## 6 Conclusions  
This paper presented a comparative analysis of the bearing performance of two large-diameter bored piles in the Huaiyang Left-Line Bridge Project of the Ljanyungang-Zhenjiang Railway, focusing on the results obtained from static load tests conducted using the self-balancing method. The analysis compared the pile foundation's performance before and after the application of combined grouting. It also examined the changes in the pile load transfer mechanism before and after grouting. Additionally, the paper integrated numerical simulation results to investigate the bearing characteristics of piles subjected to combined post-grouting and compared these findings with those from field tests. The key conclusions drawn are as follows:  
1. The application of the combined post-grouting technique significantly increased the ultimate bearing capacity of the two test piles of the railway bridge project by 32.99% and 38.42%, respectively. Moreover, the pile top displacement is significantly decreased under the same applied load. It is worth noting that the load-displacement curve at the pile top exhibited a gradual and smoothly varying trend, rather than a sharp decline, following the combined grouting treatment. This observation underscores the substantial impact of the post-grouting intervention. 2. The injection of grout into the pile sides during the combined grouting process significantly enhances the mechanical characteristics of the pile-soil interface, leading to an increase in pile lateral resistance along the entire length of the pile. The grout injected at the pile end creates a pre-compression effect on the soil within a specific proximity of the pile tip. Consequently, the most pronounced increase in pile side resistance is observed within approximately 1 m from the pile end, with an enhancement ranging from 78.72% to 80.21%. 3. Under identical pile tip displacements, test piles subjected to combined post-grouting demonstrate an augmented pile tip resistance. The pile tip resistance responds more promptly to increasing pile tip displacements after grouting. Furthermore, the application of combined tip-and-side post-grouting alters the load transfer mechanism of the pile. Specifically, there is a substantial increase in both pile lateral resistance and pile tip resistance in the vicinity of the pile end. Consequently, the proportion of pile tip resistance to the ultimate bearing capacity of the pile foundation increases, from a range of 12.49% to 12.99% before grouting to a range of 18.14% to 18.65%  
Figure 8: Self-balance test of numerical simulation load–displacement curve. after grouting. This shift signifies a relocation of the center of gravity of the pile foundation's bearing capacity towards the pile end. 4. The numerical simulation results indicate that the implementation of combined post-grouting enhances the mechanical properties of the pile-soil interface through the introduction of grout into the pile sides, thereby increasing the pile side resistance. Moreover, the consolidation of grout injected into the pile end significantly improves the pile tip resistance performance. It is worth highlighting that the development of pile lateral resistance and pile tip resistance occurs asynchronously; the pile lateral resistance reaches its ultimate capacity before the pile tip resistance is fully mobilized. This observation aligns with the findings obtained from the field testing. Methodology, R.Z. ; Software, Z.G. ; Resources, W.G. and Z.W. ; Writing--original draft, R.Z., Z.G. and Z.W. ; Writing--review & editing, R.Z., Z.G., W.G. and Z.W. ; Project administration, Z.W. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China (52008100, 52178317), Natural Science Foundation of Jiangsu Province (BK20200400), China Postdoctoral Science Foundation (2022M723534), and the Natural Science Foundation of Jiangsu Higher Education Institutions of China (23 KJA560005). And The APC was funded by [PERSON].