Theoretical and Experimental Study on Elastic–Plastic–Sliding Load Transfer Model for Pile Tip and Pile Side Postgrouting Piles

An elastic–plastic–sliding trilinear load transfer model is established for grouted piles. This model summarizes the resistance–displacement relationships at both the side and tip of the pile, describing the transition from elastic to plastic to sliding behavior, to effectively capture the hardening, softening, and elastic-perfectly plastic characteristics of shear stress that are beyond the capabilities of current models. Postgrouting improvement in the shear performance at the pile–soil interface is represented by an increase in the stiffness of the elastic segment and the ultimate resistance value. A model test is conducted on pile tip and pile side grouted piles, whose resistance–displacement relationships are fitted, and the load–settlement curves are predicted using the proposed model. The results indicate that pile–soil grouting at both the tip and side demonstrates superior effects in improving the bearing characteristics compared to a single grouting method under the same grouting volume. The prediction error for the ultimate bearing capacity of the model piles ranges from −7.7% to 3.2%. Furthermore, the applicability of the elastic–plastic–slip model has been demonstrated by two case studies. Compared to the hyperbolic and Boxlucas1 models, the proposed model provides more accurate predictions of the stiffness in the initial elastic segment of the pile top load–displacement curves, which is crucial for the settlement calculation of pile foundations in engineering projects. Additionally, the prediction error of this model for ultimate bearing capacity is smaller compared to the two nonlinear models.