An innovative device for determining the soil water retention curve under high suction at different temperatures

To characterise the water retention behaviour of fine soils, high suction values are applied. In this range of values, the vapour equilibrium technique is usually used. This paper presents an innovative device, a sorption bench that permits the determination of the water retention curve of soil. With this new testing method, the time required for testing is significantly reduced. In addition, this apparatus enables the thermal conditions of a test to be controlled; thus, the applied suction can be better controlled, and the water retention curve for different temperatures can be determined. Another valuable aspect of the device is the adopted technical solution that permits weighing of the samples inside the desiccators at any time. Consequently, the water content kinetics can be defined without disturbing the drying or wetting processe

Grain scale mechanisms for capillary collapse in a loose unsaturated pyroclastic soil

Soil collapse may occur passing from unsaturated to saturated conditions, thus causing major problems, among which one can mention a poor performance of the structures or the occurrence of landslides turning into flows. The mechanisms of the soil collapse have been studied at macroscopic scale since many years, while few observations at microscopic level are available. In this work, the mechanisms of capillary collapse were investigated for a volcanic (air-fall) pyroclastic soil of Southern Italy, which is characterized by an open metastable structure and is frequently involved into catastrophic rainfall-induced landslides. The experimental investigation was performed through X-ray Computed Tomography, which allows reconstructing 3D images of the specimen from the spatial distribution of the linear attenuation coefficient. The tests were carried out on coarse sand. During the tests, the specimens were loaded by its self-weight without any external load, and the suction was gradually reduced until the specimen collapse occurs. The aims of the experimental program were: i) follow the transformation of the specimen's microstructure; ii) evaluate the variation in terms of water content, porosity and grains spatial distribution; iii) analyse the effect of grain size distribution on the development of capillary forces and mass forces. The experimental evidences outline that, for the coarse pyroclastic sand, the collapse occurs at a very low suction, while it is not mandatory to reach the complete saturation

Identifying Material Parameters for a Micro-Polar Plasticity Model Via X-Ray Micro-Computed Tomographic (Ct) Images: Lessons Learned from the Curve-Fitting Exercises

Abstract: Unlike a conventional first-order continuum model, the material parameters of which can be identified via an inverse problem conducted at material point that exhibits homogeneous deformation, a higher-order continuum model requires information from the derivative of the deformation gradient. This study concerns an integrated experimental-numerical procedure designed to identify material parameters for higher-order continuum models. Using a combination of microCT images and macroscopic stress–strain curves as the database, we construct a new finite element inverse problem which identifies the optimal value of material parameters that matches both the macroscopic constitutive responses and the meso-scale micropolar kinematics. Our results indicate that the optimal characteristic length predicted by the constrained optimization procedure is highly sensitive to the types and weights of constraints used to define the objective function of the inverse problems. This sensitivity may in return affect the resultant failure modes (localized vs. diffuse), and the coupled stress responses. This result signals that using the mean grain diameter alone to calibrate the characteristic length may not be sufficient to yield reliable forward predictions. Key words: micro-CT imaging, micro-polar plasticity, critical state, higher-order continuum, Hostun San

An Innovative Device for Determining the Soil Water Retention Curve Under High Suction at Different Temperatures

In order to characterize the water retention behavior of fine soils, very high suction values have to be applied. In this range of values, the vapor equilibrium technique is usually used. This paper presents an innovative device, a sorption bench, which permits the determination of the water retention curve of soil. By this new testing method, the testing time required is strongly reduced. In addition, this apparatus allows the controlling of the thermal conditions of the test and thus leads to better control of the applied suction as well as to determine the water retention curve for different temperatures. Another important added value of the device is the technical solution adopted, which permits the measurement at any time of the weight of the samples inside the desiccators. Consequently, one can define the water content kinetics without disturbing the drying or wetting processes

Morphological transitions for pore water and pore air during drying and wetting processes in partially saturated sand

Water retention characteristics are important for modeling the mechanical and hydraulic behavior of partially saturated sand. It is well known that the soil water characteristic curve shows hysteresis during drying and wetting processes. For a better understanding of the water retention characteristics of partially saturated soil, a microscopic investigation of the morphological transitions for the pore water phase and the pore air phase, such as volume distribution, spatial distribution and continuity during drying and wetting processes, is crucial. In the present study, different water retention states of a partially saturated sand were visualized during water retention tests using microfocus X-ray computed tomography (CT). The CT images obtained from the tests were segmented into the soil particle phase, the pore water phase and the pore air phase. Then, a series of image processing, erosion, dilation and cluster labeling was applied to the images in this order to quantify the cluster volume distributions, the number of clusters and the continuity of both the pore water phase and the pore air phase. The morphological transitions for the pore air phase and the pore water phase, subjected to decreasing and increasing degrees of saturation, were revealed using the results of the image processing, and then, the water retention states were characterized based on the morphologies for the two phases. The influence of the morphologies on the hysteresis was discussed

Modelling of drying and cracking initiation in a gallery excavated in shale

Damage induced by desiccation (i.e. drying shrinkage and cracking) is an issue to be investigated in deep geological nuclear waste storage engineering. A dried and cracked zone can form in the underground gallery host shale due to ventilation effects. Increase of the permeability and modifications of the mechanical properties of the shale could ensue. This study first presents a modeling framework able to address the issue of desiccation in shale. The advanced constitutive model ACMEG-S, which relies on multi-mechanism hardening plasticity is used. Conditions for mode I (opening) cracks initiation such as desiccation cracks are supposed to be met when the minor principal effective stress becomes equal to a threshold value (like in Griffith criterion). A finite element analysis of the coupled hydro-mechanical transient processes is further performed. A typical excavation from Mont Terri Underground Laboratory (Switzerland) is simulated, that could be caused by ventilation of the excavation. The results of the simulation clearly show the penetration of a drying front. The resulting effective stress distribution generates a gradual plastification at the neighborhood of the excavated gallery. It is shown how stresses develop during desiccation until a cracking criterion is reached

Open-source support toward validating and falsifying discrete mechanics models using synthetic granular materials—Part I: Experimental tests with particles manufactured by a 3D printer

This article presents a new test prototype that leverages the 3D printing technique to create artificial particle assembles to provide auxiliary evidences that supports the validation procedure. The prototype test first extracts particle shape features from micro-CT images of a real sand grain and replicates the geometrical features of sand grain using a 3D printer. The quantitative measurements of the particle shape descriptors reveal that the synthetic particles inherit some attributes such as aspect ratio and sparseness of the real materials while exhibiting marked differences for sphericity and convexity. While it is not sufficient to consider the printed particle assembles a replica of the real sand, the repeatable manufacture process provides convention tools to generate additional data that supports the validation procedure for particulate simulations. Oedometric compression tests are conducted on a specimen composed of the printed particles of identical size and shape to create benchmark cases for calibrating and validating discrete element models. Results from digital image correlation on the synthetic sand assemblies reveal that the fracture and fragmentation of the synthetic particles are minor, which in return makes particle position tracking possible. As our prototype test and research data are designed to be open source, the dataset and the prototype work will open doors for modelers to design further controlled experiments using synthetic granular materials such that the individual influence of each morphological feature of granular assemblies (e.g., shape and size distribution, void ratio, fabric orientation) can be individually tested without being simultaneously affected by other variables

Multi-scale analysis of lime treated sand-bentonite mixtures

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Etude de la rétention d'eau et de la consolidation des sols dans un cadre thermo-hydro-mécanique

This work is concerned with the study of water retention and consolidation of unsaturated soils in a thermo-hydro-mechanical framework. It is organized into two parts which deal respectively with deformation and temperature effects on hydric behaviour, and suction and temperature effects on mechanical behaviour. In the first part, we point out the relevance of the characteristic surface concept for soils as opposed to the retention curve, which has limited modelling power in the case of deformable media. The characteristic surface concept is experimentally illustrated for the example of a clayey silty sand. Its modelling is based on a large sample of experimental investigations with about 240 measurements of the triplet void ratio, water content, suction. In addition, a thermo-hydric behaviour model is proposed in order to determine the characteristic surface and the retention curve for a given temperature. This model is validated for the case of two materials : a ceramic and a clayey silty sand through direct testing, and for other materials on the basis of an analysis of the literature. Finally, we present an application to the determination of the permeability of unsaturated soils taking into account deformation and temperature. In the second part, temperature and suction effects on the mechanical behaviour are studied through consolidation tests on "Sion" silt. These tests are performed for different temperatures and suctions. For each test, swelling and compression indexes, as well as the preconsolidation pressure are measured. The influence of temperature and suction on these essential parameters of mechanical behaviour is determined. Finally, we propose a theoretical model which account for preconsolidation pressure as a function of temperature and suction.Ce mémoire est consacré à l'étude de la rétention d'eau et de la consolidation des sols non saturés dans un cadre thermo-hydro-mécanique. Il est organisé en deux parties qui concernent respectivement "l'effet de la déformation et de la température sur le comportement hydrique" et "l'effet de la température et de la succion sur le comportement mécanique" de sols non saturés. Dans la première partie, on montre la pertinence du concept de surface caractéristique d'un sol par opposition à la courbe de rétention dont la portée s'avère limitée dans le cas de milieux très déformables. Ce concept est illustré expérimentalement dans le cas d'un sable limoneux argileux. La modélisation mathématique de la surface caractéristique s'appuie sur une importante campagne expérimentale comprenant la mesure d'environ 240 triplets (indice des vides, teneur en eau, succion). Par ailleurs, un modèle de comportement thermo-hydrique est proposé afin de déterminer la surface caractéristique et la courbe de rétention à une température donnée. Ce modèle est validé dans le cas de deux matériaux (une céramique à base de terre cuite et un sable limoneux argileux) testés par l'auteur ainsi que dans le cas d'autres matériaux de la littérature. Enfin, on présente une application à la détermination de la perméabilité des sols non saturés prenant en compte la déformation et la température. Dans la deuxième partie, l'effet de la température et de la succion sur le comportement mécanique est examiné à partir d'essais de consolidation du limon de Sion. Ces essais sont réalisés à différents niveaux de température et de succion. Pour chacun de ces essais, les indices de gonflement et de compression ainsi que la contrainte de préconsolidation sont mesurés. On détermine ainsi l'influence de la température et de la succion sur ces paramètres essentiels du comportement mécanique. Enfin, on propose une fonction d'approximation modélisant l'évolution de la contrainte de préconsolidation en fonction de la température et de la succion

Anisotropic features of the mechanical behaviour of Opalinus clay

The Opalinus Clay is a potential host rock for radioactive waste repositories. Triaxial tests at different load levels and directions carried out on Opalinus clay showed that the mechanical behaviour of this soft rock is anisotropic. The stiffer samples are those where the loading direction is parallel to the bedding plane so that the preconsolidation stress depends on the orientation of the loading with respect to material structure. It is proposed to interpret those features with an elasto-plastic model based on four mechanisms of plasticity which are alternately mobilized depending on the direction of loading. The predicted stress-strain response is of different nature for different directions of the space, because the hardening process depends on the number of plastic mechanisms that have been mobilized. The numerical predictions showed overall good agreement with the experimental data in terms of deviatoric stress versus axial strai