Deformation Mechanisms: Granular Flow - Materials Science for Structural Geology - page 212

expected to constitute only a ''fading memory''. Nevertheless it may be a valuable
source of information concerning the nature of the deformation processes.
7.2 Granular Flow Controlled by Frictional and Cataclastic
Effects
7.2.1 Introduction
So far in this chapter we have considered purely kinematical or geometrical
aspects of granular flow. We now introduce dynamical aspects, considering the
factors that determine the resistance to flow under applied stress. This step
immediately enables us to distinguish two broad categories of granular flow.
In the first category (microbrittle granular flow), considered in the immediately
following sections under Sect. 7.2 , the dynamics of the flow are controlled by
frictional factors, including any local fracturing necessary to allow relative sliding
of granules, that is, by local factors that are characteristic of the brittle field. In the
second category (microplastic granular flow), considered in subsequent sections
under Sect. 7.3 , the dynamics are controlled by local factors that are characteristic
of the ductile field, involving either crystal plasticity within the granules (cf. Chap.
6 ) or atom transfer (cf. Chap. 5 ) or both. The distinction between these two
categories is sufficiently profound that it is unconventional to consider them
together in the same chapter. Yet they are logically related through the kinematical
aspects considered under Sect. 7.1 . In practice, their fields of application tend to be
distinct because the first type of flow arises mainly at relatively low temperatures
and the second at higher temperatures (although, in the case of solution transfer
effects on the geological timescale, the temperature need not be very high).
In elaborating the first category further, it is useful to separate the case of pure
particulate flow, involving only friction between intact granules, and the case of
particulate flow with accompanying local fracture processes. We now consider
these cases in turn, as well as touch on the question of what is meant by cataclastic
flow and on the roles of temperature and pore fluid pressure.
7.2.2 Pure Particulate Flow
The simplest and, in many respects, archetypal model for granular flow is that of a
cohesionless assemblage of rigid particles that remain intact during the flow. Such
a model is often invoked in soil mechanics in connection with the flow of a
cohesionless dry sand. It can also be taken as a reference case in developing more
complex models in which additional physical processes are incorporated. We shall
refer to it as the model of ''pure particulate flow''.