Impact of Suction and Swell-Induced Softening on Pile Behavior in an Unsaturated Expansive Clay

This study examined the response of a single pile embedded in an unsaturated expansive clay. This was achieved by performing three-dimensional finite-element analyses, employing the Barcelona Basic Model. The study aimed to quantify the contribution of suction and swell-induced softening in dictating the mobilization of shaft resistance. Centrifuge testing was used as a benchmarking study to simulate free-swell and pile pullout response. The resulting coupled hydromechanical flow deformation analyses effectively captured swell evolution, changes in lateral swell pressure, and swell-induced softening, all of which significantly influenced load mobilization along the pile length. The results obtained identified zones of both positive and negative shaft resistance along the pile, previously overlooked by studies focusing solely on the development of positive resistance within the active zone. The development of tensile load in the pile was closely correlated with the reduction in suction and resulting swell upon wetting. During initial wetting, reduction of suction to the air occlusion value (where only 18% of total swell had been achieved) produced the maximum tensile load, after which reductions in load were observed. Furthermore, in comparison to rainfall infiltration, a rise in the water table was found to be a more critical hydraulic condition, generating maximum axial load and pile uplift. The study also highlighted conservatism in calculating pile capacity based on saturated soil mechanics, with the capacity found to be around five times lower than under unsaturated conditions. These findings enhance the understanding of pile foundation design in unsaturated expansive clays.