<p>The post-grouting enhancement coefficients prescribed in current Chinese specifications may underestimate the bearing capacity of post-grouting bored piles. The aim of this study is to investigate the enhancement coefficients for typical soil layers in the lower flood plain of the Yellow River, China. The Osterberg-Cell method was used to conduct field load tests on combined side-and-tip grouted bored piles. The bearing characteristics of composite grouted bored piles in this region were analyzed, and the enhancement coefficients for the side resistance and tip resistance in the typical soil layers of this region were adjusted. The results indicate a significant increase in the ultimate bearing capacity of the combined grouted piles, with a maximum improvement of 78.35%. Additionally, the ultimate settlement at the pile top decreased by up to 23.57%. The enhancement effect on the side resistance and tip resistance was more pronounced in the coarse-grained soil layers, such as the silty sand and medium sand. After grouting, the proportion of the tip resistance to the total bearing capacity increased, whereas the proportion of the side resistance decreased. Statistical analysis of the field tests reveals an increase in the enhancement coefficients for the lower flood plain of the Yellow River, providing valuable insights for the design and construction of combined side-and-tip grouted bored piles in this region.</p>