Stabilization
76 principles
In the previous chapter on excavation principles, we discussed the idea of
taking the rock into the post-peak region of the complete stressstrain curve
for excavation. Here, we discuss the principles of stabilization-whether
for staying in the pre-peak region, or for allowing the rock to pass into the
post-peak region and utilizing its residual strength.
Following the introduction on the form of the disturbances caused by
the excavation process, we discuss the two fundamental methods of stabi-
lization: rock reinforcement and rock support. In each case, the behaviour
of essentially continuous and discontinuous rock masses is discussed
separately. Finally, we consider the principles of stabilization when the rock
mass behaviour has attributes of both a continuum and a discontinuum
(e.g. slip on planes of weakness). These principles are amplified and their
practical application illustrated in the later chapters dealing with rock
engineering.
16.1 The effect of excavation on the rock
mass environment
There are two aspects of rock excavation that we will concentrate on here.
The first is that ‘one cannot prevent all displacements at the excavation
boundary’. The second is that ’a mistake in excavation design can be a
major problem’. In order to understand the displacements and avoid
problems, we will consider the three primary effects of excavation and then
decide on the ramifications for stabilizing excavations of all kinds.
(a) displacements occur because stressed rock has been removed, allowing
the remaining rock to move (due to unloading);
(b) there are no normal and shear stresses on an unsupported excavation
surface and hence the excavation boundary must be a principal stress
plane with one of the principal stresses (of magnitude zero) being
normal to the surface. Generally, this will involve a major perturbation
of the pre-existing stress field, both in the principal stress magnitudes
and their orientations; and
The three primary effects of excavation are: