Volume change measurement of a triaxial specimen by digital imaging

Abstract

This project aimed to evaluate the feasibility of using digital image processing to determine the volume change of a soil specimen under isotropic consolidation. While more accurate methods

of measuring the volume change of a sample may involve the use of liquid volume measurement for a saturated soil specimen or 3D laser scanning, digital image photogrammetry

would be comparatively cheaper to obtain and simpler to conduct. However, accuracy of values obtained from such an approach remains to be further verified. This can be done by comparing

the results obtained from the photogrammetry approach with any of the two more accurate methods mentioned above.

If it can be shown that the results of volume change obtained from photogrammetry is within an acceptable margin of error, this could potentially be a reliable and economical method for

volume measurement in laboratory tests, especially for unsaturated soil tests. This study involves the use of a fixed digital camera that can be revolved around the triaxial cell housing

the specimen. By taking images of the specimen at different angles and at various time intervals, a three-dimensional specimen with (x,y,z) co-ordinates can be generated with an image processing software such as EOS PhotoModeler Scanner. Subsequently, the resulting model as a point cloud representation can be analysed using a thin layer method thereby obtaining a representative volume for each layer. Volume computation is subsequently a straightforward procedure of summing up all the layers. The study showed that photogrammetry is promising in measuring volume of a triaxial specimen. After relevant corrections, the errors shown using a digital camera and handphone camera are +/- 7 mm³ and +/- 15 mm³ respectively. The digital pressure volume controller (DPVC) used to validate the experimental results was found to suffer from compliance errors showing larger volume differences at low volume (low pressure) compared to high volume (high pressure).