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Extensive research has been reported in the literature to characterize the failure mode of dry sand using various experimental techniques such as surface optical imaging, photo-elastic materials, 3D computed tomography (CT), and 3D synchrotron micro-computed tomogra-phy (SMT). However, there is a limited literature about the behavior of saturated sand. This paper presents the results of axisymmetric triaxial compression (ATC) experiments that were conducted on saturated sand specimens. The behavior of specimens composed of a uniform sand with grain size between US sieves #40 and #50 is compared to specimens conducted on the same sand that has a wider gradation. 3D SMT technique was used to acquire 3D scans while shearing the spec-imens to probe localized events that are completely missed or misinterpreted when analyzing ATC measurements based on global standard measurements. The results show a higher EPSR for the non-uniform specimen and a thicker shear band when compared to uniform specimen. 

Back pressure (BP) is used to saturate triaxial specimens before the shearing phase. A high BP is commonly used in the laboratory to ensure a high degree of saturation. Most soil deposits in the field for Civil Engineering applications have a low pore water pressure in the range of 30 to 50 kPa. The principles of soil mechanics postulate that the value of BP does not affect the behavior of triaxial specimens as long as the effective confining pressure is kept constant. This paper investigates the influence of BP on the behavior of uniform saturated sand tested using axisymmetric triaxial compression (ATC). 3D synchrotron micro-computed tomography (SMT) technique was used to acquire 3D scans while shearing the specimens to probe localized events that are completely missed or misinterpreted when analyzing ATC measurements based on global standard measurements. Specimens tested under low BP exhibited a large pore air volume change, which was not detected by ASTM standard measurements for triaxial cells. The paper discusses the influence of BP on the deformation mode of the specimens using rich SMT images and sheds light on the change of degree of saturation for specimens tested at low BP, and compares the behavior of specimens tested at high BP. 

The failure of sheared granular materials is manifested in zones of intensive shearing known as shear bands. The onset and evolution of shear bands are influenced by many factors including specimen density, particle morphology, gradation, boundary conditions, and loading conditions. This paper investigated how particle morphology and drainage condition (drained versus undrained) affect the evolution of shear bands for saturated sand.