Bearing Behavior of Postgrouting Pile Foundation in Collapsible Loess: An Experimental Study

To investigate the reinforcement effect and corresponding mechanism of the postgrouting technique on the pile-bearing behavior in collapsible loess, a series of model pile static loading tests and infiltration tests were performed using a newly designed transparent half-pile testing tank. In the slurry injection stage, the observable slurry diffusion behaviors indicated that the increasingly dense loess structures arose from slurry compression, and loess collapsible deformation greatly increased the pressure necessary to form permeation or splitting grouting. Consequently, under compaction grouting, the diameter of the enlarged pile base shaped in a semiellipsoidal form gradually increased without any noticeable upward slurry return to be observed. In the static loading stage, the postgrouting technique effectively enhanced the ultimate pile-bearing capacity by creating an enlarged pile base and reinforcing the loess beneath it. In the infiltration stage, the reinforced pile base resistance counterbalanced the negative pile shaft friction generated in the already saturated zone, thereby postponing the appearance of pile quick settlement due to the shear failure of the loess beneath the pile base. Considering the aforementioned influencing factors, a hyperbolic model was proposed to describe the mobilization of enlarged pile base resistance with pile base settlement extending unsaturated soil mechanics. The feasibility of the proposed model was validated through the experimental results and a case study collected from published literature. Additionally, this model was employed to backcalculate the grouting pressure required to form an enlarged pile base, which enabled the pile settlement to remain within the allowable range even under extreme rainfall conditions. The disciplines revealed that the model proposed could well facilitate practicing engineering in the proper design of postgrouting piles in collapsible loess areas.