Measurement of Volumetric Deformation, Strain Localization, and Shear Band Characterization during Triaxial Testing Using a Photogrammetry-Based Method

Triaxial testing has been routinely used as a standard laboratory test that allows correct determination of soil characteristics. Previously the volumetric strain of the triaxial specimen was considered to be uniformly distributed along with the specimen during the isotropic and deviatoric loading. Although this assumption might hold true under isotropic loading, the effects of restrained ends and disturbance during the procedures of specimen installation and testing can cause nonuniform strains throughout the whole specimen. This paper investigates the effects of specimen preparation and misalignment on the strain uniformity along with the soil specimen during triaxial testing. A series of consolidated drained tests at several stress paths were conducted on sand specimens. A photogrammetry-based method was applied at different stages of specimen preparation and testing to provide a three-dimensional full-field deformation measurement of the surface of the triaxial soil specimen. One commercial camera was used to capture images for the triaxial specimen, and a developed application for data processing and post-processing was utilized to ensure automatic and fast processing of the developed photogrammetric-based method. The local displacement data provided by the photogrammetry-based method enabled the evaluation of the strain localization and the volumetric strain nonuniformity analysis at different heights along with the specimen. The triaxial test results demonstrated that the soil specimen during triaxial testing has deformed nonuniformly in the axial, radial, and circumferential directions. The plots of the strain localization precisely presented the variation of local strains and the magnitude of deformation after the saturation stage. These results prove the soil specimen volume is not constant during saturation, and unavoidable disturbance had occurred during the specimen preparation steps and saturation. The results proved that the specimen misalignment during triaxial testing leads to scattering in the triaxial test results. Further discussion was presented about the shear band characterization including shear band thickness, formation, and propagation.