Experimental and Numerical Analyses of Piled Raft in Sand

Abstract

The early 20th century marked a period of significant infrastructural development in Europe, resulting in the construction of numerous bridges and structures that have lasted to the present day. Many of these foundations, often designed as piled rafts but analysed as pile groups, were constructed using simplistic methodologies. These methods neglected the interactions between the raft, soil, and piles. Despite advancements in geotechnical engineering, current standards like Eurocode 7 still rely on this outdated approach, potentially leading to unnecessarily conservative and expensive retrofitting solutions.

Recent research highlights the significant contribution of rafts to the overall bearing capacity of foundations, suggesting that current design standards may underestimate the capacity provided by rafts. This thesis investigates the behaviour of piled raft foundations compared to pile groups and rafts, utilizing both 3D finite element analysis with the SANISAND-MS constitutive model and centrifuge testing. Furthermore, it explores the feasibility of using a viscoelastic model to capture the cyclic behaviour of piled raft foundations. The study focuses on dry sand conditions and quasi-static cyclic loading to address questions regarding load-displacement behaviour, load and moment distribution, internal forces on piles, and the modelling of cyclic loading conditions.

The findings demonstrate the superior performance of piled rafts in terms of stiffness and resistance compared to pile groups and rafts. Specifically, the vertical resistance ratio (β_PR,V ) and combined resistance ratio (ξ_PR,V ) indicate that accounting for the raft nearly doubles the vertical resistance of the piled raft foundation and aligns closely with the combined resistance of the pile group and raft as separate foundations. In lateral loading, the horizontal resistance ratio (β_PR,H) and moment resistance ratio (β_PR,M ) suggest that the piled raft requires more horizontal load and moment to achieve the same displacement and rotation compared to the pile group alone. The raft contributes approximately 45% of the vertical load and 50% of the moment, reducing the axial load on individual piles by around 40%.

Analysis of internal forces reveals that shaft resistance is relatively small. Larger moments are observed in edge piles under axial loading, indicating the deformability of the raft and suggesting a likelihood of failure in the edge piles due to moments. Under lateral loading, the tension piles quickly reach maximum tensile resistance, with moments continuing to increase, highlighting a high probability of failure in the edge piles due to moments.

The study also demonstrates that modelling the elastoplastic cyclic behaviour of piled rafts using a viscoelastic model with frequency-independent viscosity is feasible. While not fully capturing all aspects of foundation behaviour, this approach adequately replicates stiffness and damping characteristics, making it useful for seismic design using a pseudostatic approach.

These findings offer potential for more efficient foundation designs, improved retrofitting strategies for existing structures, and increased understanding of the foundation behaviour of piled raft systems.