On quantifying the installation effects of displacement piles in natural clays

Doktorsavhandling, 2025

Urbanisation calls for a more efficient use of space. Historically, cities are often located close to water where deep deposits of soft natural clay are ubiquitous. This combination of factors results in construction of increasingly large buildings on less competent ground, which requires deep foundations. Displacement piles offer one of the most cost-effective deep foundation solutions. The installation of those piles, however, leads to mass-displacements and changes in state in the clay that surrounds the newly built and existing structures in the geotechnical system. Advanced numerical analysis is more routinely used by practising geotechnical engineers to quantify the soil-structure interactions in urban areas. Including the pile installation phase in such system-level analyses, however, is complex and thus often neglected. In practice, designers rely on empirical relations to consider the influence of the installation on the pile response, and the impact of mass-displacement is investigated as a separate design case in which the change of state is not
considered.

This thesis explores the possibility of directly including pile installation in the numerical analysis of the urban geotechnical system. The focus was on modelling the changes of the soil in space and time during (parts of) the pile cycle using numerical techniques. The research results show that the pile installation phase can be successfully modelled at each stage of the pile cycle, using realistic kinematics by using a numerical framework for large deformations and a constitutive model that captures the complex deformation characteristics of soft natural clays. Furthermore, the need for
complexity in the numerical analysis, in terms of modelling the kinematics of pile installation and the constitutive model, was investigated. Geotechnical engineers should avoid using advanced constitutive models while neglecting the influence of pile installation. The simplified models
perform well in comparison to the advanced models for predicting the short-term response at distances larger than five pile radii. The results reiterate that short-term mass-displacements occur in undrained conditions and that the length and cross-sectional area, in combination with the relation between the pile length and distance from the pile, govern the distribution of soil displacements. Additionally, stiff boundaries, existing foundations, as well as horizontal and vertical stiffness gradients influenced the distribution of the displaced soil. In conclusion, the work conducted in this thesis provides a basis for further numerical studies on the influence of pile installation on the urban geotechnical systems.