Load Settlement Response of Composite Foundation of Variable Pile Diameters

On condition that the upper load varies greatly from area to area, the stiffness of a composite foundation usually needs to be adjusted and optimized by changing the length or diameter of a proportion of piles to control the differential settlement within the allowable range. However, compared with the short and long pile composite foundations that have been comprehensively studied with the normalized method for settlement calculation, few research attentions have been concentrated on the load displacement response of composite foundation of variable pile diameters (CFVPD). In this study, a large-scale model test was conducted to study the load transfer mechanisms of CFVPD under static loading. The results indicated that increasing the pile diameter of large diameter piles effectively reduced the settlement of CFVPD. In addition, the large diameter pile exhibited greater pile head load and pile base resistance than the small diameter pile. During the loading process, three phases of pile-soil interaction relationship were observed for both large and small diameter piles and the neutral plane gradually shifted downward. To develop the analytical model, the CFVPD was discretized into a series of analytical elements representing individual piles, with each characterized by distinct influence areas, by establishing a zero shear stress interface to represent the location of negligible shear stress induced by each pile on the surrounding soil. Then, the stress state and deformation of soil between piles could be described by a series of differential equations and solved according to the stress equilibrium condition and deformation compatibility condition using a three-phase hardening model to consider the pile-soil interaction. Meanwhile, the compression deformation of underlying soil stratum could be conventionally calculated using a stress diffusion method. The relatively good agreement between predictions and experimental data indicated that the proposed method could be a valuable and convenient tool in the predictions of the settlement of CFVPD.