Geology Reference
In-Depth Information
7.2.3 Cataclastic Granular Flow
In the models of pure particulate flow ( Sect. 7.2.2 ) it has been assumed that the
granules or particles remain intact throughout the flow (except presumably for
some wear on the surfaces, associated with the frictional sliding). Such a situation
can only be assumed to apply at relatively small stresses, such as those commonly
involved in soil mechanics or in the handling of particulate matter. In practice,
grain crushing becomes significant when the mean normal stress exceeds a certain
level which depends on mineralogy and grain size but for medium-grained sand is
of the order of 1 MPa (Chaplin 1971 ; Hettler and Vardoulakis 1984 ; Vesic and
Clough 1968 ).
We now consider the case of granular flow in which the intergranular forces are
sufficiently large to lead to gross fracture of the granules. Such flow is here
described as cataclastic granular flow, taking ''cataclastic'', from its etymological
origins, to mean ''pertaining to breaking down by fracturing''. In the case of
extensive fracturing, the term ''comminutive granular flow'' might alternatively be
used. There are two distinct ways in which cataclastic effects can be of relevance
to granular flow:
(1) In the first case, as in that of an initially more of less intact rock, the occur-
rence of cataclasis is a precondition to the possibility of granular flow, in that
the cataclasis reduces the body to an assemblage of granules which can then
move relative to each other in producing a granular flow. In geological rock
mechanics, this combination of cataclasis and granular flow is commonly
referred to as ''cataclastic flow'', without distinguishing between the two
aspects. Often such a process is only one, possibly minor, component in a
deformation that also involves crystal plasticity. In the latter case, if the
fracturing is insufficiently pervasive to reduce the rock effectively to an
assemblage of separate granules, the main contribution of the cataclastic
component to the deformation will be dilatational rather than distortional.
(2) In the second case, that in which the body is initially already in a particulate
state, the cataclasis may not only have a direct effect, in which the relative
movement of groups of granules is facilitated by the fracturing or interlocking
grains, but also an indirect or structural effect of changing the size distribution
and packing density of the granules and hence influencing the potential ease
with which further relative movement can occur. Thus, the fracturing of
granules is important in relation to compatability or accommodation require-
ments and, consequently, to dilatancy.
The clearest example of the role of cataclastic effects is provided by the
development and deformation of fault gouge in rocks, and its study serves to
illustrate the principal aspects of cataclastic granular flow. The first of these
aspects concerns the nature and the progressive comminution of the granular
material that constitutes the gouge. From a study by Sammis et al. ( 1987 ) and
earlier papers quoted by them, the following properties appear to apply to gouge:
Search WWH ::




Custom Search