Engineering Rock Mechanics

272 Stabilization principles

Sketch of radially
reinforced annulus

stabilizing forces

within the rock mass

e.g. a ring of

pre-cast concrete

segments

(b)

Figure 16.4 Principles of (a) rock reinforcement and (b) rock support.

76.3.7 Rock reinforcement in continuous rock

It may be thought that the use of rock reinforcement, e.g. rock bolts, is only

of use in discontinuous rock masses in order to prevent discrete block

displacements. However, the use of rock reinforcement in a continuous

medium can also be of benefit because of the reinforcement effect on the

overall rock properties and hence rock behaviour. If a continuous rock is

strong, it may well be capable of withstanding the induced stresses without

further assistance. Conversely, if a continuous rock is weak, heavy direct

support may be required, such as segmental pre-cast concrete rings. The

case we are discussing here is where improvement to the intrinsic strength

by the rock reinforcement is all that is required for rock stabilization. The

mechanics of this problem are similar to those of reinforced concrete.

Consider an element of reinforced rock adjacent to the excavation

boundary (see margin sketch). The effect of the reinforcing elements is to

produce an effective confining stress of

where A' and E' are the ratios of the cross-sectional areas and the Young's

moduli of the reinforcing element to that of the rock being

reinforced, respectively,

v is Poisson's ratio for the rock, and

00 is the tangential stress.

Note that the larger the ratios A' and E', i.e. for an increased rock bolt

density and lower stiffness rock, respectively, the larger the effective

confining pressure will be. As an example, consider a chalk (E = 1 GPa)

being reinforced with 25 mm diameter steel bars at a density of four bars

per square metre of rock face, or = 0.120~ Although the induced confining

stress is only 12% of the tangential stress in this example, it will have a

profound effect on the strength and failure properties of the chalk. Figure

6.15 shows the marked effect of a small confining stress on the strength and

shape of the complete stress-strain curve of rock in compression. Use of