A novel optical method for measuring 3D full-field strain deformation in geotechnical tri-axial testing

Abstract

Soil is a complex multiphase material and has anisotropic properties; meanwhile, soil behaviour is typically evaluated in tri-axial testing. Until now, there have still been some challenges in measuring the total strain, the local strain deformation and the volume changes during tri-axial testing. In conventional geotechnical testing, it cannot acquire enough deformation characteristics. To improve the situation, a digital image measurement system has been constructed and calibrated on a geotechnical tri-axial apparatus. In the digital image measurement system, the three-dimensional digital image correlation (3D-DIC) technique was applied to measure the soil specimen’s 3D large deformation and reconstruct the 3D surface topography of the specimen. To improve the measurement accuracy, a standard spatial calibration procedure to rectify the 3D reconstruction data in tri-axial testing is proposed to improve the effect of the projective transformation in stereovision. Through rectification, the root-mean-squared error in terms of displacement measurement in the tri-axial testing was reduced by more than half. Furthermore, a subpixel edge detector was applied to estimate the volume changes based on the 3D-DIC results. Finally, comparing the results between the image measurement system and the conventional measurement method, the proposed method not only can obtain the specimen’s distribution of 3D full-field deformation, volume strain and so on, but also can provide a method for rectifying the 3D data measured by the image measurement technique in tri-axial testing. The results from 3D reconstruction can describe directly the characteristics of anisotropy and non-uniformity in geotechnical materials. The results can provide more information than conventional testing when establishing a constitutive model in soil mechanics.