Hydromechanical Hysteresis Effect-Induced Bearing Capacity Variations for Unsaturated Sand

Hydraulic hysteresis and mechanical behavior [i.e., void ratio (e) dependency] are fundamental in unsaturated soil behaviors. Applications of the upper bound theorem to account for these aspects in unsaturated soil mechanics have never been reported in geotechnical problems, such as bearing capacity, to the best of the author’s knowledge. Therefore, this paper presented modifications to and the development of a computational limit analysis method [discontinuity layout optimization (DLO)], which considered the influence of hydraulic hysteresis, mechanical behavior, and changes in soil fabric on strength. The modified approach unsaturated DLO (UNSAT-DLO) was utilized to carry out a parametric study for a bearing capacity problem, where the numerical results were compared with 1g physical modeling tests that were conducted for a surface strip footing that was placed on sandy soil. The comparison demonstrated that the UNSAT-DLO approach could capture the nonlinearity of the experimental results within a small range of suction (s) profiles. In addition, the bearing capacity scatter-induced hydraulic hysteresis, mechanical behavior, and soil fabric changes, even at approximately the same applied s, were reasonably well defined by the UNSAT-DLO approach, because the parameters were carefully assigned. In addition, differences in the failure mechanisms due to the hysteresis were investigated to provide insights into distinct changes in the failure modes due to the hydraulic hysteresis and mechanical behaviors.

The conventional design of shallow foundations was based on dry or saturated states, according to the saturation conditions, which led to a conservative design. Due to the drying and wetting cycles, soils are frequently found in unsaturated conditions. The additional strength that was imparted to soils, which was attributed to the effect of partial saturation, led to more stable geotechnical structures. The literature confirmed that suction (s) had a significant effect, and a ≤10-fold increase in bearing capacity was obtained when unsaturated cases were compared with the fully saturated conditions. Further evidence of this was the stability of vertical cuts (trenches), embankments, retaining walls, and railways. With a better understanding of the mechanics of unsaturated soils, the design of the geotechnical structures could impart additional strength due to partial saturation. The utilized (UNSAT-DLO ) approach, which accounted for the effect of s and saturation, exhibited good agreement with the experimental data. In addition, it considered hydraulic and mechanical hysteresis. Therefore, the approach was a robust tool that could help with safe structures and more economical designs.