<p>The performance of bridge foundations remains a critical challenge in geotechnical engineering, especially in riverine environments prone to seismic actions and scour. This study evaluated the vertical serviceability of composite caisson-pile foundations (CCPFs), a hybrid system combining the end-bearing capacity of caissons with the shaft resistance and redundancy of piles. Using three-dimensional finite element simulations, the behavior of CCPFs was systematically compared with conventional caisson-only and pile-only foundations. A parametric framework varying caisson embedment ratios and pile spacing was employed to quantify settlement control, stiffness contributions, and load-sharing mechanisms. To support design application, novel performance indices were introduced: the Vertical Synergy Ratio, Settlement Improvement Ratio, Critical Depth Index, and Economy Index. The analysis results demonstrated that CCPFs reduce vertical settlements and enhance load transfer efficiency compared to traditional systems. These findings advance understanding of composite foundation behavior and provide design-oriented metrics for resilient, sustainable bridge foundation solutions in complex geotechnical settings.</p>