Numerical modeling of mixed mode multiple crack propagation and its application to the simulation of nonlinear rock deformation and borehole breakout.

摘要

Rock is a very heterogeneous material, containing structural weakness at all scales. These weaknesses include grain boundaries, pores, and cracks on the small scale, and joints, faults, and bedding planes on the large scale. Nonlinear rock deformation in the low-temperature, low-confinement regime is due primarily to the growth of cracks from these weaknesses and the coalescence of cracks to form macroscopic structural features. Another important aspect of rock deformation and failure is the statistical distribution of weaknesses in the initial microstructure.; Borehole breakout is the process by which portions of a borehole wall fracture or spall when subjected to compressive stresses. Studies of borehole breakout in the past twenty years include experiments, field studies, and numerical modeling. With regards to the numerical modeling of borehole breakout, the rock surrounding the borehole is considered as a nonlinear continuum material in most of the previous approaches. Experiments and field studies, however, have shown that the heterogeneous and discontinuous nature of rock has a strong impact on the mechanics of borehole breakout.; This dissertation describes a numerical model that has been developed to simulate the damage of rock and the corresponding non-linear stress-strain behavior, and also the progression of borehole breakout in heterogeneous and discontinuous rock by mixed mode crack growth, interaction, and coalescence. The rock is simulated as an elastic material containing a random distribution of cracks. As compressive load is applied, the initial cracks grow, interact, and coalesce to form macroscopic fractures. The numerical model was developed by making a series of modifications to the displacement discontinuity code of Crouch and Starfield (Crouch & Starfield, 1983). The most important modifications include modifying the boundary element for the calculation of stress intensity factors, adding Coulomb friction for closed portions of cracks, adding a crack generator, and adding an algorithm for crack coalescence. The numerical model is used to simulate the non-linear deformation and the progression of breakout in Westerly granite, and the results are realistic.