A Digital Image-Based Deformation Measurement System for Triaxial Tests

This paper describes the development of a new digital image-based deformation measurement system for triaxial tests on soils. Three digital cameras placed on radii at intervals of 120° viewed on plan outside a transparent triaxial cell were used to capture images of a deforming cylindrical soil specimen at various instants. A digital image correlation program was written to analyze the captured images, using ray tracing to take account of image distortion due to refraction at the interfaces between the cell fluid, the cell wall, and the atmosphere. The technique is validated with reference to a sand specimen tested in drained triaxial compression. Typical implementations of the system to study surface deformation characteristics (e.g., barreling, onset of localization, and shear band evolution) of deforming soil specimens are illustrated.