Engineering Rock Mechanics

284 Stabilization principles

No support required

(a) Single set of

discontinuities in strong rock

c

.- .a

>

0

d H

fr

on standardized pattern

(c) Multiple

respect to discontinuities

(b) Single set of

discontinuities in weak rock discontinuities

Figure 16.15 Rock bolting in discontinuous rock masses. (a) Single set of discon-

tinuities in strong rock. @) Single set of discontinuities in weak rock. (c) Multiple

discontinuities.

detached.

Rock bolts can be installed on a fixed production pattern (designed

according to the rock structure), or the pattern can be varied according to

local conditions, or both. One extreme in the rock structure spectrum is a

single set of parallel, persistent and planar discontinuities: the other end

of the spectrum is the case of discontinuities at all orientations with many

idiosyncratic features. Much can be established about the optimal rock

bolting pattern from engineering judgement.

If there were only one set of discontinuites, and the rock were strong,

failure would not occur at all. The minimum number of faces required to

form a discrete rock block is four: thus, a single set of discontinuities

intersecting an excavation does not result in any rock blocks being formed,

as illustrated in Fig. 16.15(a). Conversely, if the rock mass were weak,

reinforcement would be needed and the optimal direction for the bolts

could be determined, which would not be consistently radial to the

excavation, as illustrated in Fig. 16.15(b).

If the numbers and properties of the discontinuities tend to form a

homogenous and isotropic rock mass structure, a standardized pattern can

be adopted. This case is illustrated in Fig. 16.15(c).

Shotcrete can be regarded as a rock reinforcement method, because it

inhibits block movement rather than providing structural support. This

method of operation may be understood if we consider that a thin (say 100

mm) membrane of shotcrete may be sufficient to stabilize a 5 m diameter

tunnel. Such a thin unreinforced concrete element is incapable of support-

ing large radial pressures without failing. The pressures do not develop

because the slight lateral restraint offered to the rock around the excava-

tion by the shotcrete is sufficient to inhibit the block movement, and

effectively increase the rock mass strength. It follows that the shotcreting

is most effective when a complete ring is formed, and drainage holes

should be provided if significant water pressures are likely to develop.

The structural operation of a reinforcement system consisting of ten-

sioned rock bolts and a shotcrete membrane is conceptually analogous to

that of a spoked bicycle wheel. In such wheels the thin spokes are tensioned