Calculation of bearing capacity for combined post-grouting bored piles based on settlement control

It is of great practical significance to determine the bearing capacity of post-grouting piles on the basis of the principle of settlement control. Based on the field static load tests on six large-diameter cast-in-situ bored piles of Shishou Yangtze River Highway Bridge project, the influences of combined post-grouting on the bearing and deformation behaviors of cast-in-situ bored piles in deep fine sand layer are studied by comparing the field test results before and after combined post-grouting. The range of improvement coefficient for the tip resistance and shaft resistance under different pile head settlements is obtained through statistical analysis on the basis of the field test results, and a design method for the bearing capacity of combined post-grouting pile based on settlement control criterion is presented. Finally, a case history is cited to demonstrate the validity of the design method. The results show that the bearing capacity of cast-in-situ bored pile in deep fine sand layer is significantly improved under the combined post-grouting, and the improved range of bearing capacity increases with the increasing settlement of the pile head. Moreover, the ultimate bearing capacity of combined post-grouting pile is increased by at least 66% under the ultimate loading, and the settlement of pile head can be controlled effectively. Meanwhile, the bearing performances of the pile tip and mechanical characteristics of the pile side can be improved effectively, the tip resistance and side resistance are significantly improved by the combined post-grouting, and the load transfer characteristics of the pile have a significant impact. Additionally, the proposed method can well give the range of load-settlement relationship of combined post-grouting pile. It is suggested that the calculated lower bound should be used conservatively in engineering design.