268 Stabilization principles
(c) at the boundary of an excavation open to the atmosphere, any previous
fluid pressure existing in the rock mass will be reduced to zero (or more
strictly, to atmospheric pressure). This causes the excavation to act as a
'sink', and any fluid within the rock mass will tend to flow into the
excavation.
These three primary affects are illustrated in Fig. 16.1.
With respect to the likely displacements (top right-hand diagram in
Fig. 16.1), there is the choice of allowing them to occur or providing some
method of stabilization to resist them. The engineering objective dictates
the significance of any rock displacement and its maximum tolerable
magnitude. It is important to know whether the displacements are
- Displacements
occur because
rock resistance
removed
Excavation Rock mass
In the rock
the principal
stresses become
zero at
excavation
boundary -
which becomes
a principal
stress plane
induced
Hydraulic pressure
reduced to zero,
excavation
becomes
a sink
Effect 1 : Displacements and rock failure
)ck
out
ontinuities
'\
I
Effect 2: Stress rotation
1' principal stres
U Principal stresses
rotated to become
perpendicular to
an unsupported
excavation boundary
Effect 3: Water flow
I Discontinuities
Figure 16.1 The three primary effects of excavation on the rock mass environment.