HR: 0800h
AN: S41A-0926    [Abstracts]
TI: Formation and Evolution of Strong Discontinuity in Granular Rocks
AU: * Borja, R I
EM: borja@stanford.edu
AF: Stanford University, Department of Civil and Environmental Engineering, Stanford, CA 94305 United States
AU: Foster, C D
EM: cdfoster@stanford.edu
AF: Stanford University, Department of Civil and Environmental Engineering, Stanford, CA 94305 United States
AB: Planar discontinuities (sharp) in granular rocks are characterized by shear fracture and slip surfaces forming from an initially intact state. The structure of deformation is defined by a jump in the displacement field, called strong discontinuity, in contrast to tabular deformation bands where the jump occurs in the displacement gradient field, called weak discontinuity. Mathematically, strong discontinuity may be considered as a limiting case of weak discontinuity as the thickness of the band approaches zero. We describe essential ingredients of a strong discontinuity model for granular rocks using classical theory of plasticity and the finite element method. They include: (a) a condition for the formation of a strong discontinuity in an initially intact material; (b) a transition constitutive law from a continuum state to a damaged state characterized by a fully developed strong discontinuity; and (c) a mathematical description of the progression of the strong discontinuity to residual state. Each deformation state requires a complete constitutive description of the material, including a constitutive characterization of the frictional resistance on the surface of discontinuity. We describe the role of slip speed on frictional resistance along the slip surface, and elucidate how a well-known state- and velocity-dependent phenomenological constitutive law for the coefficient of friction may be integrated into the framework of the strong discontinuity model. We next describe how the kinematics of strong discontinuity may be captured in the finite element approximation. The proposed finite element technique is based on the concept of embedded strong discontinuity. The idea is to simultaneously capture both the coarse-scale field, representing the macroscopic deformation of the rock, and the fine-scale field, representing the intense deformation along the discontinuity, without resorting to severe refinement of the finite element mesh. Further, the approximation must be insensitive to element alignment and mesh refinement. The resulting multi-scale finite element model is then used to simulate the formation and evolution of a strong discontinuity in a specimen of rock.
DE: 8010 Fractures and faults
DE: 8020 Mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting