HR: 0800h
AN: T21B-0477    [Abstracts]
TI: Granular Friction and Interparticle Force Networks; Effects of Particle Size, Size Distribution, and Strain Rate
AU: * Halpert, A
EM: ahalp@rice.edu
AF: Rice University, Dept Earth Science, MS-126, 6100 Main St, Houston, TX 77005 United States
AU: Morgan, J K
EM: mcgovern@lpi.usra.edu
AF: Rice University, Dept Earth Science, MS-126, 6100 Main St, Houston, TX 77005 United States
AB: Micromechanical simulations of granular fault gouge, conducted in 2-D, allow us to examine the spatial and orientation distributions of interparticle contact forces, their evolution with time, and influence on fault zone strength and stability. Modeled after laboratory experiments on glass rods and spheres [e.g., Mair et al., 2002, Anthony and Marone, 2005], fault gouge consists of rounded particles with diameters between 53 to 250 æm, in both well sorted and poorly sorted arrangements, sheared between smooth and rough planar walls. Applied normal stresses of 5 or 10 MPa are in the non-fracturing regime. Simulations are carried out at several different shearing velocities to examine the velocity dependence of friction. The normal and shear stresses acting on the shear zone walls are supported by complex networks of interparticle forces that span the gouge zones. Interparticle forces greater than average define inclined force chains that evolve as the gouge deforms, mapping the internal stress and deformation field. Boundary conditions strongly influence the stability of the force chains. Rough boundaries couple with the gouge zone, causing distributed deformation and continuous reorganization of the force chains. Stress drops occur when major force chains fail and, as in laboratory experiments, their magnitudes correlate directly with the mean size of the particles. In contrast, shear zones with smoother boundaries develop enduring boundary shears that preserve the internal force network; stress drops accompany the onset and cessation of boundary shearing, and are unrelated to particle size. Particle size and boundary conditions also affect the fabric of the shear zone's force network. The mean force orientation within the shear zone shows a dependence on particle size for smooth boundaries, but not for rough boundaries. At the scale of individual particle contacts, the effective radii of two particles in contact appears to dictate the orientation and magnitude of the interparticle force. In general, smaller particles support smaller forces at low angles to horizontal, while larger particles support larger forces closer to the vertical. When time-dependent contact healing is added to the simulations, velocity dependence of certain quantities is also observed, e.g., contact force magnitude, ratio of sliding contacts, and related micromechanical quantities. Velocity stepping experiments result in time-dependent stress drops and gouge strengthening behavior that generally conform to the results of laboratory experiments.
DE: 0560 Numerical solutions (4255)
DE: 8010 Fractures and faults
DE: 8012 High strain deformation zones
DE: 8020 Mechanics, theory, and modeling
SC: Tectonophysics [T]
MN: Fall Meeting 2005