Linking macroscopic failure with micromechanical processes in layered rocks: How layer orientation and roughness control macroscopic behavior

摘要

To constrain the impact of preexisting mechanical weaknesses on strain localization culminating in macroscopic shear failure, we simulate triaxial compression of layered sedimentary rock using three-dimensional discrete element method simulations. We develop a novel particle packing technique that builds layered rocks with preexisting weaknesses of varying orientations, roughness, and surface area available for slip. We quantify how the geomechanical behavior, characterized by internal friction coefficient, mu(o), and failure strength, sigma(F), vary as a function of layer orientation, theta, interface roughness, and total interface area. Failure of the simulated sedimentary rocks mirrors key observations from laboratory experiments on layered sedimentary rock, including minima sigma(F) and mu(o) for layers oriented at 30 degrees with respect to the maximum compressive stress, sigma(1), and maxima sigma(F), and mu(o) for layers oriented near 0 degrees and 90 degrees to sigma(1). The largest changes in sigma(F), (66%) and mu(o) (20%) occur in models with the smoothest interfaces and largest interface area. Within the parameter space tested, layer orientation exerts the most significant impact on sigma(F), and mu(o). These simulations allow directly linking micromechanical processes observed within the models to macroscopic failure behavior. The spatial distributions of nucleating microfractures, and the rate and degree of strain localization onto preexisting weaknesses, rather than the host rock, are systematically linked to the distribution of failure strengths. Preexisting weakness orientation more strongly controls the degree and rate of strain localization than the imposed confining stress within the explored parameter space. Using the upper and lower limits of mu(o) and sigma(F), obtained from the models, estimates of the Coulomb shear stress required for failure of intact rock within the upper seismogenic zone (7 km) indicates that a rotation of 30 degrees of s