Simplified Settlement Model for a Shallow Foundation on Composite Ground With Rigid Piles-Article

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The technical note proposes a simplified method for estimating settlement of a shallow foundation supported by composite ground, where piles (or rigid inclusions) are not structurally connected to the foundation. The foundation’s total settlement is treated as the sum of pile settlement and movement of the foundation relative to the pile heads.<br /><br />The method selects a trial relative movement, S, and estimates how much sustained load the subgrade soil carries at that movement. The remaining load is assigned to the piles. Pile settlement is then calculated using an equivalent-pier analysis to the neutral plane, and total settlement is calculated as S plus pile settlement. The process can be iterated to meet settlement limits.<br /><br />The model simplifies complex soil–pile interaction: it treats piles as rigid, estimates soil stresses using a Boussinesq distribution, and decouples soil and pile settlement calculations. Numerical analyses suggest this stress distribution is reasonable and slightly conservative for preliminary design at modest pile replacement ratios. The method is most reliable when the neutral plane lies below the foundation’s significant stress-influence zone or within a stiff layer. Cushion stiffness and thickness must also be selected to control load transfer and avoid concentrated stresses.<br /><br />Two hypothetical examples show how the approach can be applied. In one, a 3 m square foundation’s settlement is reduced from 46 mm without piles to about 25 mm using five piles and a compressible cushion. A second example considers the same foundation over shallow bedrock and adjusts the pile and cushion dimensions to achieve a target settlement.<br /><br />The author recommends validating the simplified model through large-scale plate-load testing or another suitable method before final design.

Keywords

shallow foundation

settlement estimation

composite ground

rigid inclusions

pile settlement

neutral plane

Boussinesq distribution

compressible cushion

load transfer

plate-load testing