282 Stabilization principles
Regardless of the technique used for determining the tangential stress, the
formula above shows that the optimal support pressure vanes around the
excavation boundary.
Although we have used only one discontinuity set in this example, it is clear
that the same approach can be adopted for all the discontinuity sets present
and the solutions superposed. If a particular region of excavation boundary
is found to have the potential for inter-layer slip with respect to multiple dis
continuity sets, then it will be particularly prone to failure. From the point of
view of provision of support at any point on the boundary, then the highest
calculated support pressure is the one required to inhibit slip on all sets.
16.6 Further comments on rock stabilization
methods
Following the explanations earlier in this chapter, it follows that the
simplest way to stabilize a rock mass containing discontinuities is to install
rock bolts in order to reduce the mechanical effect of the discontinuities.
In the extreme case, were we able to eliminate these effects completely, the
rock discontinuum would have been changed to a rock continuum. The
excavation surface is a special case, requiring extra reinforcement and so,
as mentioned earlier, a combined system can be used which would include
shotcrete and wire mesh. This can be either a primary operation used for
temporary protection, or it could be the final stabilizing operation.
In Fig. 16.14, there is an example of a rock mass which has been
reinforced by rock bolts alone, and an example in which rock bolts and
shotcrete have been used together. The bolts can be either tensioned or
untensioned: there are advantages and disadvantages to each.
The advantage of a tensioned bolt is that it can provide extra force across
the discontinuity surfaces and hence inhibit further block displacement.
The disadvantage of such a bolt is that the tension may not be sustained,
due to relaxation, over the design life. Hence, unless there is a continual
monitoring programme, the engineer may not have sufficient continuing
confidence that the bolting system is operating as designed and originally
installed. When rock bolt heads have to remain accessible for monitoring
following the installation of secondary reinforcement or support (in
particular, further layers of shotcrete or cast in situ concrete), the extra
complication will incur higher costs.
The advantage of untensioned rock bolts is that block displacements
induce the necessary tension within them, due to dilation of the shearing
discontinuity: thus, they respond directly as required. The disadvantage
of these bolts is that they do not possess the small amount of pre-tension
which could inhibit the initial displacements.
There are many proprietary types of rock bolt available, and many ways
in which the tension is applied through the rock bolt-rock bond to create
compression in the rock. There are also operational factors to be considered:
tensioned rock bolts require the necessary equipment to be available, and
time for its use to be scheduled; with untensioned bolts the engineer can
never be certain that the rock bolt-rock bond has sufficient strength to
allow the required tension to be induced.