Engineering Rock Mechanics

270 Stabilization principles

excavation, as determined by

the engineering objective.

t$ are introduced

the rock mass, su excavation to inhibit rock

mass is stiffened and st

with the result th

Figure 16.2 Basic categorization of rock reinforcement and support.

the engineering elements are inserted within the rock mass and with
support they are inserted within the excavation.
The hierarchical system shown in Fig. 16.2 is for the purpose of separat-
ing the stabilization concepts: in practice, two or more of the conditions
shown in the bottom row may be operating concurrently.
In the case of reinforcement, as illustrated in Fig. 16.3(a), steel cables or
bars grouted within boreholes are used in an attempt to minimize displace-
ment occurring along the pre-existing discontinuities-so that the rock
supports itself. An associated preventative measure is the spraying of
concrete or cement mortar onto the rock surface immediately on exposure,
not as a direct structural support, but in conjunction with the bolting in
order to protect the surface and inhibit minor block movements. Other
materials can be considered as reinforcing elements for particular purposes,
e.g. wood or glass fibre, if it is necessary to subsequently excavate through
the reinforced rock mass.
In the case of support, structural elements-such as steel arches or
concrete rings, as illustrated in Fig. 16.3@)-are introduced to inhibit rock
displacements at the boundary of the excavation. These elements, which are
external to the rock mass, provide load carrying capability, with the result
that the rock does not totally support itself the rock is supported. Support
does not directly improve the intrinsic strength of the rock mass, but does
alter the boundary conditions. We will be discussing later how the utility
of such support can be assessed by means of the 'ground response curve',
and how developments such as using knuckle joints with pre-cast concrete
segments (so that the applied loads are resisted as compressive hoop forces
rather than bending moments) are helpful. At this stage, we note that the