Measurements of local and global volumetric strains for wet sand specimens under uniaxial compression using digital image correlation method

Determination of volumetric strain localization regions in shear bands is significant for studies on geological hazards related to pore fluids. Possibly, compared with those of the maximum shear strain localization, the positions of the volumetric strain localization are closer to sites for the source of hazards. Stress-longitudinal strain curves of three wet sand specimens (water contents=12.7%~17.1%) under uniaxial displacement-controlled loading are measured. Using a developed digital image correlation (DIC) method based on the particle swarm optimization algorithm, the distribution of local volumetric strain in sand specimens is calculated before and slightly beyond microcracks are initiated. It is found that as deformation proceeds, the uniform distribution of the local volumetric strain at the approximately linear stage is changed to the nonuniform one at the strain-hardening stage until the local volumetric strain localization occurs. In the tangential direction of a shear band, compared with the relatively uniform distribution of the maximum shear strain, the distribution of the local volumetric strain exhibits several peaks, which is related to the periodic occurrence of microcracks. When microcracks will be initiated, the deformation in the shear band is basically dilatational; the shear bands are more apparent; and the maximum shear strain and local volumetric strain in the shear band are two and four times larger than those outside the band, respectively. However, at the earlier stage, the dilatational and contractive regions are