The unloading pile-sheet retaining wall is a new type of support/retaining for embankment slopes and has demonstrated excellent performance in engineering applications. However, its mechanical behavior and operational mechanisms are still not fully understood. This study conducted model tests on unloading piles and cantilever piles. Backfilling was conducted in four stages: 30 cm for the first three layers and 10 cm for the final one. The tests focused on the evolution of earth pressure, internal forces, and deformation in both pile types. The results show that: 1) Upon completion of backfilling, the horizontal displacement at the top of the cantilever pile reaches 81.76 mm, which is 5.45 times that of the unloading pile (14.99 mm). The maximum earth pressure on the unloading pile is 10.08 kPa, accounting for 66.40% of the 15.18 kPa recorded on the cantilever pile. The unloading effect alters the distribution pattern and magnitude of earth pressure. 2) The bending moment distributions differ significantly. The cantilever pile exhibits a “fish-belly” pattern with a maximum moment of 115.8 N· m. In contrast, the unloading pile shows an “S-shaped” profile, featuring a pronounced point of contraflexure at the unloading platform and a maximum negative moment of −60.99 N· m. 3) Incorporating an unloading platform effectively reduces earth pressure and enhances the anti-overturning moment. These effects jointly improve sliding resistance and overall structural stability. These findings offer theoretical insights and technical guidance for the practical implementation of unloading pile-sheet retaining walls.

The application of a significant additional load induces soil deformation around the pile, generating a downward drag force, commonly referred to as negative skin friction. This phenomenon significantly reduces the pile's ultimate axial load capacity. Therefore, precise estimation of negative skin friction is crucial for pile design. To accurately calculate the negative skin friction acting on the pile, it is essential to determine the stress states at the pile-soil interface under varying soil deformations. However, many existing methodologies solely consider peak or residual stresses on the shear plane, neglecting the process of stress changes in soil deformation. This approach often results in an overestimation of negative skin friction. In this investigation, we propose a novel method for calculating negative skin friction that comprehensively accounts for the whole process of stress state alterations occurring during soil deformation (pre-failure zone and peak stress, post-failure zone and residual stress state) and describes the relationship between soil deformation and stress using a hyperbolic mechanical model. On this basis, soil deformation behavior is classified into three distinct forms. The spatial distribution characteristics for negative skin friction were then explored individually for each form. Additionally, the influence of different soil parameters on the spatial distribution of negative skin friction was also investigated. Finally, the accuracy and applicability of the new negative skin friction calculation method is validated through comparison with field measurement data. It can be used as a reference for practical engineering.

The counterpressure pile sheet retaining walls are a novel type of slope support structure. Although engineering practice has demonstrated their excellent performance, systematic studies on their deformation characteristics and mechanical behavior remain limited. Through model tests comparing counterpressure piles and cantilever piles, this study investigates the evolution of pile-top displacement, pile deformation, internal force distribution, and earth pressure during the sandy backfill process. Key findings are as follows: (1) The top displacement of the cantilever pile was 81.76 mm, which is 6.69 times that of the counterpressure pile (12.22 mm), resulting in cracking in the soil mass 51 cm horizontally away from the pile top. (2) The counterpressure pile exhibits a typical S-shaped distribution of bending moment, with a distinct reverse bending phenomenon and a reduction in the peak moment. (3) The excessive deformation of cantilever piles leads to an increase in the gravitational component of soil weight perpendicular to the pile shaft direction. The soil pressure on the counter-pressure pile is about 15% lower than that on the cantilever pile, reflecting its good stress redistribution ability. (4) Three primary working mechanisms are identified: provision of anti-overturning moment through soil reaction on the counterpressure plate, enhancement of horizontal resistance via friction between the plate and soil, and increase of passive earth pressure in front of the pile. The counterpressure platform significantly enhances overturning resistance, mitigates slip, and improves long-term structural stability. These findings offer experimental validation and a theoretical basis for optimizing counterpressure pile sheet retaining wall designs.