Chemo-mechanical behaviour of non-expansive clays accounting for salinity effects

Changes 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

Effects of curing on the hydro-mechanical behaviour of cement-bentonite mixtures for cut-off walls

Cement-bentonite cut-off walls are commonly employed in geoenvironmental applications to limit ground water flow and pollutant transport. The wide diffusion of this artificial material in the current practice is not only due to its low permeability, but also to its simplicity of use. In this paper, experimental evidences about the role of curing on the hydro-mechanical behaviour of cement-bentonite mixtures are presented. Different curing times and curing conditions (representative for either water saturated or hydrocarbon polluted soils) have been considered, and their effects on both hydraulic conductivity and mechanical response in oedometer and triaxial conditions have been assessed. A unified hydro-mechanical framework, accounting for the changes of material fabric occurring with curing time and environment, is formulated. The hydraulic conductivity is very well predicted by a Kozeny-Carman like equation, whereas the mechanical behaviour is finely reproduced via an enhanced elastic–plastic constitutive model

Undergraduate geotechnical engineering education of the 21st century

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal.Peer ReviewedPostprint (author's final draft

Assessing the Integration of Electrified On-Board Systems in an MDAO framework for a small transport aircraft

The integration of on-board systems design within the aircraft design process is often considered only in the late part of the initial design. This is acceptable for civil aircraft using standard technology systems. However, facing with MEA and AEA concepts and different possible architectures, the systems design and the assessment of their effects on the overall aircraft should be moved up in the usual design process. This paper deals with evaluation of the effect of different on-board systems architecture, with a different electrification level, on the overall aircraft design. These effects have been evaluated using three different MDA workflows developed within the AGILE4.0 European research project. The workflows are defined with an increasing number of disciplines to show how the effect of a proper selection of a systems architecture is differently caught by each one. In this way it is possible to define which disciplines should be included for the systems architecture assessment. The results show a save of 1% of MTOM for the AEA applied to a small turboprop aircraft when only the OBS mass is assessed. Increasing workflow complexity, adding performance and engine design the save increase to 1.2%. Finally, the save increases to 1.3% when the effect on engine SFC is also considered

Numerical modelling of the cyclic response of a caprock material with an elasto-viscoplastic constititutive model accounting for debonding

The energy transition is a paradigm shift that affects the entire economic, social and industrial trajectory of our country. In this context, large-scale underground storage of hydrogen is seen as an effective way of overcoming the drawback associated with the seasonal nature of renewable energy sources in meeting the needs of energy demand. However, the injection/production cycles will produce a seasonal cyclic pressure of the stored gas, which in turn will induce stress changes in both the host formation and the caprock material. Such geomechanical variations can seriously compromise the sealing capacity of the caprock, increasing the risk of leakage. It is then of paramount importance to appropriately model the fatigue behaviour of the caprock material. This work investigates the ability of an existing elasto-plastic framework developed for structured soils to model the experimental response of an Italian stiff clay, which represents a typical caprock material. The existing framework is enhanced by introducing a viscous strain component to reproduce the observed time-dependency of the caprock response, which is shown to be crucial in modelling the material behaviour under different cyclic loading conditions

Preliminary investigation on the water retention behaviour of cement bentonite mixtures

Cement bentonite mixtures are often used to build slurry walls for the containment of both aqueous and non aqueous pollutants, due to their quite low hydraulic conductivity and relatively high ductility and strength. Although their hydro-mechanical behaviour in saturated conditions has been studied in the past, a part of the slurry wall is expected to rest above the groundwater level. The hydraulic characterization in unsaturated conditions is then particularly relevant to evaluate the performance of the barrier, especially when it is aimed at containing non aqueous pollutant liquids which are lighter than water (LNAPL). These non wetting fluids rest above the water table and their penetration is possible just if the barrier is unsaturated. This paper presents some preliminary results of a laboratory characterization of the water retention behaviour of three different cement bentonite mixtures. The mixtures, prepared at cement - bentonite mass ratios ranging from 4:1 to 6:1, were immersed in water and cured for 28 days. Their water retention behaviour was then determined along drying and wetting paths through different techniques, namely axis translation, filter paper and vapour equilibrium. In the high suction range, the water content - suction relationship was found to be independent of cement-bentonite ratio. In the low suction range, the water content at a given suction was found to decrease for increasing cement bentonite ratios

Aerodynamic analysis and optimization of a regional transport aircraft

The geometry of a typical regional transport aircraft is modified to reduce drag and improve performances, in particular cruise speed. Once performed a preliminary aerodynamic analysis on the original geometry, in order to detect those portions of the body shape whose modification mostly influences drag variation, an automatic procedure, manageable trough MATLAB, allows to modify those parts using interpolating curves and surfaces, espectively NURBS and NURBCOONS. Within the modification loop, each new geometry is analyzed trough a panel code solver until optimized shapes are found. Finally, the optimized body is exported into a CAD format (IGES) suitable for design and production. The optimization process has guaranteed a reduction of 3 percent of the total drag and an increase of 2 percent of cruise speed respect to the original configuration

Analisi numerica di diaframmi plastici dotati di dispositivi per il monitoraggio e la prevenzione dell'inquinamento ambientale

I diaframmi plastici costituiti da miscele di cemento e bentonite rappresentano una soluzione interessante e vantaggiosa in interventi di tutela e salvaguardia ambientale. Scopo dei diaframmi plastici in tale ambito è isolare e ridurre il più possibile la dispersione nell’ambiente di fluidi contaminanti; pertanto i loro requisiti principali sono una conducibilità idraulica molto bassa e una elevata durabilità. Questo implica la necessità di una valutazione sempre più accurata dell’efficienza dell’opera. A questo scopo, l’uso di specifici ed innovativi dispositivi inseriti durante la realizzazione stessa del diaframma evoluto si è dimostrato particolarmente adatto ad intercettare i fluidi potenzialmente contaminanti prima che fuoriescano dalla barriera e contaminino l’ambiente. In questa nota si presentano i risultati preliminari di uno studio numerico svolto con l’obiettivo di suggerire criteri di installazione di questi dispostivi, frutto di un accordo di collaborazione promosso dalla Direzione Generale per la Sicurezza anche Ambientale delle Attività Minerarie ed Energetiche del Ministero dello Sviluppo Economico con il Politecnico di Milano