Engineering Rock Mechanics

Stabilization of ‘transitional’ rock masses 279

Zero displacement

condition Zero strength - -

.

Boundary displacement

Figure 16.10 Ground response curve in discontinuous rock masses.

The two limiting cases of the suite of ground response curves in Fig. 16.10

are the linearly elastic behaviour at the left part of the figure and the zero

strength behaviour represented by the uppermost horizontal curve. Note

that in order to achieve a zero displacement condition in either case, it is

necessary for the introduced support pressure to equal the in situ rock

pressure. However, equilibrium is reached when the available support line

intersects the ground response curve, so that in most cases for a

continuous rock it is not necessary to replace the in situ rock pressure with

an equivalent support pressure. Nevertheless, considering Fig. 16.10, it can

be seen that increasingly higher support pressures are required for

equilibrium as the introduction of more and more discontinuities into the

rock mass flattens the ground response curve. So, at the other limit, there

is a zero-strength material in which it is always necessary to replace the in

situ rock pressure with an equivalent support pressure.

The circumstances are not only affected by the overall discontinuous

nature of the rock mass, but are exacerbated by the existence of discrete

rock blocks which will create point loads on the support elements. More-

over, there is the basic danger to personnel of rock blocks falling from the

roof during construction and the difficulty of localized high water flows.

16.5 Stabilization of ‘transitional‘ rock
masses
The previous four sub-sections have followed the lower boxes in Fig. 16.2,
and have concentrated on the major features of the subjects. In practice,
there will be a wide spectrum of rock media and associated rock
behaviour. The term ‘transitional’ in the heading to this section is used to
indicate that the rock mass around an excavation may have attributes
associated with continuous and discontinuous rocks. There is a wide range
of such attributes and consequential behaviour; here, we highlight one
transitional case-slip on discontinuities in a layered rock. In such a case,
the stress distribution around the opening can be found from a continuum
analysis, but the mode of failure is due to the discontinuous nature of the
rock. The ’@ theory’ described here was presented by Goodman (1989). The
parameter @ is the angle of friction between two discontinuity surfaces.