This paper investigates the validity of the interpretation of results from testing saturated axisymmetric triaxial compression (ATC) sand specimens utilizing 3D synchrotron micro-computed tomography (SMT) to probe localized events that are completely missed or misinterpreted when analyzing ATC measurements based on global standard measurements. Drained and undrained experiments were conducted at low and high back pressures (BPs) coupled with multiple in situ 3D-SMT to acquire high-resolution scans of the specimens at different axial strains. Specimens tested under low BP exhibited a large pore air volume change, which was not detected by the pump system that represents standard volume measurement. The increase in air volume caused a significant reduction in the degree of saturation leading to a possible transition from saturated to partially saturated constitutive behavior. Undrained experiments exhibited a significant volume change contrary to the assumption of negligible volumetric strain for saturated undrained experiments. Air bubbles within the shear band for drained and undrained low BP specimens showed opposite capillary pressure responses, increased for drained and decreased for undrained cases, due to the variation in the mechanism by which each of the two experiments predominantly counters the volume expansion within the shear band.
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