Experimental Characterizations of Contact Movement in Two-Dimensional Rod Assembly Subjected to Direct Shearing

This study explored the features of particle motion and, in particular, the associated movements at contacts in response to direct shearing. A tailor-made direct shear box equipped with an image system was set up. The particle image velocimetry (PIV) technique and close-range photogrammetry were used together to help with the investigation. The test sample was made of randomly packed wooden rods of three different diameters. It was found that both the rolling and sliding components were concurrent at the same contacts. Both components were simultaneously increased and gradually concentrated on some particular contacts during shearing. This suggests bimodal behavior of the movements at the contacts (i.e., a strong motion contact versus a weak motion contact). Strong motion contacts are essential to the volumetric responses; contact rolling leads to volumetric dilation, whereas to a certain extent, contact sliding produces volumetric contraction. As the sample volume continues to dilate, the averaged cumulative rolling distance gradually prevails over the cumulative sliding distance, and therefore, the difference between these two distances is steadily increased. Such a difference, however, is reduced as the applied vertical stress (sigma) increases because a higher confining pressure hinders the development of rolling-induced dilation by simultaneously promoting the countereffect from contact sliding between the rollover particle pair. The analyses on an individual pore in response to shearing reveals a similar trend between the evolution of pore size and the associated movement of contacts among the surrounding particles.