Numerical simulations of granular shear zones using the distinct element method: 1. Shear zone kinematics and the micromechanics of localization

Abstract

Abstract. Two-dimensional numerical simulations were conducted using the distinct element method (DEM) to examine the influences of particle size distribution (PSD) and interparticle friction /at, on the nature of deformation in granular fault gouge. Particle fracture was not allowed in this implementation but points in PSD space were examined by constructing assemblages of particles with self-similar size distributions defined by the twodimensional power law exponent D. For these numerical "experiments," D ranged from 0.81 to 2.60, where D = 1.60 represents the two-dimensional equivalent of a characteristic PSD to which cataclastically deforming gouge is thought to evolve. Experiments presented here used /at, values of 0.10 and 0.50 and were conducted using normal stress c• n on the shear zone walls of 70 MPa. Shear strain within these simulated assemblages was accommodated by intermittent displacement along discrete slip surfaces, alternating between broadly distributed deformation along multiple slip planes and highly localized deformation along a single, sharply defined, subhorizontal zone of slip. Slip planes corresponded in orientation and sense of shear to shear structures observed in natural gouge zones, specifically Riedel and Y shears; the oblique Riedel shears showed more extreme orientations than typical, but their geometries were consistent with those predicted for lowstrength Coulomb materials. The character of deformation in the shear zone varied with PSD due to changes in the relative importance of interparticle slip and rolling as deformation mechanisms. A high degree of frictional coupling between large rolling particles in low D (coarse-grained) assemblages resulted in wide zones of slip and broadly distributed deformation. In higher D assemblages (D >= 1.60), small rolling particles selforganized into columns that separated large rolling particles, causing a reduction in frictional resistance within the deforming assemblage. This unusual particle configuration appears to depend on a critical abundance of small particles achieved at D = 1.60 and may enable strain localization in both real and simulated granular assemblages.

Direct simulation of fluid‐solid mechanics in porous media using the discrete element and lattice‐Boltzmann methods

“…[7] In this paper we use a traditional DEM approach similar to that presented by Cleary and Campbell [1993]; Campbell et al [1995]; Morgan [1999]; Morgan and Boettcher [1999], based on an existing two-dimensional DEM application [Rege, 1996;Williams and Rege, 1997].…”

“…[6] This work investigates hydromechanical behavior of porous media using a DEM technique for solid mechanics first presented by Cundall [1971] and Cundall and Strack [1979], which has successfully approximated the behavior of noncohesive, granular systems under low stress conditions [Cundall et al, 1982;Cleary and Campbell, 1993;Campbell et al, 1995;Morgan, 1999;Morgan and Boettcher, 1999], and lithified sedimentary rocks [Bruno and Nelson, 1991;Potyondy et al, 1996;Hazzard et al, 2000;Boutt and McPherson, 2002]. Extensive comparisons of the DEM to theoretical, numerical, and experimental systems have been performed [Pande et al, 1990].…”

Breaking Up: Comminution Mechanisms in Sheared Simulated Fault Gouge

“…Two-D DEM simulations (e.g. MORGAN, 1999) also suggest that grain size distribution may influence friction. In addition, it has been shown that grain shape has a first order influence of the sliding friction of the fault gouge.…”

Micromechanical modeling of cracking and failure in brittle rocks

“…It is possible that these stress waves may effect rock fracture on all scales from microcrack formation to fault slip around mine workings [Hildyard et al, 1995] The distinct element method has been used successfully to examine several different geological applications. Shear band formation in granular material has been examined in depth [Antonellini and Pollard, 1995;Cundall, 1989;Morgan and Boettcher, 1999], earthquake faults with gouge have been simulated [Mora and Place, 1998], and Donz• et al [1996] have used it to model an explosive source in a brittle rock mass. However, there is less published information on the use of distinct element codes to simulate failure of fully competent rocks under compression.…”