Insights into the links between microstructure and Bishop's X parameter for unsaturated soils

International audienceIn recent years, Bishop's proposal for a generalized stress extending Terzaghi's stress to unsaturated states gained a large audience. Despite the complexity introduced in some aspects of modelling or stress representation, such an approach allows to model in a simple manner particular features of unsaturated soils behaviour. In this paper, an alternative choice for this parameter is proposed and validated using either shear strength or elastic stiffness experimental data from a set of soils ranging from almost granular materials to high plasticity clays. A physical interpretation in terms of microstructurally trapped water is provided. It is shown that the amount of trapped water is closely linked to the definition of X. When the amount of non-free water increases (increasing plasticity indices), a significant deviation of X values from the degree of saturation is observed. It is thus concluded that some care must be taken when assuming as generally done that Bishop's X parameter equals the degree of saturation of water

A microstructurally-based effective stress for unsaturated soils

International audienceCurrent alternative choices of stress state variables in unsaturated soils are described and compared, with a special focus on the use of an effective stress. Experimental data on stiffness and shear strength evolution with suction suggest that the proportion of suction contributing to the effective stress is often much smaller than predicted by the term ‘suction times degree of saturation' generally used in effective stress expressions of the Bishop type. It is suggested that effective stress in unsaturated soils should be related to soil microstructure. An effective degree of saturation is defined as describing the volume of water partially filling the soil macroporosity. This effective degree of saturation defines the proportion of the prevailing suction that actually contributes to the effective stress. Two alternative expressions (piecewise linear and non-linear) are proposed for the effective degree of saturation. They offer a similar performance. Available data on stiffness and shear strength variation with suction of a few different soils, ranging from a markedly granular material to high-plasticity clay, have been analysed. The analysis supports the proposed microstructural interpretation of the effective stress. Indeed, for granular soils the effective degree of saturation is almost equal to the total degree of saturation, and therefore the Bishop-type expression generally used as an effective stress is recovered. As the soil becomes more plastic, the proportion of free water reduces, and the contribution of suction to the effective stress reduces. At the limit, when the proportion of free water is negligible (this is the case of high-plasticity clays at high values of suction) the proposed effective stress reduces to the net stress (excess of total stress over the air pressure). The proposed effective stress equation may be identified if information on the amount of immobile water is available for a given soil. Water retention or porosimetry data provide this information. This has been shown by comparing the present proposal with independently obtained information about immobile water in high-plasticity clays

From zoonotic models to humanized chimeras the importance of experimental pathophysiology of infectious diseases at the post-genomic era

L’étude des mécanismes de déclenchement et de l’évolution des maladies infectieuses nécessite une approche à la fois épidémiologique et physio- pathologique. De l’étude des zoonoses où les pathogènes ont une spécificité d’hôte peu restreinte à celle des infections opportunistes où par définition, les pathogènes évoluent selon les conditions environnementales, la physiopathologie expérimentale révèle le potentiel adaptatif des micro- organismes par leur plasticité génétique. Le décriptage des génomes des pathogènes et de leurs hôtes montre que les interactions entre les déter minants infectieux et les molécules réceptrices de l’hôte n’impliquent souvent que quelques gènes, facilement modifiés par des mécanismes de mutation et de recombinaison. La connaissance de ces interactions précises grâce aux modèles in vivo permet de mieux appréhender la biodiversité et l’évolution des génomes et d’en déduire de nouvelles stratégies thérapeutiques et préventives des maladies infectieuses.From zoonotic models to humanized chimeras, the importance OF EXPERIMENTAL PATHOPHYSIOLOGY OF INFECTIOUS DISEASES AT THE POST-GENOMIC ERA Understanding the mechanisms of infectious diseases both requires epidemiological and pathophysiological approaches. From the study of zoonoses, where the pathogens have low-restricted host specificity, to that of opportunistic infections where, by definition, the pathogens evolve according to environment, experimental pathophysiology reveals the adaptative potential of microorganisms due to their genetic plasticity. Deciphering the genomic sequences of the pathogens and of their hosts show that interactions between infectious determinants and host-receptors often involve only some genes, easily altered by mutation and recombination. The precise identification of these interactions by the use of in vivo models allows better understanding of biodiversity and genomic evolution and to set up new therapeutic and preventive strategies of infectious diseases

Is the degree of saturation a good candidate for Bishop's X parameter?

International audienceIn unsaturated soil mechanics, the quest for an effective stress playing the same role as Terzaghi's effective stress does for saturated soils has introduced a long standing debate, dating back to the 1960s. Several contributions have been proposed since the early work of Bishop. It is well recognized to date that a single constitutive stress is not sufficient by itself to catch the main features of the behaviour of unsaturated soils and it is often combined with matric suction. In this paper, focus is given to a largely used formulation for such a constitutive stress, based on the use of an averaged pore pressure. In particular, this paper discusses on thermodynamics bases the validity of the choice of the factor X weighting the fluid pressures contribution to the constitutive stress. This factor is usually assumed to be equal to the degree of saturation of water. In this work it is shown that the choice of this natural candidate implies restrictive assumptions on the plastic flow rule. As shown from experimental data obtained from a literature review, this choice may not be pertinent for certain classes of materials, particularly high plasticity clays

Recent Increase in Meningitis Caused by Neisseria meningitidis Serogroups A and W135, Yaoundé, Cameroon

From 1991 to 1998, Neisseria meningitidis serogroups A, B, and C represented 2%-10% of strains isolated from cases of bacterial meningitis in Yaoundé. During 1999 to 2000, the percentage of meningococci reached 17%, a proportion never reported since recordkeeping began in 1984. The increase of serogroup A meningococci and the emergence of W135 strains highlight the need for increased surveillance for better diagnosis and prevention

Revisiting the thermodynamics of hardening plasticity for unsaturated soils

A thermodynamically consistent extension of the constitutive equations of saturated soils to unsaturated conditions is often worked out through the use a unique 'effective' interstitial pressure, accounting equivalently for the pressures of the saturating fluids acting separately on the internal solid walls of the pore network. The natural candidate for this effective interstitial pressure is the space averaged interstitial pressure. In contrast experimental observations have revealed that, at least, a pair of stress state variables was needed for a suitable framework to describe stress-strain-strength behaviour of unsaturated soils. The thermodynamics analysis presented here shows that the most general approach to the behaviour of unsaturated soils actually requires three stress state variables: the suction, which is required to describe the invasion of the soil by the liquid water phase through the retention curve; two effective stresses, which are required to describe the soil deformation at water saturation held constant. However a simple assumption related to the plastic flow rule leads to the final need of only a Bishop-like effective stress to formulate the stress-strain constitutive equation describing the soil deformation, while the retention properties still involve the suction and possibly the deformation. Commonly accepted models for unsaturated soils, that is the Barcelona Basic Model and any approach based on the use of an effective averaged interstitial pressure, appear as special extreme cases of the thermodynamic formulation proposed here

The Spectroscopic Orbit of the Planetary Companion Transiting HD209458

We report a spectroscopic orbit with period P = 3.52433 +/- 0.00027 days for the planetary companion that transits the solar-type star HD209458. For the metallicity, mass, and radius of the star we derive [Fe/H] = 0.00 +/- 0.02, M = 1.1 +/- 0.1 solar masses, and R = 1.3 +/- 0.1 solar radii. This is based on a new analysis of the iron lines in our HIRES template spectrum, and also on the absolute magnitude and color of the star, and uses isochrones from four different sets of stellar evolution models. Using these values for the stellar parameters we reanalyze the transit data and derive an orbital inclination of i = 85.2 +/- 1.4 degrees. For the planet we derive a mass of Mp = 0.69 +/- 0.05 Jupiter masses, a radius of Rp = 1.54 +/- 0.18 Jupiter radii, and a density of 0.23 +/- 0.08 grams per cubic cm.Comment: 11 pages, 1 figure, 2 tables, LaTex, aastex, accepted for publication by ApJ Letter

The SARS algorithm: detrending CoRoT light curves with Sysrem using simultaneous external parameters

Surveys for exoplanetary transits are usually limited not by photon noise but rather by the amount of red noise in their data. In particular, although the CoRoT spacebased survey data are being carefully scrutinized, significant new sources of systematic noises are still being discovered. Recently, a magnitude-dependant systematic effect was discovered in the CoRoT data by Mazeh & Guterman et al. and a phenomenological correction was proposed. Here we tie the observed effect a particular type of effect, and in the process generalize the popular Sysrem algorithm to include external parameters in a simultaneous solution with the unknown effects. We show that a post-processing scheme based on this algorithm performs well and indeed allows for the detection of new transit-like signals that were not previously detected.Comment: MNRAS accepted. 5 pages, 3 figure