Offer 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 particular at the interface with a substrate on which the drying soil mass rests and consequently fracturing occurring because of tensile conditions, see for example [4], the phenomenon that we plan to reproduce in the laboratory should instead occur even in the absence of tensile stresses, when a drying soil mass cracks superficially due to capillary-induced grain rearrangement. Validation of such a hypothesis can only be performed if the effect of the presence of the substratum is negligible and therefore on a sufficiently thick sample.
The nucleation and propagation on the surface will be filmed using a high-speed/high-resolution camera in order to guarantee a wide field of view and stable control with high resolution even on targets of large size compared to the characteristic size of the microstructure. Methods of digital image correlation will be adopted to get a full field measure of displacement and strain all along the test so as to characterize fracture pre-triggering conditions as well as fracture nucleation and propagation. Water content will also be monitored by weighing the drying soil mass.
A study of the samples, once the cracking process is completed, will also be carried out using the Micro-tomograph available at GeM (Processes and Composite Materials Platform (PMC)) in order to characterize the cracking path in the volume and to relate it to the rearrangement of the grains constituting the skeleton of the soil.
References
[1] IPCC, 2023: Sections. In: Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, H. Lee and J. Romero (eds.)]. IPCC, Geneva, Switzerland, pp. 35-115
[2] H. Shin, and J. C. Santamarina, 2010 : Fluid-driven fractures in uncemented sediments: Underlying particle-level processes. Earth and Planetary Science Letters, 299(1):180–189.
[3] A. K. Jain and R. Juanes, 2009 : Preferential mode of gas invasion in sediments: Grain-scale mechanistic model of coupled multiphase fluid flow and sediment mechanics. Journal of Geophysical Research: Solid Earth, 114(B8).
[4] H. Peron, L., Laloui, L.B., Hu, T. Hueckel, 2013 : Formation of drying crack patterns in soils: a deterministic approach. Acta Geotechnica, 8, 215-221.
