Measurement of Soil Shrinkage Curve Using Photogrammetry

Photogrammetry techniques have been widely used in geotechnical engineering both in the field and laboratory. In this study, photogrammetry is used to determine the shrinkage curves of an expansive soil. The shrinkage curve is a plot of volume of soil with water content as a soil dries. While, it is relatively easy to determine the gravimetric water content during drying, it is difficult to determine the volume of the soil specimen during drying. Using photogrammetry, the volumetric change of a soil specimen can be obtained by constructing a 3D model of the soil specimen using 2D images at various times. The difference in volumes of the soil specimen between two different times gives the volumetric change. Unlike previous studies, low-cost camera, and freeware are used to make the proposed method easily accessible. Moreover, this approach is non-contact and non-destructive and hence the soil specimen remains undisturbed during drying. In this study, a novel setup is developed which allows a handphone camera to move 360° around the soil specimen for capturing images. Considerations in the setup include lighting, number of images, and camera position. Proper lighting is essential to avoid shadow and distortion in the 3D model. A minimum number of images are required to ensure that the 3D model is constructed successfully. The camera can be fixed at one orientation (elevation and tilt). Two different handphone cameras of different specifications are evaluated. It is observed that 5-megapixel and 8-megapixel cameras gave similar results which show that camera having lower specification (5-megapixel) can be used to obtain an accurate volume measurement of the soil specimen. The absolute error in volume determination given by the two handphone cameras is about 0.3%. Generally, the volume of soil specimen with asymmetric deformation can be determined more accurately by photogrammetry compared to the conventional method.