Pile foundations are widely used in high-rise buildings, transportation hubs, bridges, ports, and other major infrastructure projects. Their performance is directly related to structural safety and the overall economic efficiency of construction. Existing studies on the bearing capacity of post-grouting bored piles have primarily focused on uniform or single-layer geological conditions. However, many engineering projects involve complex strata composed of alternating soft and hard layers. Under such conditions, the reinforcement mechanism of post-grouting and the differences in unit side resistance between adjacent soil layers remain insufficiently understood and have not been systematically verified through field investigations.

The Shili Heyuan project in Yunnan Province was selected as the engineering case study. Nongrouted and post-grouting bored piles were designed and constructed in geological strata characterized by alternating soft and hard layers, and static load tests were performed on both pile types. Fiber Bragg grating sensing technology was employed to monitor strain distribution along the entire pile length. The load-settlement response, axial force distribution, and mobilization characteristics of pile-soil unit side resistance were systematically analyzed.

Post-grouting substantially improved the bearing performance of bored piles. The ultimate bearing capacity of post-grouting bored piles was substantially higher than that of nongrouted piles, whereas their settlement under the same load was considerably lower. Comparative analysis of representative nongrouted and post-grouting bored piles showed that nongrouted piles relied predominantly on side resistance during progressive loading. In contrast, the load-bearing mechanism of post-grouting piles gradually shifted from side-resistancedominated behavior to the combined mobilization of side and end resistance. Under ultimate loading, side resistance contributed 89% and 77% of the total bearing capacity of nongrouted and post-grouting bored piles, respectively, whereas end resistance contributed 11% and 23%, respectively. At lower load levels (≤2 800 kN), the unit side resistance of nongrouted piles was higher than that of post-grouting piles in shallow soft soil layers but lower in deeper, harder layers. At higher load levels (≥4 000 kN), however, the unit side resistance of nongrouted piles was substantially lower than that of post-grouting piles across all soil layers. The measured ultimate bearing capacities of nongrouted and post-grouting bored piles were 26% and 86% higher, respectively, than the corresponding values estimated using empirical methods.

Unit side resistance exhibits distinct load-transfer characteristics along the pile shaft, and the mobilization of resistance in upper soil layers directly influences the bearing contribution of underlying layers. The increase in unit side resistance is more pronounced when a pile transitions from a soft layer to a hard layer than when it transitions from a hard layer to a soft layer. This effect is also more pronounced in post-grouting bored piles than in nongrouted piles. At soft-hard interfaces, the unit side resistance of nongrouted piles is constrained by the mud skin effect, which restricts the improvement of frictional resistance at the pile-soil interface. Post-grouting effectively improves the interface condition, promotes more coordinated load transfer between soft and hard layers, and thereby substantially enhances the overall bearing capacity of the pile. In complex strata comprising alternating soft and hard layers, the measured compressive bearing capacity of post-grouting bored piles substantially exceeds the empirical estimation limit. Compared with nongrouted piles, post-grouting piles exhibit a greater overall increase in unit side resistance, with particularly notable improvement observed in deep coarse sand layers.