For his internship CIFRE PhD thesis, Tomasz Daktera, design engineer in the design office of Soletanche Bachy International, conducted an intense three-year monitoring programme of three worksites on Line 15 Southbound for the Grand Paris Express, in order to assess the calculation methods used for retaining walls based on feedback from recent large projects.
We look back on the three years of research.
Why did you decide to do a PhD?
I developed the outline of my PhD project while I was at the Ecole des Ponts. I wanted to carry out research that would contribute to geotechnical works, especially applied geotechnics.
Why did you choose this thesis?
This subject was extremely interesting and very wide-ranging. The topics covered span the installation of monitoring sensors, analysis of data, digital analyses, as well as understanding the issues involved in dimensioning real projects.
After three years of study, what has emerged from your research?
The work analyses the results of numerous measurements taken during the excavation of three stations on Line 15 Southbound for the Grand Paris Express, and compares the behaviour observed during these multi-struts deep excavations and the results of calculations carried out using the finite elements method and reaction coefficient method.
Without going into the technical details, the research improved our knowledge of the behaviour of deep excavations supported by diaphragm walls in the types of terrain found in Paris, as well as specifying use of the calculation methods, identifying their limits and the precision we can expect, while also clearing up some inconsistencies between the different approaches.
What are the benefits of this study?
The engineers from the Soletanche Bachy design offices are always looking for the most advanced, as well as the most economical, technical solutions. Being able to better identify the limitations of the calculation methods used for dimensioning retaining structures enhances the control Soletanche Bachy has over its response to projects.
The thesis also allows validation of tried and tested calculation methods commonly used in the sector. These are sometimes rejected in favour of more complex methods for the sole reason that a single model does not necessarily take everything into account. But as well as being safe and economical, they have the obvious advantage of allowing engineers to maintain greater control over their calculations, with a better understanding of the various failure modes.
Is there anything else you would like to add?
I wanted to thank the Société du Grand Paris without which this construction and research work would not have been possible. And thank you to the FNTP (National Federation of Public Works) for partially funding the measuring equipment.
Thank you, Tomasz and congratulations on your PhD!