Over the last few years we have carried out a number of scientific studies, in particular on the topic of soil-foundation interaction. These developments, all motivated by practical issues encountered in the geotechnical studies that we work on every day, have been applied through new calculation tools incorporated step by step into the Foxta software suite. Here is a brief summary.
The development of the Tasplaq module was a major step forward in the modelling of rafts and slabs, providing a practical calculation method breaking away from the “slab on springs” representation while avoiding the use of complete 3D finite-element processing. The method has demonstrated its effectiveness in a number of major projects (high-rise buildings at La Défense, EPR in the United Kingdom, etc.) and has benefited from several recent updates, such as the processing of a non-horizontal multilayer and the integration of unloading/ reloading effects.
The lessons learned from the ASIRI research project have been used to extend the Taspie+ module, which applies a simplified approach to process the complex interactions involved in the behaviour of soil reinforced by rigid inclusions. The power of this method lies in its ability to account for the effects of soil/inclusion relative rigidity in the modelling of load transfer mechanisms.
The latest update of the module incorporates edge effects in the case of a limited number of inclusions. The general application of the “t-z” and “p-y” models as implemented in the Taspie+ and Piecoef+ modules provides a satisfactory and rigorous solution to the issue of parasitic effects, taking into account the soil-foundation interaction (negative skin friction, lateral thrusts, swelling, etc.). Buckling is another parasitic aspect specific to the design of micropiles. The mathematical formulation of Piecoef+ provides a new approach for direct estimation of the critical buckling load, an approach that also evaluates the second-order effects in the case of a non-zero initial curvature.
The new Groupie+ module for pile groups enables easy processing of the 3D equilibrium of the pile/soil/foundation system in a single model, taking account of the non-linearities related to the soil behaviour. Development is continuing, with the aim of introducing the footing flexibility and processing pile/soil/pile interaction effects.
The recent development of geothermal foundations has motivated the development of a new module, Thermopie+, for evaluation of thermal-mechanical loads in a group of geothermal piles connected at the top by a structural element of any stiffness. The proposed approach accounts for the effects of pile/structure/pile interaction, unlike conventional modelling which is limited to an isolated pile with a single elastic connection at the top.
In parallel, the recent increased awareness of the challenges related to the seismic risk has encouraged us to work on additional analysis methods in order to take full account of the dynamic character of seismic loads. In this context, we are currently focusing on the following topics:
- Piles under seismic conditions: a rigorous treatment of the seismic response of a pile foundation necessitates taking into account the (dynamic) effects of pile/soil/ pile interaction, which are ignored implicitly in the usual ‘pseudo-static’ analyses. This was the motivation for the initiation at the beginning of 2016 of a PhD thesis together with AP Consultants, Ecole Centrale de Nantes and IFSTTAR Nantes. This project is aiming to develop a “macro-element” able to represent the non-linear response of a pile group under seismic conditions, taking frequency-related effects into account.
- Rigid inclusions under seismic conditions: going beyond the calculation of internal loads under inertial and kinematic effects, the seismic design of a foundation on rigid inclusions requires the evaluation of the irreversible displacements related to the “fuse” effect played by the granular layer. A “generalised Newmark model”, integrating the effects of soil-structure interaction, has been developed and subjected to practical evaluation as part of an on-going R&D partnership with Hyundai Engineering & Construction.