<p>The stability of foundation piles in the secondary terrace of the Wei River is of great significance for engineering construction in northwestern China. To investigate the bearing behavior of piles and the shear characteristics of the pile–soil interface, three cast-in-place piles were constructed at a representative site, with reinforcement stress meters installed in two of them. Static vertical compression tests were performed using a sparse-to-dense loading strategy, and comparative interface shear tests were carried out under controlled laboratory conditions. The results show that the pile side friction resistance accounts for the majority of the bearing capacity, while the contribution of end resistance is limited. The shear stress–displacement curves of the pile–soil interface can be classified into three types—strain-hardening, strain-softening, and ideal elastic–plastic—depending on soil properties and stress levels. These findings provide a clear classification of the support–deformation relationship and reveal the mechanisms governing load transfer in the secondary terrace deposits. The outcomes offer practical guidance for the design and optimization of pile foundations in alluvial terrace areas.</p>