274 Stabilization principlesoptimal orientation for the anchor as that which enables the anchor tension
to be a minimum, then the angle between the anchor and the slope surface
is equal to the friction angle between the block and the slope. Many other
factors may be involved in this analysis: these will be covered in Chapter
- The intention here is to indicate the fundamental philosophy.
The key point to be made is that, if the reinforcement inhibits block movement,
and sufficient stress can be transmitted across the interface, then in principle the
rock reinforcement has changed the rock discontinuum to a rock continuum.
In practice, when rock anchors are installed in a discontinuous rock mass,
the rock surface is often covered with wire mesh and then covered in shot-
Crete (sprayed concrete). It is emphasized that the wire mesh and shotcrete
are part of the rock reinforcement system: the purpose of the shotcrete is
to provide a stiff coating to inhibit local block rotation and movement.
Before rotation, forces may be being transmitted across complete block-to-
block interfaces; after even very small rotation, these forces become con-
centrated at the edges or vertices of the blocks, with high local stresses
being developed. It is this sequence of block rotations that leads to the
progressive failure of a discontinuous rock mass and subsequent loss of
integrity of the engineered structure.
16.4 Rock support
The term 'rock support' is used for the introduction of structural elements
into a rock excavation in order to inhibit displacements at the excavation
boundary. As in the case of rock reinforcement, rock support is considered
separately for continuous and discontinuous media. In reality, the
distinction between continuous and discontinuous rock masses may
not be quite as clear as implied; the transitional case is discussed in
Section 16.5.76.4.7 Rock support in continuous rock
Consider the stresses and displacements induced by excavating in a CHILE
material. For example, the radial boundary displacements around a circular
hole in a stressed CHILE rock in plane strain areu, = (R/E)[q + 02 + 2(1 - ?)(GI - O~)COS 28- VG~]
where R is the radius of the opening,
q and o2 are the far-field in-plane principal stresses,
o3 is the far-field anti-plane stress,
8 is indicated in the margin sketch, and
E and v are the elastic constants.
Recall that the stress concentrations around an opening in similar
circumstances are independent of both R and the elastic constants-the stress
concentrations around circular openings of different diameters and in
different CHILE materials are the same. However, the magnitude of the
radial displacement must depend on both the radius of the opening and the
values of the elastic constants, as indicated in the equation above: