Observations of multi-scale granular kinematics around driven piles in plane strain condition

The behavior of granular media around driven piles is not well understood. Current design guidelines still remain empirical and highly approximate. Plane-strain calibration chamber tests have been conducted to provide visual observations of the penetration mechanisms occurring during driving. Tests were performed to investigate the effects of confinement, relative density, and soil type. Digital image correlation (DIC) combined with other advanced image analysis tools were used to obtain meso-scale displacement, finite strain maps, as well as micro scale particle kinematics during pile installation. The observed shear strain map for the confined dense Ottawa sand test shows a wedge-type soil failure plane with a rigid cone of sand beneath the pile tip. The obtained volumetric strain maps show an intense comprebion zone directly below the pile tip, followed by a dilation zone to accommodate shear. High degree of sand compaction at the pile tip during driving also created a thin dilation strip along the pile shaft. High hoop strebes could then be sustained in the surrounding denser sand by arching. Large rotation and chaotic particle motion were also observed in the areas where there were large shear and volumetric strains. The random motion of particles near pile boundaries was further confirmed through affine/non-affine analysis of grain kinematics.