## 5 Conclusions  
In this study, a gradient pile-concrete slab composite foundation was employed for the initial time in the expressway culvert-subgrade transition section located in the soft-soil region of China. This treatment aimed to address the issue of significant differential settlement resulting from the abrupt stiffness transition at the junction of the transition section. Additionally, a field excitation test was conducted using SBZ30 variable-frequency torque vibrators to establish the vertical distribution patterns of dynamic responses under various vehicle axle loads and speeds. Long-term roadbed settlement was also monitored. The primary findings are summarized as follows:  
1. The gradient pile-concrete slab composite foundation has demonstrated its ability to effectively decrease the amplitude of dynamic response parameters and roadbed settlement. It offers valuable guidance and serves as a reference for settlement control and foundation treatment in similar expressway-transition-section projects;
2. Dynamic stress exhibits exponential attenuation characteristics, with the pavement and base layers dissipating a significant amount of dynamic load energy. When the burial depth is shallow, an increase in the vehicle speed and axle load results in higher dynamic stress amplitudes. However, when the burial depth is deeper (beneath the road base), higher speeds lead to an overall increase in the dynamic stress, albeit with some fluctuations, while an increase in the axle load generally leads to a reduction in the dynamic stress;
3. Acceleration amplitudes tend to increase with higher axle loads and speed. As the vehicle speed increases, acceleration decay accelerates, resulting in a shallower depth of influence. Conversely, as the load increases, acceleration decay slows down, leading to a greater depth of influence;
4. Dynamic displacement amplitude decreases with depth, with slower decay at greater depths. Surface-layer displacement increases with the velocity and load, but deep-layer displacement is largely unaffected by velocity changes. The load has a more significant impact on the displacement amplitude, particularly in deeper subgrade layers, compared to velocity;
5. The cumulative deformation of the roadbed and the number of excitations exhibit characteristics consistent with exponential functions, ultimately stabilizing within 4.5 mm. The cumulative deformation process can be divided into three stages: rapid accumulation (0-4.7 \(\times\) 10\({}^{5}\) vibration cycles), slow increase (4.7 \(\times\) 10\({}^{5}\)-1.2 \(\times\) 10\({}^{6}\) vibration cycles), and settlement stabilization (1.2 \(\times\) 10\({}^{6}\)-1.6 \(\times\) 10\({}^{6}\) vibration cycles). In addition, numerous additional factors influence the dynamic response and settlement values of roadbeds. In future research, it becomes imperative to conduct field tests encompassing a broader spectrum of load configurations and rainfall conditions. Furthermore, the integration of numerical modeling can be employed to comprehensively investigate the overall performance of the roadbed. **Author Contributions:** Conceptualization, Z.L. and L.X. ; methodology, Z.L. ; software, Y.C. ; validation, Z.L., Y.C., and Y.L. ; formal analysis, N.S. ; investigation, Z.Y. ; resources, L.X. ; data curation, Y.C. ; writing--original draft preparation, Y.C. ; writing--review and editing, Z.L. ; visualization, K.D. ; supervision, L.X. ; project administration, Y.C. ; funding acquisition, L.X. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the National Natural Science Foundation of China (grant nos. 42172322 and U2268213), the National Key Research and Development Program of China (grant no. SQ2022 YFB2300030-06), the Transportation Construction Scientific Research Plan Project of Department of Transportation, Zhejiang Province, China (grant no. 2019001), and the Postgraduate Scientific Research Innovation Project of Hunan Province (grant no. QL20230104).