Fine reconstruction of full-surface of soil specimens during triaxial testing

Noncontact image-based methods are becoming increasingly popular for characterization of geomaterials deformation during laboratory loading. However, surface reconstruction of a specimen inside a triaxial chamber is still challenging due to difficulties in handling optical ray refraction point cloud acquisition. A new system, which includes six cameras, an ultra-thin transparent membrane, the 3D-DIC technique, and multi-media photogrammetric methods and the associated software package GeoTri3D, is proposed to reconstruct soil surface during triaxial loading. Tests on a stainless-steel cylinder and silt specimens are performed to evaluate the proposed system. The multi-media photogrammetric method is able to rigorously correct the effect of optical ray refraction and extends the application of 3D-DIC from single medium to multi-media conditions. Meanwhile, the introduction of 3D-DIC significantly increases the resolution of the multi-media photogrammetric measurement and reduces the time and effort required for generation of dense point clouds. With the proposed system, the specimen surface can be photo-realistically reconstructed, which offers an accurate and data-rich solution for advanced material characterization under multi-optical-media conditions. Furthermore, for a specific region of interest (ROI) on the specimen surface, the measurement resolution can be significantly increased up to an ultra-high level (i.e.6034.7 dots/cm²), which is particularly favorable for localized deformation characterization.