Discrepancy in the Critical State Void Ratio of Poorly Graded Sand due to Shear Strain Localization

Abstract

The critical state (CS) concept is a theoretical framework that models the constitutive behavior of soils, including sand and other granular materials. It supports the notion of a unique postfailure state, where the soil ultimately experiences continuous shearing with no change in the plastic volumetric strain. However, the published literature has frequently noted the nonconvergence of sand specimens with different initial densities to a unique CS in the compression plane due to many factors such as specimen fabric, particle morphology, breakage, and grain size distribution. This paper examines the CS for poorly graded (uniform) glass beads and 3 different types of silica sands using 50 conventional triaxial compression (CTC) experiments, 12 oedometer tests, and in situ synchrotron microcomputed tomography (SMT) scans for 10 CTC experiments. The results of the 50 CTC experiments revealed a diffused CS zone in the compression plane, which was further examined using the in situ SMT scans. A thorough three-dimensional image analysis of the SMT scans accurately quantified the evolution of the local void ratio (e local ) versus axial compression within zones of intensive shearing toward the center of the specimen. The evolution of the void ratio was also measured using the entire volume of the specimen (e global ). At the CS, the e local =e global ratio was assessed to be ∼1.25 when a single shear band developed within the scanned specimens and ∼1.1-1.15 for specimens that failed via external bulging that was internally manifested by the development of multiple shear bands. This finding suggests that the CS zone in the compression plane can be attributed to the common wrong consideration of e global evolution in lieu of e local within the developing shear bands. Furthermore, the lack of shear band development in uniaxial compression has made the results of the oedometer test reliable in quantifying the CS parameters in the compression plane.

Characterizing Sediment Flux of Deforming Glacier Beds

“…Greater void ratio corresponds to a smaller number of contact points between interlocked grains and a decreased resistance to rolling. This point is particularly relevant in the context of our experimental results, since the critical state void ratio in a shear band is greater at low effective stresses (Imseeh et al, 2020). These particle kinematics suggest that the decreasing limb of the double-valued N-Q sed relationship may arise due to suppressed grain rotation along shear band boundaries past the threshold N. This would lead to a decrease in shear band thickness with increasing effective stress, which is precisely the behavior we observe.…”

Strain localization in the standard triaxial tests of granular materials: Insights into meso‐ and macro‐scale behaviours

“…In other words, the critical state line (CSL) measured at the mesoscale shifted upward compared to the CSL traditionally measured for the macro RVE, as shown in Figure 21B. This explains why there is a discrepancy in the convergence of soil specimens to a unique CSL in the compression plane 64–66 . The soil specimen is not homogeneous, therefore, the global behaviour does not represent the intrinsic material behaviour.…”

“…This explains why there is a discrepancy in the convergence of soil specimens to a unique CSL in the compression plane. [64][65][66] The soil specimen is not homogeneous, therefore, the global behaviour does not represent the intrinsic material behaviour.…”