Structural capacity of slender piles in very soft clay

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Driven long, slender displacement piles are commonly employed in soft soil areas. The ultimate load-bearing structural capacity depends on the relative stiffness between the soil and the pile, and the dimensioning of their structural capacity remains under debate. In soft soils, especially close to the surface, the support stiffness is low, potentially leading to buckling of the pile. Current design methods are somewhat crude and poorly validated against field data. For example, in Sweden the Winkler model is used to estimate the ultimate failure load of a hinged pile, with constant subgrade reaction and without vertical spring component. A major concern is that this approach leads to overly conservative designs.

To overcome these limitations, in the present work we expand on the work by Heelis (2004) for soft soil conditions in Gothenburg, Sweden. Semi-analytical solutions to calculate the critical buckling where boundary conditions, linear increasing subgrade reaction and skin friction is considered. The sensitivity of different calculations cases and boundary conditions are studied in a Sensitivity Analysis and quantified using Design of Experiments. The results show that the Swedish design method is conservative. When second order effects are considered, where constant initial imperfection along the pile, limiting lateral resistance of soil and a corresponding steel pile is assumed the limiting property remains material yielding of the pile.

Key words: Piling, buckling, instability, soft soil, ultimate failure load, subgrade modulus

International Conference on Advances and Innovations in Soft Soil Engineering (ICAISSE2026)

Infrastructure Performance and Monitoring