Buckling Strength of steel piles Influence of imperfections, soil support, and elasto-plastic soil behaviour

Abstract

Long slender displacement piles are widely used in soft clay deposits in Sweden, where the length-to-diameter ratio may exceed 100. Under these conditions, pile stability is strongly influenced by initial geometric imperfections, soil–structure interaction, and boundary conditions. Conventional analytical design approaches in Sween typically idealise the pile as a beam on a Winkler foundation, assume classical boundary conditions, adopt sinusoidal imperfection shapes, and represent the lateral subgrade stiffness as constant with depth. However, such simplifications may not adequately capture the behaviour of piles embedded in soft clay deposits with depth-dependent stiffness characteristics. This thesis investigates the geometrically non-linear behaviour of slender piles embedded in soft soil. The study focuses on evaluating the influence of initial geometric imperfections, imperfection mode shapes, soil stiffness distributions, and boundary conditions on the instability response of the pile–soil interaction system. Both constant and depth-dependent soil stiffness profiles are considered in the analyses. The influence of different boundary-condition configurations on the linear buck ling response is also examined. However, the geometrically non-linear analyses are limited to the pinned–pinned (P–P) configuration, since this boundary con dition consistently produces the lowest critical buckling resistance and therefore considered the most critical case for instability assessment. Theanalyses are performed numerically using the finite element software ABAQUS in combination with a Python-based automation script developed within the study. The pile is modelled using beam element, while the surrounding soil is represented using linear and bilinear Winkler-type springs distributed along the pile. Both lat eral and vertical soil springs are considered to represent the interaction between the pile and the surrounding soil, commonly referred to as soil–structure interac tion (SSI). This interaction governs how loads and deformations are transferred between the pile and the surrounding ground and has a major role in the stability behaviour of piles embedded in soft clay. Since there are no experimental data existing for direct comparison, the numerical models were first verified against analytical solutions for cases both with and without constant lateral and vertical soil support. Initial geometric imperfections were introduced using eigenmode shapes obtained from the linear buckling analyses. These imperfections were subsequently in corporated into the geometrically non-linear analyses in order to evaluate their influence on the instability response and ultimate load-carrying capacity of the pile–soil system. The elasto-plastic post-buckling behaviour was captured using the Static RIKS method, enabling investigation of the effect of imperfections and soil–structure interaction on the ultimate load-carrying behaviour of the analysed pile configurations. The results provide insight into the sensitivity of pile stability to imperfection shape and amplitude, soil stiffness distribution, and boundary conditions. The study highlights limitations associated with simplified design assumptions and contributes to a more realistic assessment of buckling resistance for slender piles embedded in soft soil