Samenvatting

This thesis investigates the development of internal shear stresses in stiffened-core deep composite pile foundations, with particular focus on the FUNDEX Combi Pile system. These piles are widely used in urban environments due to their vibration-free installation and ability to perform under challenging geotechnical conditions. Despite their increasing application, there is currently no generally accepted design method to assess internal shear stresses at the interface between the precast concrete core and the cast in-situ concrete shaft, which governs the structural behaviour of the system.

The study aims to improve the understanding of shear-transfer mechanisms at this concrete-to-concrete interface and to evaluate modelling approaches capable of describing shear stress development along the pile length. The research focuses on Mode 1 loading conditions, in which the external load is applied exclusively to the precast core, leading to the mobilisation of shear stresses at the internal interface.

A theoretical framework is established based on existing literature on composite pile systems and concrete interface behaviour. Shear transfer is governed by adhesion, mechanical interlock, and friction, with adhesion identified as the dominant mechanism for the smooth interfaces typical of Combi Piles. The interface response is nonlinear and depends on the level of slip, which forms the basis for the modelling approach adopted in this study.

To analyse the behaviour of the system, three modelling approaches are developed: a one-dimensional nonlinear numerical model based on load-transfer theory, a two-dimensional linear finite element model, and a simplified Strut-and-Tie Model for the pile head region. These approaches are evaluated through a variant study using a Multi-Criteria Analysis, considering their accuracy, applicability, and suitability for engineering practice.

The results show that shear stresses are highest near the pile head, where a discontinuity region forms due to the localised introduction of load into the core. The numerical and finite element models capture peak shear stress distributions along the interface, while the Strut-and-Tie Model provides an average representation of stresses within this critical region. The analysis indicates that shear stress levels may exceed values prescribed by design codes, highlighting the importance of properly accounting for interface behaviour in design.

It is concluded that the concrete-to-concrete interface is a governing component in the structural performance of composite piles. The modelling approaches presented in this thesis provide insight into shear stress development and can support engineering judgement, although they remain simplified representations and are not intended to replace detailed design calculations. Recommendations are provided for improving construction practices, experimental validation, and future modelling strategies.