Low enthalpy geothermal energy is a green and local source of energy. Traditional geothermal systems have high cost of installation and energy geostructures, i.e. geostructures equipped with the facility to exchange heat with the ground, represent a promising alternative. However, they generate a thermal loading to the ground, which might affect its hydro-mechanical response and eventually the geotechnical performance of the structure. The aim of this project is to investigate experimentally the mechanical response of clays subjected to thermal loading and to provide recommendations for the design of energy geostructures in clayey soil. The project proposes an innovative fundamental and multidisciplinary approach involving soil mechanics, clay science and physical-chemistry. The first Work Package aims to address the interplay between microstructure and the macroscopic volumetric response of clayey soils subjected to drained thermal cycles under ‘standard’ fully saturated conditions. The microscopic analyses will be obtained through Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscopy (SEM), carried out at different key stages of the stress-strain path to monitor the evolution of the pore-size distribution and, hence, microstructure. Both kaolinite and illite will be tested, to investigate a relatively broad range of clay types. The second Work Package will be devoted to elucidate key aspects of the micro-mechanisms behind the thermal response at the macroscale via ‘non-standard’ tests. First, selected experiments will be repeated under dry and partially saturated conditions. Then, to explore the role of electro-chemical forces generated between the negatively charges particles faces (mainly repulsive Coulomb forces), samples will be prepared using pore-fluid with different dielectric permittivity. Comparison with previous results where double-layer interactions were modified via the temperature, will allow assessing the role of electro-chemical forces on thermal behaviour at the macroscale. Finally, the role of the mechanical forces (mainly attraction Coulomb forces) developing at the edge-to-face contacts will be explored by preparing samples with alkaline pore-water, which ‘deactivates’ the edge-to-face contact. Again, the macroscopic tests will be combined with MIP and SEM data. In the last Work Package, a constitutive model selected from the literature to simulate the response of soils under thermo-hydro-mechanical (THM) loading will be reconsidered in the attempt to give the constitutive parameters a physical meaning based on the findings from micro-scale investigations. In turn, this would allow the constitutive parameters to be estimated by practitioners via ‘accessible’ routine experimental tests rather than complex and excessively time-consuming THM tests. To support the development of relationships to estimate constitutive parameters, advanced discrete numerical models calibrated against the experimental results will be used as a virtual laboratory to explore more complex loading paths. The final goal is to provide a concrete support to engineers in the design of energy geostructures in clays by providing guidance on parameter selection.

Bio-cementation is a process to improve structures and soils which appears to be more environmentally friendly and to need less energy than classical techniques. Previous studies have shown its potential for a broad spectrum of applications: internal erosion, soil liquefaction, landslides... One of its interest is to increase the mechanical properties, while keeping a high permeability. Indeed, only a tiny volume fraction of calcite, essentially localised at the contact between grains, is needed to increase dramatically the macroscopic properties. Thus, this project aims to characterise the mechanical properties of the bio-cemented contacts designing original micro-mechanical experiments coupled to high resolution 3D imagery. The contact properties will be then used to predict macroscopic laws and effective properties using DEM (Discrete Element Modelling) simulations. Moreover, one work package of this project will be dedicated to the study of the improvement durability by measuring the evolution of the mechanical properties with the exposure time to acid conditions, representative of an acid rain.

Cementitious materials and concrete in particular are the most widely used building materials and their global daily consumption is second only to water. Although a broad body of research has extensively investigated mechanical properties and durability, the predisposition to explosive spalling, when subjected to high temperatures, is often ignored or underestimated. The term spalling refers to the detachment (violent or not) of layers of the surface of concrete exposed to high temperatures; this process reduces the working cross section of the structural element and can expose the reinforcements, leading to structural failure. Spalling is a complex phenomenon that is currently beyond the reach of the typical structural design. The interaction between the thermal load, the evolving moisture content and the evolving permeability of the medium is poorly understood at the fundamental level and their influence on the structural response on the lacks a coherent formalisation into practical criteria. This project aims to further our understanding of the fundamental mechanisms driving spalling at different scales (from molecular to material scale) through a synergic combination of advanced experimental investigations and state-of-the-art numerical modelling. We believe that standard procedures based on point-wise measurements or mass-loss experiments have intrinsic limitations and that only with a strong interconnection of numeric and innovative experiments, efforts will succeed in understanding the spalling phenomenon.

Plastic products are present everywhere in daily life with 370 million metric tons produced in 2020. Raising concerns about oil depletion, end-of-life management, microplastics effects on life and land have led institutions to take severe measures to avoid their use such as Europe’s directive on SUP from 2018. Today, the necessity to find more sustainable solutions for everyday life materials and the recent advances in our comprehension of cellulosic biomass opens up new challenges in materials science and manufacturing. In particular, one possible direction is to explore more sustainable processes to produce materials using biomass that could substitute plastic products. In this context, DRYBIOMAT proposes to evaluate the potential of dry processing methods to manufacture bio-based materials with reduced environmental impact. The main purpose is to succeed in processing bio-materials that are energy and cost-efficient, sustainable, economically competitive and up-scalable. The dry processes identified are ultrasonic compression molding (UCM) and thermocompression. UCM is a derivative of the ultrasonic welding and inspired from sintering processes. It consists in forming materials through the welding of compressed bio-elements induced by local heating resulting from inter-particle friction and viscous dissipation. Dry-processing methods have not been much considered so far to manufacture bio-based materials. Nevertheless, the topic is highly innovative and responds in many ways to the challenges of tomorrow. Historically, pulp, papers or cardboards are obtained through wet-processes which consume energy (2.9 MWh/ton of paper) and water (15-25 m3/ton) thus increasing their environmental footprint. Only a few recent studies have started to focus on the obtention of binderless boards and all-cellulose composites through thermocompression. UCM has only been used to process 100% bio-composites from a starch powder and wood pulp fibers by researchers involved in this project. Resulting properties were found to be quite close to conventional polymers and even suitable for structural applications. However, some obstacles remain preventing further development of these solutions. The technical barriers are associated to the lack of information regarding (i) the actual conditions (temperature, humidity, stress, strain, structure) within the material during dry-processing, (ii) the influence of elements size, shape, structural properties, and stress/strain states on heat generation, propagation, and dissipation in UCM, and (iii) the overall energetic efficiency of ultrasonic processes. These phenomena are of prime concern because they drive the establishment of adhesion between elements that result in the formation of bulk materials. The objective of DRYBIOMAT is to lift these technical barriers through four research axes. The 1st one will concern the development of UCM and thermocompression at lab-scale adapted to biomass specificities. The 2nd axis will evaluate the capability of bio-resources and by-products to form materials using dry-processing methods. The 3rd one will investigate on the understanding of adhesion mechanisms leading to the formation of materials and associate them to the changes in properties during the process. At last, the 4th axis will focus on the characterization of dry-processed materials and analyze their performances in respect of the associated environmental footprint. The development of dry-processed methods could lead to the obtention 100% bio-based and bio-degradable materials with properties similar to plastics and composites. The direct use of agriculture and wood by-products could lead to bio-materials with significantly reduced environmental footprints while promoting circularity. It would prove the existence of alternative methods to process biomass into materials and accelerate the development of sustainable solutions in materials science and engineering.

The MHICA project focuses on the experimental and numerical study of the performances of bilayer armour configurations under multi-hit loading of small- and medium-calibre bullets. Bilayer armours composed of a ceramic plate as front face associated with a ductile backing (made of metal or composite) are nowadays considered as the most performing systems against these threats. Indeed, thanks to their high hardness and compressive strength, ceramics materials favour projectile blunting and/or breakage, thereby limiting the penetration capacity of the bullet. However, few microseconds after impact, the shock wave propagating through the ceramic tile initiates the onset and the growth of numerous oriented microcracks. Thus, most of the dynamic interaction of this type of ballistic event comes down to the penetration of a damaged or broken projectile into an intensively fragmented target. The mechanical behaviour of the fragmented ceramic is supposed to play a major role in the penetration process as well as in case of multiple impacts. This aspect of the behaviour remains largely unexplored and one cannot find in the literature a robust modelling approach based on reliable experimental data. To date, numerical tools are still underutilized in the design process of ceramic-based protective systems. Indeed, it rather relies on empirical approaches with series of ballistic tests. However, considering the numerous parameters to take into account (projectile characteristics, impact velocity, angle of incidence, thickness of constitutive parts …), numerical simulation represents a promising way to optimize the protective systems. Nevertheless, to be predictive, the models implemented in finite element codes have to be reliable, robust and validated by using accurate experimental data. The MHICA project proposes: - to perform instrumented ballistic tests against bilayer armour configurations made with dense or porous silicon carbide front plate, - to study the damage induced in impacted bilayer armour configurations by using CT-tomography analysis, - to develop a new experimental method to characterize the behaviour of fragmented ceramics and to identify a constitutive model to be implemented in a finite element code, - to numerically simulate a single-impact of AP projectiles with both finite-element method and discrete element method, - to numerically simulate a multiple-impact of AP projectiles with the discrete element method taking into account for the initial cracking state provided by the tomographic analysis, - to compare the computational results with the experimental data. Finally the present work should make possible to better understand the relationship between ceramic's microstructure and their dynamic fragmentation on the one hand, and the impact behaviour of the fragmented ceramic one the other hand. The global approach proposed herein, composed of experimental characterization, modelling and numerical simulations, constitutes an innovative way to improve the understanding of the links between material characteristics, mechanisms activated at high strain-rates and performance of an armour system. The works carried out in this project will benefit to the DGA by providing tools to optimize the protective solutions such as body armour of the foot soldiers or the police task forces as well as armour configurations used in vehicles operating on the battlefield