Multiphysical Study of The Response of Soils to

**Multiphysical study of the response of soils to extreme drought conditions with respect to their hydraulic and mechanical properties**:

- Réf **ABG-127259**
- Stage master 2 / Ingénieur- Durée 6 mois- Salaire net mensuel 600€- 03/12/2024- École Centrale de Nantes- Lieu de travail- Nantes Pays de la Loire France- Champs scientifiques- Sciences de l’ingénieur
- Matériaux
- Mots clés- Drying, Cracking, Two-phase flow, Experimental mechanics.-
- 17/01/2025**Établissement recruteur**:
**Site web**:
Research Institute in Civil and Mechanical Engineering (GeM)

The activities of the different research groups are focused on 'Materials, Structures and Processes' with the main objective of developing and implementing integrated approaches covering the design, production and operation of industrial components (in aeronautics, automotive, energy, civil engineering sectors etc). A wide range of materials is covered: geomaterials, metals, polymers, composites. Research into 'Processes' focuses particularly on rheology and assembly and more generally on the use of materials.

The research teams are also all involved in analysing the in-service functionality of structural components: performance under extreme mechanical and environmental conditions (earthquake, crash), sustainability in the broadest sense (especially risk management, lifecycle analysis).

A specific objective of the laboratory is to develop experimental methods with regard to the implementation of full-scale test benches, or replicating real industrial conditions, and equipped with multi-physical instrumentation. Work is also underway to develop surveillance and measurement methods based on optical methods (fibres etc). The modelling methods developed in the laboratory are particularly focused on non-linear, multi-scale and multi-physical aspects, including the integration of uncertainty. Innovative numerical methods are also developed with a direct link to this modelling and experimental characterisation work, in particular with regard to coupling and durability. In addition, considerable emphasis is placed on model reduction techniques.

**Description**:
According to the Intergovernmental Panel on Climate Change (IPCC) AR6 Synthesis Report: Climate Change 2023 [1], human influence has likely increased the risk of compound extreme events since the 1950s. These combined extreme events include increased frequency of heatwaves and simultaneous droughts. The global area affected by extreme drought increased from 18% in 1951-60 to 47% in 2013-2022, endangering water security, sanitation and food production. The most obvious manifestation of extreme drought events is the formation of desiccation crack networks in soils, which negatively alter soil properties and compromise the integrity of soil structures, a dominant factor in many potential geotechnical hazards. Analyzing the impact of changes in soil surface moisture, induced by extreme drought episodes, on the intrinsic properties of the soil is therefore the main objective of the project in order to understand how land and biodiversity conservation is affected by such phenomena.

The most significant feature of the proposed approach and therefore the most important scientific obstacles to overcome concern the study of the desiccation cracking process in positive effective stress regime, i.e. in soil compression conditions, which corresponds to the natural state of granular materials without cohesion. This phenomenon is a special case of invasion by an immiscible fluid, where air invades water-saturated sediments. Desiccation cracks generally form at the surface and propagate laterally and vertically, forming vertical planes. When the grain size is large, the invasion seems to follow the classic pore-by-pore invasion where the pressure of the invading gas exceeds the air entry pressure at the pore throat, without generating any reorganization of the soil microstructure. However, as the grain size is reduced, the capillary pressure limit for gas entry at the pore throats is much higher and the increasing pressure of the invading gas reaches a fracturing limit at which frictional sliding and grain rearrangement develop. This in turn widens the pore throats, allowing the advancement of the air-water interface; the grain network no longer behaves as a rigid medium and fractures start to grow at the surface and propagate vertically in the network in the direction normal to that of the minor principal effective stress [2,3].

In this context, crack nucleation and propagation in the opening mode do not seem to be associated with tensile forces induced by boundary conditions that limit the shrinkage of the entire soil mass, and thus to the transformation of elastic energy into cracking energy, but to a loss of adhesion between soil particles due to a loss of capillary energy.

Even if existing experimental tests suggest a strong dependence of surface crack intensity on boundary conditions, in particul