Chemo-mechanical couplings in compacted argillite submitted to high-pH environment

AbstractIn the French concept of deep nuclear wastes repository, the galleries should be backfilled with excavated argillite after the site exploitation period. After several thousands of years, the degradation of the concrete lining of the galleries will generate alkaline fluid (pH > 12) that will diffuse through the backfill. The objective of the paper is to describe the influence of such solute diffusion on the microstructure and the mechanical behavior of compacted argillite. Saturated-portlandite water was circulated through compacted samples for 3, 6 and 12months at 20 °C or 60 °C, respectively. The microstructures before and after fluid circulation were determined with mercury intrusion porosimetry. Since it was planned to introduce additives (bentonite or lime) in the remoulded argillite to backfill the deep galleries, such mixtures were also studied. The results show that the influence of the alkaline fluid on the properties of the argillite is a function of the nature of the additive. The pure argillite undergoes slight modifications that can be related to a limited dissolution of its clayey particles. Conversely, intense alteration of the bentonite-argillite mixture was observed. Lime addition improves the mechanical characteristics of the argillite through the precipitation of cementitious compounds

Swelling Soils Behaviour in Cyclic Suction-Controlled Drying and Wetting

Cyclic drying and wetting phenomena of the expansive clayey soils cause the progressive settlements which could affect principally the foundations of buildings, the drainage channels and the buffers in radioactive waste disposals. In order to better understand the coupling between these hydraulic cycles and the mechanical behaviour of the swelling soils, this article presents an experimental study performed on two different expansive soils (molded and natural) using oedometer tests by imposing suction variations with the osmotic technique. Several successive swelling and shrinking cycles were applied under different constant vertical net stresses. During the suction cycles, the compacted samples showed cumulative shrinkage strains. On the other hand, the natural samples presented cumulative swelling strains. At the end of the suction cycles, the volumetric strains reached an equilibrium stage which indicates an elastic behaviour of the samples. We can relate these elastic behaviours to the soil fabric and especially to the microstructural content of soil

Mise en sécurité des carrières souterraines

International audiencePillar failure in underground quarries can generate surface subsidence with public safety and environmental implications can be severe. In this context, various preventing and protecting methods such as the backfilling of cavities, should be used to control and to settle the hazards linked to underground quarries. The lines to select the most appropriate solution are the cost of the operation, the technical data (accessibility and stability of the site), the planned land use, the aims of the prevention. This article synthesizes the economical and technical advantages or disadvantages for each preventing or protecting method.Les manifestations en surface des désordres se produisant dans les carrières souterraines peuvent engendrer de graves conséquences en termes de sécurité publique, de dégâts sur le bâti et d'impacts sur l'environnement. Dans ce contexte, différentes techniques de prévention et de protection, telles que le remblayage des vides, sont alors à mettre en oeuvre pour maîtriser et traiter les risques induits par ces travaux. Le choix entre ces différentes méthodes dépend des conditions générales d'accessibilité et de stabilité du site, de la destination de l'ouvrage souterrain, du niveau de protection souhaité et du coût de l'opération. Cet article présente une synthèse des techniques de traitement des cavités souterraines, en indiquant leurs avantages et leurs inconvénients, autant d'un point de vue technique qu 'économique

Influence of building stiffness and foundation embedding on clays shrinkage-swelling hazard consequences

Hazard

Thermo-mechanical behaviour of clay-structure interface

International audienceThe mechanical behaviour of the soil-structure interface plays a major role in the shear characteristics and bearing capacity of foundations. In thermo-active structures, due to non-isothermal conditions, the interface behaviour becomes more complex. The objective of this study is to investigate the effects of temperature variations on the mechanical behaviour of soils and soil-structure interface. Constant normal load (CNL) and constant normal stiffness (CNS) tests were performed on soil and soil-structure interface in a direct shear device at temperatures of 5, 22 and 60 o C. Kaolin clay was used as proxy for clayey soils. The results showed that, in clay samples the temperature increase, increased the cohesion and consequently the shear strength, due to thermal contraction during heating. The temperature rise had less impact on the shear strength in the case of the clay-structure interface than in the clay samples. The adhesion of the clay-structure interface, is less than the cohesion of the clay samples

An effective constitutive model for lime treated soils

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.The effect of lime on the yield stress, and more generally the presence of structure in the soil, is usually not accounted for in the design of geotechnical structures. As a result the potential of lime treatment or of a structured soil has not been fully exploited. This paper presents a new formulation to account for the effect of structure on the mechanical behaviour for structured soils. A constitutive model is proposed in the framework of the Modified Cam Clay model to describe the behaviour of lime treated soils. The new formulation introduces a limited number of additional parameters, all of which have a physical meaning and can be obtained from an isotropic compression test. Due to similarity in behaviour of lime treated soils and naturally structured soils, the formulation can be applied to both types of soil. It is shown that the proposed model can successfully reproduce the main features of both structured soils such as maximum rate of dilation at softening and degradation at yield. The model can be applied for any structured material regardless of the origin of cementation

Comportement thermo-mécanique de l'interface sable-structure

International audienceLe comportement mécanique de l'interface sol-structure est d'une grande importance en raison du rôle de l'interface dans la résistance due au frottement et la capacité portante des structures. Dans les structures thermo-actives, du fait de la variation de la température, le comportement de l'interface devient plus complexe. L'objectif de ce travail est d'étudier l'effet des variations de température sur le comportement mécanique de l'interface sable-structure. Des essais avec des conditions de charge normale constante (CNL) et de rigidité normale constante (CNS) ont été réalisés dans une boîte de cisaillement direct à différentes températures, 22 et 60 °C sur des éprouvettes sable-sable et sable-structure. Le sable de Fontainebleau a été utilisé. Les résultats ont montré que les variations thermiques appliquées ont un effet négligeable sur la résistance au cisaillement des interfaces sable-sable et sable-structure dans les conditions CNL et CNS et que le comportement du sable peut être considéré comme étant indépendant de la température.The mechanical behaviour of the soil-structure interface plays a major role in the shear characteristics and bearing capacity of foundations. In thermo-active structures, due to non-isothermal conditions, the interface behaviour becomes more complex. The objective of this study is to investigate the effects of temperature variations on the mechanical behaviour of soils and sand-structure interface. Constant normal load (CNL) and constant normal stiffness (CNS) tests were performed on sand and sand-structure interface in a direct shear device at temperatures of 22 and 60 °C. Fontainebleau sand was used. The results showed that the applied thermal variations have a negligible effect on the shear strength of the sand and sand-structure interface under CNL and CNS conditions, and the sand and sand-structure interface behaviour could be considered thermally independent

Influence de sollicitations hydriques et mécaniques complexes sur le comportement d'un sol gonflant compacté

International audienceThe main objective of the paper is to present a study on compacted expansive soil strains generated by complex hydric or mechanical loadings and their consequences on tested soil compressibility. In the first part of the paper, the suction controlled testing devices are described (osmotic and salt solutions methods). All the presented tests were performed in the range of suctions comprised between 0 and 40 MPa. The consequences of the strains induced by wetting as a function of the applied mechanical stress were analyzed. The results showed that, under these conditions, strain depends on the followed hydraulic path whereas the slope of the plastic compression line λ(s) was only affected by the suction applied during themechanical loading. The second test series demonstrated that the stress path influenced both deformation and slope λ(s) if suction higher than the initial sample suction was imposed during the test. As a conclusion, it could be stated that the slope λ(s) depends on the suction applied during a mechanical loading and on the maximum suction experienced by an expansive material during its past “history”.L’objectif de cet article est de présenter une étude sur les déformations d’un sol gonflant provoquées par des sollicitations hydriques et/ou mécaniques complexes ainsi que leurs conséquences sur sa compressibilité notamment. Dans la première partie de l’article, les dispositifs expérimentaux à succion contrôlée employés (méthodes osmotique et solutions salines) pour conduire l’ensemble des essais sont décrits brièvement. Les essais ont été réalisés dans la gamme des succions comprises entre 0 et 40 MPa. Les conséquences d’une sollicitation hydrique monotone (humidification) conduite sous différentes contraintes ont d’abord été caractérisées. Il est apparu que dans ces conditions d’essai, les déformations dépendent du chemin de contrainte contrairement à la pente de compression plastique λ(s) qui ne dépend que de la succion imposée durant le chargement mécanique. Une deuxième série d’essais a permis de montrer l’influence du chemin de contrainte sur les déformations ainsique la pente λ(s) si une succion supérieure à la succion initiale des éprouvettes est imposée au cours de l’essai (dessiccation). En conclusion, la valeur de la pente λ(s) dépend de la succion sous laquelle est effectué le chargement mécanique mais aussi de la plus grande succion connue par le matériau au cours de son « histoire »