Effects of consolidation history and shear rate on the critical state of two sands

Critical state serves as the basis for failure criteria and post-failure behavior for many constitutive models for soils, so accurate determination and overall validity of critical state behavior are important. According to critical state theory, for a given soil there exists a unique void ratio for each state of effective stress at the critical state. It is well known that localized strains, or shear bands, form in dense sands at or near peak stress, posing difficulty in quantifying critical state using conventional, globally based laboratory measurement techniques. Results from recent biaxial tests, conducted in an apparatus that enables void ratio evolution within shear bands to be recorded and analyzed photographically, did not indicate the existence of a unique void ratio-effective stress relationship at critical state, but tended to suggest that uniqueness was a function of consolidation history. To confirm this hypothesis, drained biaxial compression tests were conducted on dense sands of different, distinct consolidation histories. Digital Image Correlation (DIC) was used to quantify localized displacements, yielding a high level of accuracy in determining the void ratio evolution to critical state within shear bands. Results indicate that a unique void ratio-effective stress relationship is realized only for sands consolidated from similar initial void ratios.; Experimental studies of the behavior of fault gouge, the granular product of wear along slipping surfaces in faults, have provided evidence that critical state sliding friction is velocity dependent, and that this dependency may be coupled with dilatancy. However, experimental issues such as indirect measurement of displacement and dilation within the deforming zone of simulated gouge, often sand, have caused difficulty in quantifying behavior. In conjunction with the current experimental program, shearing rate changes were imposed on specimens after critical state was attained, and the friction and volumetric response of the shear band material were recorded. The results have enabled correlation of constitutive model parameter with the physical evolution of the shear band, and provide input to rate- and state-dependent constitutive models for simulated fault gouge.