1,166 research outputs found
Modelling the role of pore water salinity on the water retention behaviour of compacted active clays
No full textCompacted active clays are used as construction material for engineering barriers in many geotechnical and geo-environmental applications. Their very low permeability makes them particularly suitable as liners for the containment of both municipal and radioactive waste, since it allows very limited fluxes of contaminants. Experimental data show that the water retention behaviour of active clays is very dependent on pore water chemistry, since for a given matric suction the mass of stored water decreases with water salinity. This issue can have relevant effects on the performance of clay liners, considering that active clays are usually cast in place in unsaturated conditions and that the chemical composition of their pore water after compaction may be different from the one of the surrounding environment. In this paper, the effects of water content and water salinity on compacted clay fabric are firstly reviewed, through existing formulations which consider them separately. An enhancement of the existing formulations is then proposed, by merging both contributions. The model is finally validated against experimental water retention data from the literature, showing good prediction capabilities. In the case of the active clays considered, at given water ratios, the model predicts that the matric suction can vary between one and two orders of magnitude with salt concentration. As a counter fact, at a given matric suction and initial dry density, the model predicts that the water retained by samples inundated with distilled water is more than twice with respect to the water retained by samples saturated with a concentrated solution
Elasto – plastic modelling of the behaviour of non - active clays under chemo – mechanical actions
No full textEnvironmental variables such as temperature, matric suction and pore fluid composition are well known to influence the hydro-mechanical behavior of clays and shales. The type and the relevance of this influence depends on the mineralogical composition and on the fabric of the material. Soil activity is an engineering proxy for mineralogical composition which can be used for a preliminary characterization of the expected type of behaviour under chemical actions, if those do not imply very significant cation exchange or pH variations. Very large chemo-mechanical effects occur in highly active soils used in engineering works such as barriers for nuclear waste or landfills, however concentration changes also impact on the mechanical behavior of non – active soils and rocks, such as illitic or natural blends of clays. Such materials are widely distributed in nature and their mechanical response upon chemical changes can be problematic in many cases. Examples of engineering relevance include vast slope instabilities promoted by fabric changes due to desalinization in Scandinavian quick clays, and instability or convergence issues for boreholes drilled in shales exposed to muds with a different chemical composition from the one of the pore fluid. An elastoplastic model is formulated to simulate the volumetric behaviour of such materials along chemical and mechanical loads. In addition to the parameters of the Modified Cam Clay, it requires defining the dependency of the elasto-plastic compliance and reference void ratio on pore fluid salinity. The model performs well against experiments from literature where complex chemo-mechanical histories were imposed
Chemo-mechanical behaviour of non-expansive clays accounting for salinity effects
No full textChanges in the chemistry of the pore fluid are known to impact on the hydro-mechanical behaviour of clays. Experimental evidence collected in the last decades led to the formulation of constitutive chemo-mechanical models for expansive soils used in engineering practice for the containment of pollution, such as bentonite. Less attention has been paid to modelling the chemo-mechanical behaviourof non-expansive clays, less frequently used for geoenvironmental applications, but equally exposed to chemical changes. Key differences between the impact of salinity on the fabric of expansive and non-expansive clays are pointed out. At the macroscopic scale, an increase in salinity causes a translation of the Normal Compression Line of non-expansive clays to higher void ratios, which in some cases is also accompanied by an increase in compressibility. The opposite occurs for expansive clays. These experimental evidences provide the basis for a chemo-mechanical model formulated in the frame of elasto-plasticity with generalised hardening, whose yield surface expands with pore fluid concentration. The model is validated against experimental results, both original and from the literature. Simulation results compare very well with those of tests performed on reconstituted, compacted and intact samples
A water retention model for compacted clays subjected to salinization and desalinization processes
No full textEnvironmental actions are known to induce relevant effects on the fabric of compacted active clays, which are successfully described by adopting a double porosity framework. In particular, the role of aggregate deformation has been recognized as fundamental to interpret the water retention behavior and the transport properties. These aspects are particularly relevant in the context of clay liners, being the material cast in place in unsaturated conditions and subjected to wetting process by pore fluids characterized by a chemical composition that is different from the one of compaction. Experimental data evidence that the water retention properties of active clays evolve as a function of pore water chemistry, since for a given matric suction the mass of stored water changes with water salinity. In this paper, a double porosity water retention model is proposed, capable of reproducing the variation of matric suction with water content accounting for the salinity of pore fluid. The role of salinity changes is accounted for by a suitable evolution law for aggregate deformation, which in turn affects the inter-aggregate porosity and thus the storage properties of the material
Giulia Veronica Varisco
No full textThe headword explains the biography and the contribution of the author Giulia Varisco to the children's literatur
An Elasto-Plastic Framework for the Chemo-Mechanical Behavior of Low to Medium Activity Clays
No full textPore fluid composition strongly influences the mechanical behavior of clays, impacting both on their volumetric and shear response. Accounting for this aspect is crucial for engineering applications where changes of the chemical composition of the pore fluid are anticipated, such as transport through engineered barriers for the containment of pollutants, or slope stability of natural formations rich of clay minerals subjected to freshwater infiltration. In this work, a chemo-mechanical model capable of reproducing the response of medium to low activity clays under both mechanical and chemical loading paths is presented. The model is developed starting from the interpretation of experimental evidences in an elastic-plastic framework. Chemo-mechanical coupling is introduced both in terms of stress variables and hardening law. In particular, the formulation is specialized to variations of salt concentration, introducing osmotic suction as a chemical stress variable. The model was implemented in a constitutive driver for the integration at the REV level of the incremental constitutive equations, thus allowing for its validation against literature data
Electrochemotherapy as a new therapeutic strategy in advanced Merkel cell carcinoma of head and neck region.
No full textMerkel Cell Carcinoma (MCC) is a rare and aggressive tumour, arising from a cutaneous mechanoceptor cell located in the basal layer of epidermis, with poor prognosis. The treatment of choice for the initial stage of the disease is surgery and/or radiotherapy. The treatment of recurrent or advanced disease is still controversial.We report a case of 84 years old woman with a recurrent MCC of the chin treated with electrochemotherapy (ECT). During the period of 20 months, four sessions of ECT were employed, which resulted in an objective response of the tumour and good quality of residual life.Our case shows the effectiveness of ECT in the treatment of locally advanced MCC of the head and neck region in a patient not suitable for standard therapeutic options
Un modello di danno fragile per mezzi porosi: esempi di applicazione
No full textSi presenta un approccio accoppiato per modellare il danneggiamento indotto da sollecitazioni idrauliche e meccaniche in ammassi rocciosi. Il danneggiamento del materiale è legato alla formazione a scala microstrutturale di diverse famiglie di fratture parallele, annidate una nell’altra, ciascuna caratterizzata da una propria orientazione e spaziatura. La semplicità della geometria delle fratture permette di esprimere analiticamente la variazione di porosità e di permeabilità causate dal progressivo danneggiamento del materiale. Si illustrano alcune simulazioni, sia a livello di punto di volume sia come problema al contorno, per mettere in evidenza i potenziali campi di applicazione del modello, tra i quali si individuano la stabilità di perforazioni in roccia e l’ottimizzazione di processi di fratturazione idraulica
Optimization of the Geometry of Monitoring Devices for Contaminant Detection in Cement-Bentonite Cutoff Walls
No full textCutoff walls represent an interesting solution for the containment of
the pollution of superficial groundwater. For polluted sites, the purpose of a
cement-bentonite cutoff wall is to minimize contaminant transport and the primary
design requirement for such materials is the low hydraulic conductivity.
Despite these barriers are often cast in place as provisional tools, recently their
wide use imparted the need for a better understanding of cement-bentonite walls
also in the long-term. This certainly implies not only the need to study the time
evolution of the cement-bentonite hydro-mechanical properties in a contaminated
environment, but also the necessity of a continuous monitoring of the
efficiency of the system. To this aim, the use of dedicated devices cast in place
inside the wall when the mixture is still fluid proved to be particularly suitable to
intercept and analyse the fluids flowing through the barrier. In this paper, the
results of a numerical study are presented, with the goal of suggesting criteria
about the optimum spacing and geometry of these device
Ytterbium Disilicate/Monosilicate Multilayer Environmental Barrier Coatings: Influence of Atmospheric Plasma Spray Parameters on Composition and Microstructure
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Open AccessArticle
Ytterbium Disilicate/Monosilicate Multilayer Environmental Barrier Coatings: Influence of Atmospheric Plasma Spray Parameters on Composition and Microstructure
by Giulia Di Iorio,Laura Paglia *ORCID,Giulia PedrizzettiORCID,Virgilio GenovaORCID,Francesco MarraORCID,Cecilia BartuliORCID andGiovanni PulciORCID
INSTM Reference Laboratory for Materials and Surface Engineering, Sapienza University of Rome, Eudossiana 18, 00184 Rome, Italy
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(9), 1602; https://doi.org/10.3390/coatings13091602
Original submission received: 10 August 2023 / Revised: 31 August 2023 / Accepted: 11 September 2023 / Published: 13 September 2023
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Abstract
SiC/SiC ceramic matrix composites (SiCf/SiC CMCs) are regarded as the new materials for the hot-section components of aircraft gas turbine engines, since they have one-third of the density of metallic superalloys, a higher temperature capability, good mechanical strength, and excellent thermal shock resistance. However, high-temperature water-vapor-rich combustion gases can induce severe surface recession phenomena in SiC/SiC leading to component failure. For this reason, it is necessary to design protective coatings, i.e., environmental barrier coatings (EBCs), able to protect the SiC/SiC surface in combustion environments. In the present work, ytterbium monosilicate (Yb2SiO5), stable when exposed to water vapor at high temperatures, and ytterbium disilicate (Yb2Si2O7), characterized by a thermal expansion coefficient closer to that of the substrate, were selected for a multilayer EBC system. EBCs were processed using the atmospheric plasma spray (APS) technique. A set of deposition parameters were tested, varying the power of the torch, and the composition and microstructure of the deposited coatings were studied in terms of porosity, crack density, and post-deposition phase retention by performing SEM, EDS, and XRD analysis. The results allow for the definition of the influence of deposition parameters on the final properties of multilayer EBC coatings
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