Passive Earth Pressure in Unsaturated Narrow Backfills: Impact of Soil Arching and Interlayer Shear Effects

Traditional earth pressure theories assumed that soil was either fully saturated or completely dry, neglecting the effects of wall displacement and unsaturated soil properties on passive earth pressure. This assumption led to limitations in practical engineering applications. To address this issue, this study examined unsaturated narrow backfill, in which the soil behind the retaining wall was modeled as an arc-shaped sliding mass. Interlayer shear stress was incorporated, and a passive earth pressure formulation for unsaturated narrow backfill was developed using the thin-layer element approach. The proposed method’s validity was verified through comparisons with existing theoretical and experimental results. A parametric study revealed that passive earth pressure is significantly affected by key backfill properties. Specifically, an increase in the effective internal friction angle and surcharge pressure resulted in average increases of 191.36% and 15.68%, respectively. The interface friction angle led to a 20.12% increase in the lower part of the backfill while slightly reducing pressure in the upper section. Matric suction also contributed to a 23.81% rise in passive earth pressure within a certain range. In contrast, effective cohesion reduced the pressure by 16.26%, and an increase in the backfill aspect ratio caused a notable 65.85% reduction. These results provide valuable insights for optimizing the design of retaining walls with unsaturated narrow backfill.