Experimental and numerical investigation of environmental factors affecting grain crushing

Thèses - 3 years
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.

Experimental and numerical study on the hydromechanical-gas behaviour of clay materials

Thèses - 3 years
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.

Design, mechanical optimization and digital fabrication of reinforced concrete beams.

Thèses - 3 years
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.

Rheology and Microstructure of unsaturated wet granular materials

Thèses - 3 ans
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.