Geology Reference
In-Depth Information
Fig. 7.3 A computer model of the distribution of forces between particles in a granular
assemblage after 3 % shear strain. The lines represent the orientation of forces between particles
and the thickness of the lines represent the magnitudes of the forces (copy of Fig. 3 in Thornton
and Barnes
1986b
)
major way to the deformation. Thus the granules along a chain tend to rotate
alternately in opposite senses, leading to the eventual destruction of the chain
and transfer of load. The rotations may be exaggerated in certain locations,
giving a hinge-like effect with the appearance of a local strain discontinuity.
(4) During straining the number of contacts tends to decrease, especially in the
direction of the least compressive principal stress, accompanied by the
development of dilatation and anisotropy. The dilatation seems not to depend
immediately on the intergranular friction, although this will have an eventual
effect through its influence on the development of fabric.
(5) Upon unloading, residual stresses tend to remain, due to locked-in shear forces
at contacts.
We now come to the microdynamical problem of integrating up from the
behaviour at the grain scale to the macroscopic behaviour of the aggregate, that is,
of deriving a constitutive relation or flow law from a given set of grain-grain
forces. Such a step involves not only the mathematical difficulties arising from the
complexity and variability of the pattern of contact forces and their proper
description, but also the structural or geometrical problems of what this complex
