Influence of transient flow during infiltration and isotropic/anisotropic matric suction on the passive/active lateral earth pressures of partially saturated soils

In this study, the influence of transient flow on the active and passive lateral earth pressures of variably saturated backfills is thoroughly evaluated employing the lower bound theorems of the finite element limit analysis with second-order cone programming. In order to account for the tempo-spatial variations of suction stress within the soil stratum during infiltration, a closed-form solution derived for one-dimensional transient flow through variably saturated porous media is adopted. The unsaturated ground condition is simulated by incorporating the well-established unified effective stress approach into the soil yield function. The soil medium is considered to be both isotropic and anisotropic, due primarily to the various distributions of suction stress along different directions within the partially saturated porous medium. In order to model the soil suction stress anisotropy, an iterative procedure is employed by differentiating between the mobilized suction stress within the horizontal and vertical planes. In general, the influence of the transient flow condition is magnified for cohesive soils compared with granular backfills. In either case, such an effect is more noticeable for the active lateral earth pressure as compared to the passive counterpart. In the anisotropic condition, while the suction stress anisotropy within the soil medium has fairly negligible influence on the contribution of transient flow to the lateral earth pressure coefficient in the active state, the influence of elapsed infiltration time on the passive earth pressure coefficient turns out to be more at play at lower anisotropy ratios. • Soil-environment-structure interaction is examined with emphasis on lateral earth pressures. • FELA analyses are performed incorporating transient flow of the unsaturated backfill. • The influence of anisotropic matric suction is accounted for based on grain-scale considerations. • The results highlight the important influence of transient-flow and suction-stress anisotropy. • The effect of elapsed infiltration time is magnified in the presence of higher suction anisotropy.