Validation of Photogrammetry-Based Method to Determine the Absolute Volume of Unsaturated Soils

Although many engineering practices are based upon a simplifying assumption that all soils are fully saturated, most pavement and geotechnical projects are typically constructed on compacted soils that are usually unsaturated. Triaxial tests are widely used to investigate the stress–strain behavior of both saturated and unsaturated soils. Many image-based methods have been developed to overcome the limitation of conventional triaxial testing methods. Recently, a photogrammetry-based method was proposed to measure the saturated and unsaturated soil volume change during triaxial tests. The triaxial test results and validation tests proved that this method is highly accurate, simple, and cost-effective. However, all previously suggested methods have a major limitation associated with their inability to determine top and bottom boundaries between the soil specimen, and the top and bottom pedestals. Although a simple and very precise approach has been proposed to determine the top and bottom boundaries in triaxial tests, the absolute soil volume calculation during triaxial testing included the volume of the membrane. This paper proposes a photogrammetry-based approach to overcome this limitation by developing a simple and precise method to calculate the membrane volume during triaxial testing and thus to accurately determine the absolute volume of soils. In this paper, the detailed procedures are presented, and the validation test was performed to demonstrate the accuracy of the proposed method. The results proved that the proposed method could determine the membrane volume for the calculation of the absolute volume of saturated and unsaturated soils with 0.062% error.