## 1 Introduction  
Super-long and large-diameter bored piles are frequently utilized in large-scale bridges due to their exceptional single-pile capacity. Nonetheless, conventional bored piles frequently confront challenges, including the formation of mud cakes along their perimeters, the accumulation of drilling residues at their termini, and the disruption of the underlying bearing stratum, all of which negatively influence their load-bearing capabilities. The quality of pile formation is crucial in determining the bearing capacity of bored piles, leading to a relatively variable vertical bearing capacity. A substantial body of research has consistently shown that the application of post-grouting methodologies to pile foundations can proficiently mitigate these aforementioned issues [1, 2, 3, 4, 5]. Specifically, pile end post-grouting creates an enlarged pile head at the pile bottom, which pre-compresses and reinforces the surrounding soil, significantly enhancing the single-pile bearing performance. Conversely, pile side post-grouting strengthens the soil adjacent to the pile through upward and downward seepage of the pressurized grout. The formation of a grout-soil blend at both the pile's terminal and lateral regions effectively removes the residual slag at the base and the mud cake adhering to the pile sides. The resultant benefits of employing post-grouting techniques notably enhance the load-transfer efficiency of the pile foundation, leading to a considerable augmentation in its bearing capacity while concurrently mitigating settlement. Consequently, post-grouting techniques have found widespread application in high-rise buildings, bridges, harbors, railways, and water conservancy projects [6; 7; 8; 9; 10; 11]. To investigate the bearing characteristics of post-grouting bored piles, numerous scholars have conducted extensive research. [PERSON] et al. [10] investigated the effects of post-grouting at the pile bases by performing static load tests on six drilled piles, and they combined finite element analysis software with an exploration of how the cement grout's diffusion radius at the pile tip, along with its penetration depth, influences the pile's load-bearing capacity. [PERSON] et al. [12] detailed the construction process and results of field static load tests conducted on an innovative jet-grouted precast pile in cohesionless soil conditions, and their findings revealed that, when utilizing this grouting technique, the bearing capacity of these pile foundations surpassed that of similar-sized bored piles by a factor of over 2.5 times. However, there is a paucity of reports on the application of combined tip-and-side post-grouting in railway bridge projects. Traditional static load testing typically encompasses the accumulation load method and anchor pile method. However, in railroad bridge projects, site constraints often render both methods impractical. Furthermore, large-diameter post-grouted bored piles exhibit substantial bearing capacities, rendering traditional static load testing inadequate in providing sufficient upper loads to fulfill testing requirements. Consequently, numerous scholars have adopted the self-balancing method for static load testing in pile foundation projects with significant loading demands. [PERSON] and [PERSON] [13] observed superior outcomes from a self-balancing static load test conducted on two large-diameter piles that underwent post-grouting at their terminals for the Binh Le Bridge in Vietnam. Similarly, [PERSON] et al. [14] conducted an on-site self-balancing static load test on large-diameter post-grouted piles at the Guanyin Temple Yangtze River Bridge, showing that the post-grouting technique enhances the ultimate bearing capacity and reduces pile top settlement. In summary, the self-balancing static load test is a proven method for assessing the bearing capacity of large-diameter post-grouted piles with substantial loading requirements in field conditions.