ABSTRACT
Grain-boundary microcracking, sliding, indentation, and healing have been shown to impact salt-rock bulk deformation. An improved understanding of the combined action of grain-boundary processes is necessary for accurate interpretation of salt-rock mechanical behavior in both natural and engineering contexts. We prepared granular, low-porosity, work-hardened salt-rocks (∼300 ppm water) for triaxial stress-cycling experiments at low confining pressure to investigate semibrittle behavior and effective stress. We used optical microscopy to characterize grain-scale structure. Semibrittle flow involves coupled grain-boundary sliding and wing-crack opening accommodated by indentation via intragranular dislocation glide. Grain-boundary sliding is frictional at higher strain rates, but the associated dispersion of water from fluid inclusions along boundaries can activate linear-viscous, fluid-assisted, diffusional sliding at lower strain rates (<10-8 s-1). The combined action of these mechanisms leads to pressureand time-dependent behaviors including anelasticity and hysteresis. In addition, we conducted cyclic poreand confining-pressure tests to demonstrate that during semibrittle flow, strength depends on differential pressure consistent with the Terzaghi’s effective stress law. This behavior may be explained by combined operation of pressure-independent intracrystalline-plastic mechanisms and transmission of pore pressure at grain boundaries via thin fluid films. Our study indicates coupled microprocesses are key to understanding semibrittle behavior of salt-rocks.
