Numerical study and theoretical interpretation of the role of rafts in piled foundations on cohesive soils

Numerous European infrastructures have reached their intended lifespan and require reassessment according to the recent design standards, nowadays accounting seismic actions for. The current standard verification approach for piled bridge foundations still relies on oversimplifying assumptions, potentially leading to excessive over safety and to unsustainable design solutions. With the aim of improving the design process, without introducing any additional modelling complexity, in this paper the mechanical behaviour of a real piled foundation located on homogeneous clay layer have been numerically studied. Non-linear finite elements numerical analyses have been performed by assuming the soil to behave under undrained conditions. The role of pile-raft-soil coupling, typically neglected in the common design practice, is discussed and numerical results are employed to derive the interaction domain of the piled raft system. By interpreting the failure mechanisms and by using the limit equilibrium method, a simple analytical procedure is proposed to predict the interaction domain under general loading conditions, without performing any numerical analyses. The method is demonstrated to be suitable for pile lengths comparable with the raft major dimension. The procedure has been validated over different load combinations and design geometries.