High-precision and high-accuracy stereophotogrammetric image analysis for small to large strain deformation measurement in triaxial apparatus

Hokkaido University, Faculty of Engineering, Kita-8 Nishi-13, Kita-Ku, Sapporo, Japan

^* Corresponding author: nishimura@eng.hokudai.ac.jp

Abstract

A deformation measurement system for laboratory soil testing based on stereophotogrammetric image analysis was developed with a purpose of characterising soil behaviour continuously from small to large strains. The system is applicable to triaxial tests with pressure chamber and confining fluid by correcting the refraction-induced distortion of images by ray tracing. In comparison to similar existing studies, the present study put priority in precision and accuracy in relative displacement tracking, which is the key feature in resolving small strain in soil specimen and characterising its stiffness. The displacement tracking is performed with a hybrid algorithm combining Particle Tracking Velocimetry (PTV) for primary target matching and subpixel Digital Image Correlation (DIC) for more precise image matching. The deformation of clay samples with and without end lubrication was compared by visualising the deformation at different levels of strain. This paper is basically a summary of Nishimura (2022). Another example of application in laboratory testing involving fine particle suffusion is also shown. Some notes on implementation of the technique for good practice and on future development are additionally described.