The stabilization strategy 269associated with entire rock blocks moving into the excavation, or whether
the rock mass is deforming as a whole, or whether failure is occurring in
the rock. It is possible for all three of these mechanisms to be operating
simultaneously, and it is necessary to understand the mechanisms in order
to decide on the stabilization strategy.
The most significant consequence of the second effect-disturbunce of the
stress field (middle right-hand diagram of Fig. 16.l)-is that the rock is more
likely to fail, owing to the increased magnitude of the deviatoric stresses. The
increase in the deviatoric stresses arises from the change in the magnitude
of the major principal stress, together with the fact that an arbitrary polyaxial
stress state has become an effectively uniaxial or biaxial stress state.
The third effect, that of increased water flow (bottom right-hand diagram
of Fig. 16.1), is significant because there will be higher differential heads
within the rock mass which will tend to push rock blocks into the
excavation, with the attendant possibility of increased weathering and time
dependent deterioration as the water flow increases.
These three primary effects, and the optimal way in which the rock
engineering strategy is developed to account for them, have one thing in
common: we should not blindly attempt to maintain the original conditions
(e.g. by installing massive support or reinforcement and hydraulically
sealing the entire excavation); rather, we should treat these effects as pro-
viding the opportunity to understand the rock behaviour and to develop
the engineering sympathetically.
A5 the displacements occur, engineering judgement may determine that
they can be allowed to develop fully, or be controlled later. Similarly, per-
haps the engineer can utilize the alteration of the stress field to advantage
when designing the shape of the excavation to minimize the induced
deviatoric stresses. Knowing that water will flow into the excavation means
that the flow can be controlled in accordance with the engineering
objective: some tunnels may be excavated as drainage or water-gathering
systems; others may have to be totally dry.
16.2 The stabilization strategy
A categorization of rock reinforcement and rock support in continuous and
discontinuous rock is required because rock reinforcement and rock sup-
port are not the same. In Fig. 16.2 we show this categorization based on
the engineering viewpoint.
If failure around an excavation, whether at the surface or underground,
is due to blocks of rock moving into the excavation, two approach
philosophies can be considered for stabilization:
(a) the block displacements are occurring because the rock mass is a
discontinuum, and hence the rock is reinforced so that it behaves like
a continuum; or
(b) direct support elements are introduced into the excavation in order to
maintain block displacements at tolerable levels.
The first option is known as rock reinforcement; the second is known as
rock support, as presented in Fig. 16.2. Note that with rock reinforcement