Bio-mediated methods, such as microbially induced carbonate precipitation, are promising techniques for soil stabilisation. However, uncertainty about the spatial distribution of the minerals formed and the mechanical improvements impedes bio-mediated methods from being translated widely into practice. To bolster confidence in bio-treatment, non-destructive characterisation is desired. Seismic methods offer the possibility to monitor the effectiveness and mechanical efficiency of bio-treatment both in the laboratory and in the field. To aid the interpretation of shear wave velocity measurements, this study uses the discrete element method to examine the small-strain stiffness of bio-cemented sands. Bio-cemented specimens with different characteristics, including properties of the host sand (void ratio, uniformity of particle size distribution) and properties of the precipitated minerals (distribution pattern, content, Young’s modulus), are modelled and subjected to static probing. The mechanisms affecting the small-strain properties of cemented soils are investigated from microscopic observations. The results identify two mechanisms controlling the mechanical reinforcement associated with bio-cementation, namely the number of effective bonds and the ability of a single bond to improve stiffness. The results show that the dominant mechanism varies with the properties of the host sand. These results support the use of seismic measurements to assess the mechanical efficiency and effectiveness of bio-mediated treatment.

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The hydro-mechanical properties of the vadose zone are strongly influenced by seasonal cycles. The hydraulic behavior of this zone is determined by the coupling of biotic and abiotic factors. The biotic factors are controlled by the physiology and anatomy of the vegetation growing in the area, while the abiotic factors depend on the local soil characteristics, such as water content, void ratio, and matrix structure. In this laboratory-scale investigation, we assess the influence of active biomass, water content, and suction on the particle and pore structure rearrangement. We use x-ray computed tomography and 3D digital image correlation to quantify plant roots at different stages of growth, soil deformation, and water content fluctuations. Our results show that the bulk porosity of vegetated soil is strongly affected by the induced water cycles. The global micro-structure rearrangement due to the double effects of plant water uptake and induced drying-wetting cycles translates into a final bulk porosity increase. @en

Climate change strongly affects the hydro-mechanical properties of soil. Due to drought and heavy rains the soil is subjected to severe hydro-mechanical loads, that, in turn, alter the microstructure of the soil. The most affected area is the so-called vadose zone, the layer of soil situated between the ground surface and the water table. Here the presence of vegetation has a strong impact, related to the elongation/expansion of the root architecture and the hydro-mechanical interactions with soil. Additionally, the presence of plant roots facilitate the evapotranspiration process from deeper soil layers. The research presents an experimental investigation, aimed to reproduce the typical hydro-mechanical conditions as found in the vadose zone in controlled laboratory conditions. Drying-wetting cycles are induced in soils samples, where maize plants are free to sprout and develop as well as in reference non-vegetated samples. The water content and distribution within the soil matrix are studied through 4D (3D+time) in-vivo x-ray computed tomography and effects on the soil-root microstructure are quantified with 3D image analysis. Those are correlated with above ground measurements such as fluorescence (through a spectroradiometer) that, in turn, provides leaf water potential, and the stomatal conductance that controls the evapotranspiration.@en

Vegetated soil plays an essential role in confronting climate change. Soil, together with its ecosystem, stores vast amounts of carbon; it is also the construction material most widely used for the built environment. The expected impacts of climate change, such as extreme wetting-drying cycles, pose an urgent need to understand the interplay between soil deformation, root growth, and water/solute uptake. The key to this challenge lies in the extension of unsaturated soil mechanics to incorporate bio-hydrological processes, such as root growth and water uptake. In this paper, we first provide an overview of the state-of-the-art knowledge of root-zone mechanics and bio-hydrology. We identify the main knowledge gaps and suggest an integrated, bottom-to-top approach to develop a multidisciplinary understanding of soil-water-root interaction. We explain how emerging experimental and numerical methods can be used to study rooted soil under wetting–drying cycles. We focus on the biophysical processes at the scale of plant roots, soil particles and their interfaces, and discuss potential up-scaling to the continuum/field scale. An outlook on possible further research involves effects of temperature and microbial activities.

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This paper aims to understand the effect of different particle/contact properties like friction, softness and cohesion on the compression/dilation of sheared granular materials. We focus on the local volume fraction in steady state of various non-cohesive, dry cohesive and moderate to strong wet cohesive, frictionless-to-frictional soft granular materials. The results from (1) an inhomogeneous, slowly sheared split-bottom ring shear cell and (2) a homogeneous, stress-controlled simple shear box with periodic boundaries are compared. The steady state volume fractions agree between the two geometries for a wide range of particle properties. While increasing inter-particle friction systematically leads to decreasing volume fractions, the inter-particle cohesion causes two opposing effects. With increasing strength of cohesion, we report an enhancement of the effect of contact friction i.e. even smaller volume fraction. However, for soft granular materials, strong cohesion causes an increase in volume fraction due to significant attractive forces causing larger deformations, not visible for stiff particles. This behaviour is condensed into a particle friction—Bond number phase diagram, which can be used to predict non-monotonic relative sample dilation/compression.

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Processability of powders in high load compaction constitutes a challenge due to particle rearrangement, compression and breakage occurring simultaneously. Although tableting is a central operation in pharmaceutical technology, a better understanding of the link between the macroscopic powder behaviour and its micro-mechanical properties is still required. In the present study, a dual focus on the powder compaction behaviour and the quality properties of final tablets using a compaction simulator is presented. Tableting has been performed for a wide size range of limestone powders from 10 to 400 MPa, in order to understand and compare the powder compaction behaviour at both low and high confining stresses. Compactibility of limestone, the relation between porosity and stress, has been assessed with both the classical (logarithmic) Heckel model and the newly proposed (double logarithmic) Wünsch model, confirming the improvement of the latter to enhance the description of the porosity change during compaction, as well as the model robustness towards non-pharmaceutical powders. The qualitative effect of particle size and thus cohesion on the bulk density at high pressure compaction is found to be very similar to the low pressure regime. However, the geometrical interlocking influence of large size powders found in a previous study becomes irrelevant at such high pressures. For d₅₀<10 μm, the tablet tensile strength remains almost insensitive to the size variation. However, for the coarsest grades, the tensile strength decreases with increasing d₅₀ at all compaction stresses. In addition, the tablet tensile strength is found to follow a non-monotonic trend with median particle size.

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The bulk properties of powders depend on material characteristics and size of the primary particles. During storage and transportation processes in the powder processing industry, the material undergoes various modes of deformation and stress conditions, e.g., due to compression or shear. In many applications, it is important to know when powders are yielding, i.e. when they start to flow under shear; in other cases it is necessary to know how much stress is needed to keep them flowing. The measurement of powder yield and flow properties is still a challenge and will be addressed in this study. In the framework of the collaborative project T-MAPPP, a large set of shear experiments using different shear devices, namely the Jenike shear tester, the ELE direct shear tester, the Schulze ring shear tester and the FT4 powder rheometer, have been carried out on eight chemically-identical limestone powders of different particle sizes in a wide range of confining stresses. These experiments serve two goals: i) to test the reproducibility/ consistency among different shear devices and testing protocols; ii) to relate the bulk behaviour to microscopic particle properties, focusing on the effect of particle size and thus inter-particle cohesion. The experiments show high repeatability for all shear devices, though some of them show more fluctuations than others. All devices provide consistent results, where the FT4 powder rheometer gives lower yield/steady state stress values, due to a different pre-shearing protocol. As expected, the bulk cohesion decreases with increasing particle size (up to 150 μm), due to the decrease of inter-particle cohesion. The bulk friction, characterized in different ways, is following a similar decreasing trend, whereas the bulk density increases with particle size in this range. Interestingly, for samples with particle sizes larger than 150 μm, the bulk cohesion increases slightly, while the bulk friction increases considerably—presumably due to particle interlocking effects—up to magnitudes comparable to those of the finest powders. Furthermore, removing the fines from the coarse powder samples reduces the bulk cohesion and bulk density, but has a negligible effect on the bulk friction. In addition to providing useful insights into the role of microscopically attractive, van der Waals, gravitational and/or compressive forces for the macroscopic bulk powder flow behaviour, the experimental data provide a robust database of cohesive and frictional fine powders for industrially relevant designs such as silos, as well as for calibration and validation of models and computer simulations.

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Granular materials and particulate matter are ubiquitous in our daily life and they display interesting bulk behaviors from static to dynamic, solid to fluid or gas like states, or even all these states together. To understand how the micro structure and inter-particle forces influence the macroscopic bulk behavior is still a great challenge today. This short paper presents stress controlled homogeneous simple shear results in a 3D cuboidal box using MercuryDPM software. An improved rheological model is proposed for macroscopic friction, volume fraction and coordination number as a function of inertial number and pressure. In addition, the results are compared with the locally averaged data from steady state shear bands in a split bottom ring shear cell and very good agreement is observed in low to intermediate inertia regime at various confining pressure but not for high inertia collisional granular flow.

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Particulate systems and granular matter display dynamic or static, fluid‐ or solid‐like states, respectively, or both at the same time. The mystery of bridging the gap between the particulate, microscopic state and the macroscopic, continuum description is one of the challenges of modern research. This book chapter gives an overview of recent progress and some new insights about the collective mechanical behavior of granular, deformable particles.@en

We study the effect of both particle size and shear testers on the failure (yielding) and the steady state shear strength of granular materials. Physical experiments are carried out on four fine limestone powders using a geotechnical direct shear tester and the standard Schulze ring shear tester to explore both material behaviors and device influence. The direct shear tester is validated by comparing the yield locus measurements with the Schulze ring shear tester. A good agreement is found between these two testers and this unveils the possibility of using different testers across different scientific communities/regimes. The termination locus is then studied in direct shear tester for the four limestone powders. A pronounced particle size effect is evident on both the angle of internal friction and the steady state angle of internal friction, which highlight the effect of the inter-particle cohesion@en

We study the effect of particle cohesion on the steady state shear strength of a granular material. For cohesive powders, the steady state shear loci (termination loci) from DEM simulations are nonlinear with a peculiar pressure dependence due to the non-linear increase of contact adhesion with pressure in the contact model. Physical experiments are carried out on fine limestone powder in the direct shear box to validate the interesting non-linearity in the material behavior, as detected by the simulations and to provide a basis for calibration of DEM. In order to enhance the reproducibility, the standard test procedure from Jenike shear tester is ameliorated for the direct shear tester. Finally, the difference between the yield (transition from static to flow) and the steady state shear stress (required to maintain shear motion) will be addressed.@en