Engineering Rock Mechanics

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.