Verification of an Internal Close-Range Photogrammetry Approach for Volume Determination during Triaxial Testing

Accurate strain and volume measurements are critical to phase relationships and strength determination for saturated and unsaturated soils. In recent years, laboratory-based photographic techniques of monitoring soil specimens have become more common. These techniques have been used to reconstruct 3-D models and to determine strain and volumetric changes of triaxial specimens. A new technique that used digital photographs of the soil specimen, captured from within a triaxial testing cell, was utilized. Photographs were processed using photogrammetric software to reconstruct 3-D models of the soil specimens at various times during the triaxial test. By placing camera equipment within the cell, the technique eliminated the need to account for optical distortions that were due to (1) refraction at the confining fluid-cell wall-atmosphere interface, (2) the curvature of the cylindrical cell wall, and (3) the pressure-induced deformation of the cell wall. Previously unreported results from sensitivity studies and accuracy assessments for the internal photogrammetry approach are documented herein. Furthermore, through undrained triaxial compression and extension tests, the viability of determining total and local strains, volume changes, and total volume at any stage of triaxial testing was demonstrated. By comparison with other volume-determination methods that are presented herein, including digital single-lens reflex camera photogrammetry, 3-D scanning, manual measurements, and water displacement techniques, a relative error of 0.13 % was assessed for the internal photogrammetric volume determination technique.