which can also be noted
in relation to the ground response curve; two of these are also illustrated
in Fig. 16.7. First, we remember that the support cannot be installed con-
temporaneously with excavation, and so some initial displacement must
occur before the support is installed (and, strictly speaking, displacements
will have occurred even before the excavation reaches the point in ques-
tion-because of the alteration of the complete stress, and hence strain,
states within the rock mass). Thus, the available support line starts, as
shown in Fig. 16.7, with a displacement offset.
Another useful aspect of this approach is illustrated in Fig. 16.7, i.e. the
concept of a yielding support. As shown in the diagram, the available
support line for a yielding support has a maximum strength, which cannot
I\ ; Stiff support
\ Yielding support //
Boundary displacement
Figure 16.7 Ground response curves and available support lines.