The virus spread from person-to-person contact in a pandemic is similar to the particulate dynamics associated with granular materials. Thus, a fundamental and deterministic modelling approach that combines granular mechanics with epidemiology, immunology and sociology is developed.

A population of individuals is seen as an assembly of point masses upon which we apply equations of motion constrained by socio-economic agenda. As the distance between individuals is a key factor of virus transmission, the contact law between individuals allows for a ‘social’ repulsive force which prevents people from being too close. Once contact is established, a transfer of viral load occurs, the degree of which is governed by protective measures. The evolution of viral load within an infected individual is antagonized by the immune response according to demographics, among others. This is mathematically described by a set of prey-predator ordinary differential equations.

Numerical simulations of a large, heterogenous population reveal episodes of structured waves of infected regions in space that are suggestive of transient localized deformation patterns in sand and clay. Other phenomena analogous to granular material behaviour are also discussed.

We report on simulations of microstructure development in assemblies of monodisperse spheres in a tapped container modeled through discrete element simulations. The average solids fraction of an assembly was computed at a tap completion when its kinetic energy was essentially zero. An ensemble of 25 realizations was evolved over the span of taps from which evolution curves of the solids fractions were obtained. Drastically different progressions of the individual realizations were observed that featured sporadic jumps in solids fraction over the duration of a small number of taps. This behavior is consistent with a collective reorganization process that has been previously reported in the literature. Visualizations further revealed the formation of crystalline regions separated by dislocations facilitating bulk sliding motion in the system through periodic boundaries. Simulations conducted at a higher tap acceleration promoted a larger frequency of jumps in density over the taps, resulting in more of the realizations attaining an apparent final saturation density.

A recurrent neural network model developed with a 60% training set was used to forecast the ensemble-averaged density in the limit of large numbers of taps. The model appeared to be able to capture jumps exhibited in the simulations beyond the training set. Our findings suggest that it may be possible to analyze the evolution of granular microstructure by applying deep learning methods. The inclusion of physics-informed quantities into the learning feature space may provide an enhanced ability to understand the process towards the development of predictive surrogate models.

Climate changes induces (among others) rising sea level, intensification of the rainfall events and more frequent drought periods. These changes in the environment will have an impact on coasts morphodynamics. Coastal dunes, which are typical natural structures, will be more often subjected to higher hydro-mechanical loadings, in comparison with the last decades situation. In particular, we expect to observe more frequent imbibition/drainage cycles, which may lead to the increase in their erosion kinetics. As a matter of fact, from a microscopic perspective, the mechanical strength of geomaterials is strongly affected by that of the capillary bridges between the grains. From a macroscopic perspective, this typical feature is usually understood as a dependancy of the mechanical strength on the suction (or equivalently saturation degree). In order to capture this specific behaviour through macroscopic numerical modelling, it remains mandatory to address first the hydrodynamics of the partially saturated soils. Having in mind this problematic, we propose to simulate the response of a sandy structure to environmental loads and evolving water table height, solving a partially saturated poro-elastic model by FEM. We propose here to have a particular look at the air phase, now considered as an active phase, say characterized by a non-constant pressure. If this hypothesis seems physically realistic, and constitutes a crucial point of our approach, it brings down several numerical challenges to be able to tackle the coexistence of saturated and partially saturated zones, or capturing the nucleation of drainage or imbibition fronts at the external boundaries. In order to address these two problems, we propose (i) a revision of the variational formulation of the problem to incorporate a moving saturation front, and (ii) a dedicated algorithm to capture the shift in the boundary conditions nature when the soils undergoes a full saturation.

The mechnical behavior of granular materials is depending on many parameters like the grain size distribution, the grains shape etc. The aim of this paper is to study the influence of each parameter independently. In order to reach this objective, samples are preapred with grains with controlled shapes and sizes. At first the influence of shape is studied, the samples contain only one size of grains. After that, mixtures are preapred, in this paper, only mixtures made with the same shapes will be presented.

Tests are carried out on samples made only with spherical and pyramidal grains. The size of the grains are fixed to 1 mm and 10 mm. After that, mixtures are made with these shapes and sizes in order to start to understand how the shape and the size of the grains influences the mechanical behaviour of the granular materials. In all cases, the samples are compacted to a relative density of 50%.

The triaxial tests showed that in the case of spherical grains, the maximum deviator stress is not depending on the radial a stress, indeed, it is constant whatever, the value of the applied radial stress. For the pyramidal grains, the results show an increase of the maximum deviator stress when the value of the radial stress increases. This behavior is due to the friction forces between the angular grains.

The study of the behavior of mixtures containing different sizes of spherical grains, a slight evolution of the mechanical behavoir is observed only when the amount of fine grains filled the spaces between the coarser grains. However, this evolution is not very important. The evolution of the mechanical properties of mixtures made with pyramidal grains with different sizes is more important than that with spherical grains.

Geo-structures are subject to hydraulic flows varying in time and space. Water passing through these porous media can cause the detachment and transport of certain particles from the soil constituting the structures and their foundations. This problem is generally referred to as "internal erosion". The term suffusion, a type of internal erosion, refers to the detachment and transport of finer particles through a coarser porous soil matrix due to hydraulic flow. The temporal evolution of suffusion can modify the hydraulic and mechanical properties of soils and can lead to significant changes in the behavior of structures which can eventually cause their failure. Based on the theory of porous media, a new numerical model was formulated to take into account both erosion and filtration during suffusion. In order to carry out analyzes at the scale of a structure, an elastoplastic model for granular soils was coupled to the suffusion model. The hydromechanical model was implemented in the Abaqus finite element code and used to evaluate the impact of internal erosion on the stability of structures. The evolution of suffusion within a dike was analyzed, as well as the effects of the location of a leak cavity. The results showed that two damage mechanisms are possible: a sliding of the downstream part of the dike and a sinkhole developing in the upper part of the dike. The phenomenon of internal erosion can also affect the stability of natural slopes. On an example studied in the laboratory, we show that the mechanism of instability of the material constituting the slope depends on the percentage of fines and that its reduction by suffusion can lead to the rupture of the slope during imbibition of water by addition from the surface or rise of the water table.

Granular materials are now known to be an illustration of complex materials as they display emergent macroscopic properties when loaded. An initially homogenous response can bifurcate into a heterogeneous one with the appearance of a rich variety of structured kinematical patterns. The shear banding that ensues illustrates a symmetry-breaking transition with multiple choices of macroscopic behaviours, a common feature of dynamical complex systems. Even though the phenomenon has been studied for decades, this regime transition remains mostly mysterious in geomaterials, with no convincing arguments that could link it to the underlying microscopic mechanisms. This contribution revisits this issue by invoking fundamental extremal entropy production principles to seek any connection with the second-order work theory in the mechanics of failure. Our findings are verified through discrete element simulations that highlight the fundamental role played by the elastic energy stored within a granular material before a bifurcation occurs, which also corresponds to a minimization of the entropy production. The analysis suggests a new interpretation of the intriguing shear banding phenomenon as a bifurcation with the emergence of optimal dissipative structures germane to nonequilibrium thermodynamics of open systems.