The seepage study of transient caisson pull-out in sand

Abstract

Under transient pull-out, caisson foundations will exhibit resistance from considerable suction, the magnitude of which is identified in the literature to be mainly the result of a complex interaction between the caisson dimensions, loading rate, ambient pressure, soil type, and load history. This paper investigates the seepage conditions developed around suction caissons in cohesionless soils under transient pull-out. Validations with benchmark tests are performed, using an advanced numerical simulation in Abaqus with coupled pore-fluid-diffusion/stress analysis, and employing the SANISAND constitutive model. The pore fluid response is evaluated by parametric studies on soil permeability, pull-out rate, and foundation aspect ratio, based on which a simplified expression is proposed for estimating the peak suction, with modification factors given for varying normalized pull-out rates and aspect ratios. It was identified that exponentially larger suctions can be anticipated when adopting faster pull-out rates, lower permeabilities, and higher aspect ratios. Qualitative assessment of the seepage regime indicates that higher suction developments are noticeably linked to the shortened drainage paths that pass around the skirt tip, as the pressure loss contours align closer together around the tip of skirts. Further, the distribution of mobilised friction angles inside and around the bucket is demonstrated as contributions of the outer wall friction and a bulb representing a zone of mobilised friction formed beneath the caisson during pull-out.