Rock reinforcement 273this type of analysis provides a rapid means of determining the value of
reinforcing continuous rock-which will clearly be most effective in low-
stiffness, low-strength, brittle rocks.
16.3.2 Rock reinforcement in discontinuous rock
The mode of action of the reinforcement in a discontinuous medium is
somewhat different to that described in Section 16.3.1 because, not only
are we considering improvement of the rock structure properties, but
also the avoidance of large displacements of complete blocks. The method
of analysis for the kinematic feasibility of rock blocks will be discussed
in Chapters 18 and 20 for surface and underground excavations, respec-
tively: here we will highlight the factors relating to rock reinforcement
principles.
Two of the most important factors are whether the blocks are free to
move, given the geometry of the rock mass and excavation, and the
character (quantity, length and orientation) of the reinforcement. In Fig.
16.5 we show the simplest case of reinforcing a discontinuous material a
single block on a rock surface is reinforced by a tension anchor. The tension
anchor should be installed such that the block and the rock beneath act as
a continuum, and block movement is inhibited. We may wish to know the
optimal bolt length, orientation and tension, and indeed whether the
reinforcement is required at all, cf. the engineering objective referred to in
the uppermost box in Fig. 16.2.
For the simple geometry shown in Fig. 16.5 without the rock bolt, basic
mechanics indicates that the block will slide if the angle of the slope exceeds
the angle of friction of the rock surfaces for a cohesionless interface. This
is therefore the first criterion for indicating the potential for failure.
Considering now the length and diameter of the bolt, these have to be
sufficient to ensure that the strength of the bonds across the anchor-grout
and grout-rock interfaces are capable of sustaining the necessary tension
in the anchor, which in turn will depend on the fracturing of the rock mass.
Finally, the anchor diameter may also be determined on the basis of the
tensile strength of the anchor material.
With respect to the bolt orientation and tension, it is not obvious at what
angle the anchor should be orientated for optimal effect, taking into
account the basic mechanics and the rock structure. If we regard the
Figure 16.5 Optimized rock reinforcement for the case of a block on a rock surface.