Geology Reference
In-Depth Information
Fig. 7.2 Depicting how a
granular body can be
undergoing local transient
shearing events, the pattern of
which will be changing with
time
(2) Localized shearing aspect. The relative granular displacement events such as
idealized in Fig.
7.1
are unlikely to be distributed uniformly in space or time
because of the irregularity and evolving character of the real granular struc-
ture. At any given instant, certain local structural configurations will be more
favourable to relative granule movement than others, and, for reasons of
movement compatibility, these relative movements will tend to occur as local
shears extending through quite large groups of granules. That is, at any instant,
the granular body will appear to be undergoing local shearing at isolated local
sites, as depicted in Fig.
7.2
but the pattern or distribution of the local activity
will tend to change with time. Such a continually changing kinematic picture
has been emphasized by Rowe (
1962
) and (Horne
1965
). Sometimes it may be
useful to view the local shears as resulting from the passage of dislocations
but, due to the absence of a long range periodic structure in the granular
medium and to the continually changing pattern of local flow events, such
dislocations will tend to be of an ephemeral nature.
So far, we have only considered the relative translations of the granules, or,
specifically, of their representative points, insofar as these translations give rise to
macroscopic change of shape, expressible in terms of deviatoric strain compo-
nents. However, the relative translations may also contribute a macroscopic
change of volume, or dilation, expressible in terms of a change in the mean
