Axial Bearing Behavior of Distributed Postgrouted Drilled Shafts: Field In Situ and Laboratory Model Tests

This paper presents the results of in situ static load tests conducted on two superlong and large-diameter drilled shafts to investigate the influence of distributed post grouting on bearing behavior, side resistance, and base resistance. Model shaft tests were also carried out to deepen the understanding of grout diffusion in clay and sand. Furthermore, through a comparative analysis of bearing capacity of the distributed postgrouted shaft and the combined postgrouted shaft, a calculation method for the ultimate bearing capacity design value of distributed postgrouted shaft was proposed. The results indicate that the ultimate bearing capacity of the combined postgrouted drilled shaft and distributed postgrouted drilled shaft increases by 39.6% and 52.2%, respectively. Under ultimate load, both the side and the base resistances were significantly enhanced for both combined and distributed postgrouted drilled shafts. Additionally, the distributed postgrouted drilled shaft exhibited a slightly higher enhancement coefficient for ultimate side resistance compared with the combined postgrouted drilled shaft under the same conditions, with a noticeable depth effect observed in the enhancement of ultimate side resistance. During the excavation of model shafts, it was observed that grout diffusion primarily occurred in a splitting manner in clay and through compaction in sand. The grout injected by the distributed postgrouted drilled shaft along the shaft body was more evenly distributed, resulting in a more stable shaft–soil interaction system. Based on the comparative analysis, it is recommended to increase the calculated ultimate bearing capacity of combined postgrouted shafts by 10% as the design value for the ultimate bearing capacity of distributed postgrouted shafts. These research findings have significant implications for engineering practice and theoretical research on distributed postgrouted drilled shafts.