Force networks in evolving granular layers
Abstract
The strength and stability of fault zones can be significantly affected by the presence of a dynamically evolving layer of gouge. With increasing displacement, some laboratory simulations of fault gouge can develop shear localization and significant spatial heterogeneity in grain size, and may undergo multiple transitions between velocity-strengthening and velocity-weakening behavior. To search for the micromechanical mechanisms responsible for this complexity, we have performed several suites of numerical experiments using the discrete element method. In our model, grains are approximated by circular disks which interact through elastic and frictional contact forces. We define sets of 500 to 5000 grains with a variety of size distributions. A layer of grains is confined between rigid boundaries on which shear and normal forces are imposed. We calculate the forces on each grain-grain contact, and a representative stress state within each grain. When the shear or tensile stress inside a grain exceeds a preset limit, the grain shatters into a set of smaller grains. We are interested in the redistribution of forces in the gouge in a locked fault and the triggering mechanism that allows a slip event.. Two important networks of contact forces can be recognized: an interconnected network of force chains, in which each contact transmits a relatively large force, and a less connected network of bridging contacts which transmit small normal forces but have high shear to normal force ratios. The number, distribution and orientation of the contacts in these two networks is diagnostic of the type of deformation occuring in the system. In systems with very strong grains, the triggering mechanism that allows the whole layer to fail in shear appears to be a reorientation of the network of bridging contacts. If the grains are weak (or the confining pressure is high) then the fracturing of a grain within a force chain can trigger a slip event. As comminution proceeds, the stresses in any remaining large grains become near hydrostatic (so the grain is unlikely to fracture), even though these large grains are statistically very likely to be part of one or more force chains. We are currently exploring how the force networks and layer strength change for different types of grain size distributions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.T11F..06S
- Keywords:
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- 8010 Fractures and faults