3D Imaging of particle motion during penetrometer testingFrom microscopic to macroscopic soil mechanics

We present the results of direct observation of material rearrangement due to penetration of a solid rod (penetrometer) through a granular medium. Two different techniques and their advantages are discussed in this paper. We investigate the motion of material within the bulk around the rod. Transparent, polydisperse, and irregularly shaped silica particles immersed in index matching fluid are used for detailed imaging of the interior of a granular pile. Motion of material is observed by confocal microscopy from the bottom boundary up to 100 particle diameters in height. Image analysis indicates that rearrangements spread furthest not directly under the penetrometer but in a ring around the penetrometer. In addition, the direction of preformed stress chains in the material influences the particle rearrangements. Material compressed from one side exhibits anisotropic particle rearrangements under penetrometer testing. Laser sheet scanning allows for direct imaging of individual particle motion with greater accuracy, but works best for spherical particles only.