Characterisation of soil deformation over wide strain ranges in triaxial test with high-precision stereophotogrammetry

Stereophotogrammetry was adopted as a means to measure three-dimensional displacements of a soil specimen surface in triaxial tests. New developments include an efficient algorithm that resolves the relative displacements with high precision to the order of 10 −3 mm, and strains in common-sized soil specimens to the order of 10 −3 %, while correcting for ray refraction effects caused by the pressure cell wall and water. The system, requiring only sets of compact-type digital cameras as hardware, allows a stiffness–strain curve to be determined over wide strain ranges spanning from 10 −3 % to virtually any large strain with a fixed configuration. This paper explains the proposed image analysis processes, which combine a ray tracing formulation by Zhang and co-workers, particle tracking velocimetry and sub-pixel digital image correlation in efficiently deriving accurate and precise relative displacements. Rigorous assessment of the accuracy and precision was conducted. As a demonstration, two undrained triaxial compression tests on reconstituted clay were performed with and without end lubrication. Both for small-strain (<0·05%) axial loading–unloading cycles and for monotonic loading to large axial strain (15%), the strain development was tracked and the specimen behaviour was characterised. These tests demonstrate that the new technique can be a useful option in a soil laboratory both for research and practice.