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Naturally frozen soils (permafrost and seasonally frozen soils) cover around 20% of the land surface on Earth. Over the past decades, these areas have been subjected to warming, which affects the equilibrium conditions of frozen soils, causing major ground perturbations compared to other regions. These changes pose a threat in terms of natural hazards affecting infrastructure in the periglacial, high altitude environment around the globe (e.g. warming induced progressive failure of frozen slopes) or in high latitude regions such as areas of Canada, Alaska, Russia or China, to name but a few. A thawing permafrost has critical impact on the stability of overlying structures in inhabited regions or areas exploited for mineral and energy resources. Despite their prevalence and growing importance, to date, most approaches to explain frozen soil mechanical behaviour have focussed on phenomenological models that cover its macroscopic behaviour. However, it is known that the microstructure of frozen soil changes during loading and with temperature variations but the processes underpinning these microstructural changes are not fully explained yet and thus not included in predictive models. Hence, an approach considering ice-particle interaction at the solid particle scale can help providing better and novel tools to understand and predict the behaviour of frozen soils. Such a revolution has been recently happening for cemented soils, where the cementation agent is not ice, but a mineral precipitation or cohesive material that bond particles together.

This PhD thesis aims at investigating the behaviour of frozen soils under mechanical loading and freezing/thawing processes by using laboratory soil microstructural observations. The test programme will focus on sandy soils with various fines contents. First, the freezing and thawing characteristics of frozen soils will be investigated by using Nuclear Magnetic Resonance (NMR) relaxometry. Second, frost heave tests combined with Magnetic Resonance Imaging technique will be performed. Third, the freezing and thawing processes will be further investigated at the particle scale via observations by X-ray microtomography (XRµCT). Finally, the microstructural change of frozen soil at particle scale under thawing will be investigated in using an XRµCT-based triaxial apparatus. 3D image processing of the XRµCT tests will be performed to quantitatively investigate the behaviour of frozen soils, using volumetric digital image correlation.

This PhD thesis is part of REFROZEN project, a collaboration between Ecole des Ponts ParisTech (France) RWTH Aachen University (Germany). The work will be performed at the Navier Laboratory (https://navier-lab.fr/en/).

• Funding: French National Research Agency
• Net salary: 1,700 € /month (36 months)
• Supervision: A.M. Tang, M. Bornert, J.M. Pereira, P. Aimedieu, R. Sidi Boulenouar, B. Maillet.
• Candidate profile: MSc degree in Civil Engineering or Geotechnical Engineering

Application (CV, letter of motivation, letters of recommendation) to be sent to Dr. A.M. Tang (anh-minh.tang@enpc.fr) before April 1, 2023.

The growing energy needs of urban areas and the environmental context lead to the development of new energy technologies. In particular, since the 1980s, a new geothermal method has been developed: energy geostructures, consisting in fixing heat exchanger pipes to the reinforcement cages of geotechnical structures like foundations to extract/inject the heat from/into the ground with the purpose of meeting the building heating and cooling demands. Among them, energy piles have been widely studied because their thermal behaviour is quite similar to the one of usual Ground Source Heat Pumps, with the specificity of a dual function: structural support and energy exchanger. These studies provide knowledge about mechanical behaviour mainly upon the axial direction and about the assessment of the energy performance of the system, but energy piles installation is still held back by the uncertainty of their thermo-mechanical behaviour despite all economic and ecological advantages of this technology. Furthermore, their dual role sparks some apprehension among the stakeholders. Among uncertainties, one of the questions still unanswered concerns the adaptation of design under combined lateral and axial loads, meaning the mutual effect between lateral (respectively axial) loading and axial (respectively lateral) behaviour of energy piles, coupling to volumetric thermal loading acting on surrounding ground and along the pile. This thesis aims at characterizing the effect of combined loading on energy piles and at the development of open-source and easy-to-use design tools for energy piles, capable of constructing an overall failure envelope including thermal cycling loading effects.

Two methods will be used. First, tests will be performed on two experimental energy piles (0.42 m in diameter and 12 m in length) in the campus of Ecole des Ponts ParisTech. The piles will be initially subjected to axial static compressive load (10, 20 or 40% of the ultimate axial load) and then in the subsequent stage, horizontal static load will be incrementally applied (30, 50 or 70% of the ultimate horizontal load), while the axial load is kept constant. At each level of horizontal load, to simulate the actual operating condition of energy piles, ten thermal cycles with temperature variation of -/+ 10 °C will be repeatedly applied to the piles while the mechanical loads are maintained constant. Second, the results from the experiments (and other results of the project ANR COOP) will be used to develop an open-source simplified design tool to bridge the gap between research and engineering practice. The principal goal of this application will be to determine a 3D failure envelope corresponding to the energy pile combined axial and lateral response and accounting for thermal cyclic loading. This envelope will then be used to design foundation systems including energy piles.

Funding: French National Research Agency (project ANR COOP)
Net salary: 1,700 € /month (36 months)
Supervision:
- Ecole des Ponts ParisTech : Anh Minh Tang, Jean-Michel Pereira ;
- Univ. Lille: Hussein Mroueh;
- Univ. Gustave Eiffel: Fabien Szymkiewicz, Thibault Badinier, Jean de Sauvage ;
- Pinto : Roxana Vasilescu.

Candidate profile: MSc degree in Civil Engineering or Geotechnical Engineering.
Application (CV, letter of motivation, letters of recommendation) to be sent to Dr. A.M. Tang (anh-minh.tang@enpc.fr) before April 1, 2023.

Salt crystallization in porous media is a major cause of degradation of heritage, construction materials, and geomaterials. Yet, the crystallization pressure at the origin of these damages is poorly understood, and this project aims at providing a fine description of this phenomenon, necessary to find solutions to prevent or mitigate salt weathering. This Ph.D. will aim at studying the phenomenon at small scales by combining micro-fluidics expérimentations and molecular simulations. See the description for more details.

Joint PhD position at UCLouvain and Ecole des Ponts in Geomechanics

Grain breakage is a common occurrence in granular media when the intergranular forces exceed the individual particle strength and is of major importance in many areas of geosciences. Experimental evidences suggest that particle breakage may significantly influence the mechanical behavior of the material promoting strain localization and favoring chemical interaction between reactive fluids and minerals. These effects are of major importance in geotechnical engineering for the design of geotechnical structures such as deep foundations or embankments, but also in geology for the understanding of fault mechanics and the propagation of landslides.

The objective of this PhD will be to understand and characterize the effect of environmental conditions like degree of saturation, stress-path, ambient temperature and chemical environment on the mechanism of grain crushing. To achieve this goal, the project proposes a multidisciplinary integrated research strategy that combines experiments using a state-of-theart high pressure and high temperature triaxial device at Ecole des Ponts ParisTech, together with numerical modelling using an open-source parallel Finite Element framework specifically designed to study different geomechanical problems involving multi-physical couplings.

Clay materials have been considered as potential candidate for closure structures in high-level radioactive waste repositories. In spite of several studies investigating its hydro-mechanical behaviour, the kinetics of the re-saturation process and the development of swelling pressure of clay materials, observed in the large-scale experiments during long periods (e.g. several years), can still not be accurately predicted by the existing numerical models. Besides, its behaviour under the development of gas pressure, induced by corrosion of ferrous materials under anoxic conditions, is still not well understood.

This project aims at investigating the hydromechanical behaviour of clay materials under re-saturation following injection of gas at high pressure. Advanced laboratory experiments (using X-ray microtomography and magnetic resonance imaging) will be first used to observe these processes at various scales. The experimental results will be then used to develop and validate numerical models to predict the behaviour of clay materials at the field structure scale.

Cifre Thesis with Vinci-ISC/Ecole des Ponts ParisTech

Reinforced concrete construction is sometimes compared to craftsmanship in the sense that many tasks remain essentially manual. Heavy and time-consuming site activities have a direct impact on the design of the elements, oversizing and the risk of manufacturing errors. It is clear that reinforced concrete will nevertheless remain in use in the years to come. Minimizing the environmental impact of structures built with this material means minimizing quantities, optimizing its use, but certainly also having a more ambitious vision, taking into account the reversibility of our actions, for example the demountability and possible reuse.
Several works are going in this direction in the Navier laboratory and in relation with the digital construction platform Build'In of the Ecole des Ponts.
The work to do is part of this context and on parametric precasting in reinforced concrete. New techniques, robotics and software development are at the heart of the subject, for reinforcement, assemblies, handling, mass-customization, tracing and quality control. The work also concerns the development of original mechanical optimization methods based on ultimate state behaviour with consideration of two materials, steel/concrete, and the different tensile and compressive behaviour of concrete, making the problems non-linear.
The main deliverable of the PhD should be a demonstrator of a concept and a specification of a potential industrialization.

This project addresses the rheology of unsaturated granular materials, in a generic framework, using model materials. These model systems will first be slightly polydisperse assemblies of macroscopic spherical grains (with diameters between 0.1 and 1 mm), mixed with (mostly) non-volatile, wetting, Newtonian liquids. In a second step, we will study complex shaped grains, a wider polydispersity of grains, and non-Newtonian liquids.
Thus, within a multi-scale approach, our goal is to establish the fundamentals of the capillary and/or viscous phenomena involved in these materials. Our project is structured around three components objectives of which is to:

• define the different rheological regimes in the parameter space.
• set up an experimental methodology allowing for the detailed characterisation of the microstructure of such materials in the various regimes previously established. To do so, a rheometer inserted into the X-ray microtomography setup available at laboratoire Navier and specific image processing tools will be developped.
• apply constitutive laws of such materials therefore described and predicted to progressively more complex configurations such as inclined plane flows. The experimental results will be confronted with predictions from continuous numerical simulations integrating the previously identified rheology.