Footprints on the beach: visualizing dilation-induced air entry

The dry fringe that appears beneath a footprint on a beach is a visual manifestation of dilation. Dilation causes the sand to attempt to increase in volume on shearing, resulting in negative pore pressure and enhanced bearing capacity. Conventional theory cannot explain this increase in strength, suggesting instead that bearing capacity with the phreatic surface at the sand surface should be approximately half that of the dry case. To explore this apparent contradiction, ten load-controlled bearing capacity experiments were performed on transparent soil with phreatic surfaces ranging from 100 mm below to 30 mm above the footing. For phreatic surfaces deeper than the footing width, failure occurred within the dry material. In shallower phreatic surface conditions, bearing capacity increased as the phreatic surface approached the surface. Air entry was observed to initiate along the top surface of the saturated layer and extend to the depth of the bearing capacity mechanism. For the case of the phreatic and soil surfaces being coincident, the bearing capacity was observed to be more than double that of the dry case. In submerged cases bearing capacity was less than values at the surface, as there was no air–fluid interface to enhance the development of negative pore pressure.