Length Calculation of the Negative Skin Friction Zone of Rigid Pile in Composite Foundation Based on Double Equal Settlement Surface Model

Installing a cushion layer atop rigid pile composite foundations leads to the formation of a negative skin friction (NSF) zone along the upper portions of the piles, thereby reducing the bearing capacity and posing potential risks to structural safety. Based on the double-equal-settlement model of composite foundations, a segmented analytical model for pile-soil load transfer was developed, considering the interaction among the cushion layer, piles, and surrounding soil. A theoretical method for calculating the critical length of the NSF zone was derived by considering the deformation compatibility among the cushion layer, piles, and soil. To validate this method, comparative analyses were conducted using existing field test data, numerical simulation results, and the outcomes of the proposed analytical model. Additionally, a parametric study was carried out based on a specific engineering case to investigate how the thickness of cushion layer, pile diameter, pile-soil compression modulus, and pile spacing affect the depth of the NSF zone. The results indicate that thickness of cushion layer and pile-soil compression modulus have a significant impact on the NSF zone depth, followed by pile diameter and pile spacing. These findings provide a theoretical foundation for the design and application of rigid pile composite foundations.