ARMA/USRMS 06-1069 Embedded discontinuity finite element modeling of three-dimensional strong discontinuities in rocks

Localized deformation such as shear bands, compaction bands, dilation bands, combined shear/compaction or shear/dilation bands, fractures, and joint slippage are commonly found in rocks. Thus, modeling their inception, development and propagation, and effect on stress response is important with regard to making informed engineering design decisions involving these materials. Following a GulfRocks04 paper (ARMA/NARMS 04-520) that focused on modeling bifurcation to these localized deformations, this paper focuses on modeling numerically the post-bifurcation regime, specifically strong discontinuities in rock whose bulk response is governed by a three-invariant, isotropic/kinematic hardening cap plasticity model. We develop a simple post-bifurcation constitutive model and implement it using an enhanced strain finite element method, an approach used to embed discontinuities within the coarse scale finite element response. The post-bifurcation model takes the form of a simple Mohr-Coulomb failure model along the discontinuity, but with internal variables cohesion, dilation/compaction, and friction allowed to degrade linearly or exponentially as a function of jump displacement across a discontinuity/crack. Because the dilation/compaction internal variable evolves, the enhancement function for the enhanced strain finite element formulation evolves as well. A hexahedral Embedded Discontinuity finite Element (EDE) is implemented to model three-dimensional strong discontinuities in rock. Numerical examples will demonstrate the model.