Experimental Study on the Horizontal Bearing Performance of Pile–Soil Composite Foundation Under Coupled Action of Active and Passive Loads

The pile–soil composite foundation system, highly acclaimed for its remarkable load-bearing capacity and limited deformation characteristics, has emerged as a fundamental element in geotechnical engineering practices. In the applications of adjacent slope engineering, such composite foundations are influenced by intricate loading scenarios. These scenarios involve both active vertical–horizontal combined load and passive soil-displacement forces generated due to the alteration of soil constraints. In this study, a self-designed movable retaining wall model box was employed. By applying different vertical and horizontal loads and controlling the rotation of the retaining wall around its base, a systematic investigation was conducted on the horizontal bearing mechanisms of single-pile and four-pile composite. The experimental data indicate that for every increment of 15 kPa in the vertical load, the horizontal bearing capacity experiences an average growth of approximately 18.9%, and the extreme value of the bending moment shows an average increase of 19.6. The analysis reveals coupled effects in internal force distribution and deformation patterns within load-bearing pile segments under concurrent active–passive loading conditions, while the embedded sections remain unaffected. Among four-pile composite foundations, the horizontal bearing mechanism of the front-row piles is consistent with that of a single-pile system. However, the maximum bending moments of the front-row and rear-row piles, compared to the single-pile system, have reached 0.68 times and 1.74 times, respectively. Notably, the bending moment of the front-row piles under the translational mode of the retaining wall is approximately 2.9 times that under the rotational mode, posing a potential risk of damage to the retaining structure, and necessary intervention is required. The results of this study provide a scientific basis for the force and deformation mechanism of piles at different positions in the composite foundation near foundation pit engineering, as well as their design for bending and shear resistance.