Geology Reference
In-Depth Information
requirements (homologous with the compatibility constraints discussed in
Sects.
In contrast to the negligble or very small volume change during deformation of
often an important aspect of deformation by granular flow mechanisms. Assuming
that the solid volume of the granules themselves remains constant, the dilatancy
derives from variation in the interstitial volume during the deformation. This
variation can occur for two essentially different reasons, associated, respectively,
with the contact stresses in the granules and with the pressure in the interstitial
medium:
(1) as an accommodation mechanism associated with the relative translation of
granules, as discussed above; thus, a looser packing of granules locally may be
required to permit them to pass one another in the neighbour-changing process.
(2) as a direct response to a difference between the pressure in the interstitial
medium (for example, pore fluid pressure) and the mean total macroscopic
stress, assuming that the former pressure is independently controlled.
In the case in which the material of the granules can also be transported into or
out of the specimen, there may be an additional dilatation or compaction associ-
ated with the net transfer.
7.1.4 Fabric and Memory
On general symmetry principles (Curie
1894
; Paterson and Weiss
1961
) it can be
expected that in a granular body undergoing deformation the structural arrange-
ment of the granules will be in some way anisotropic. The appropriate description
of the structural arrangement, usually called a texture or fabric, will depend
somewhat on the nature of the deformation mechanism, that is, on the dynamical
factors governing the deformation. In the case of the low-temperature deformation
of particulate media such as unconsolidated sediments, the fabric may be specified
in terms of the contact normals between the particles; see, for example, Field
(
1963
), Brewer (
1964
), Oda (
1972a
,
c
), Nemat-Nasser (
1982
) and Satake (
1982
).
Thus, while a random dense packing of spheres may model some aspects of such a
granular medium, the random character will lack the essential anisotropy of the
medium actually undergoing deformation. In the case of polycrystalline material
undergoing high-temperature granular (''superplastic'') flow, the most important
element of the fabric may be the configuration of the grain boundaries at which
sliding is occurring.
Since the anisotropy of the fabric reflects the nature of the deformation process,
the fabric serves as a ''memory'' of this process. However, as in the case of the
crystallographic preferred orientation resulting from deformation by crystal plas-
ticity, where later phases of a complex deformation history can overprint and
obliterate the effects of the earlier phases, so in granular materials the fabric can be
