Engineering Rock Mechanics

A note on tirne-dependency and weathering 359

possibility of discrete blocks developing as a consequence. This is especially
likely in the case of excavations in stratified rock, when the perturbed stress
field causes interlayer slip, which in turn could lead to joints opening and
the resulting possibility of blocks falling. This is illustrated in the suite of
diagrams in Fig. 19.21. The slip is similar to the case described immediately
above, except that the discontinuities may be regarded as ubiquitous, with
the location of the slip being at the corners of such an excavation, where
the angle between stress trajectories and the discontinuities is at its most
adverse.

19.3 A note on time-dependency and

weathering

We have concentrated on the mechanisms and associated solutions for
simple cases in order to demonstrate the value of understanding the basic
principles. We have utilized idealized cases for a CHILE rock. The rock is
actually discontinuous, but in the last two mechanisms we did consider the
influence of introducing discontinuities. The effects of inhomogeneity and
anisotropy will have to be explicitly studied through the use of numerical
analysis, although in many cases the trends would be similar.
The last of the differences between CHILE and DIANE rocks is that the
latter do have mechanisms which are time-dependent. We would expect
that the influence of time, creep and stress relaxation might assist
engineering because stress concentrations would be reduced and displace-
ments are not instantaneous. However, the more insidious aspect of time-
dependency is that the material itself might lose strength: whilst the stress
concentrations might reduce asymptotically to a given level, the strength
of the rock might continue to decrease over the months and years through
many processes, collectively termed weathering.

Perturbed stress field can lead to interlayer slip:

‘Jn

+r;

-T

With low slt, discontinuities may open: With high slt, strata may sag and separate

I I I

Figure 19.21 Composite instability around excavations in stratified rock.