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 at-surface, as there was no air-fluid interface to enhance the development of negative pore pressure.海滩脚印下方出现的干边，是剪胀（dilation）的可视化表现。剪胀会使砂土在受剪过程中试图增大体积，进而产生负孔隙水压力，并提升地基承载力。传统岩土理论无法解释这一强度提升现象，反而认为当潜水面（phreatic surface）位于砂土表面时，地基承载力约为干态工况的一半。为探究这一表观矛盾，研究人员针对透明土（transparent soil）开展了10组荷载控制式地基承载力试验，试验的潜水面埋深范围覆盖基础（footing）下方100mm至基础上方30mm。当潜水面埋深大于基础宽度时，剪切破坏发生于干燥土体内部；在潜水面埋深较浅的工况下，地基承载力随潜水面趋近地表而升高。研究观测到，进气（air entry）现象沿饱和层顶面起始，并延伸至地基承载力机制的作用深度。当潜水面与土体表面相重合时，实测地基承载力超过干态工况的两倍；在完全浸没的工况下，地基承载力低于地表处的工况，原因是此时不存在气-液界面以促进负孔隙水压力的发育。