Discrete modelling of capillary mechanisms in multi-phase granular media

A numerical study of multi-phase granular materials based upon micro-mechanical modelling is proposed. Discrete element simulations are used to investigate capillary induced effects on the friction properties of a granular assembly in the pendular regime. Capillary forces are described at the local scale through the Young-Laplace equation and are superimposed to the standard dry particle interaction usually well simulated through an elastic-plastic relationship. Both effects of the pressure difference between liquid and gas phases and of the surface tension at the interface are integrated into the interaction model. Hydraulic hysteresis is accounted for based on the possible mechanism of formation and breakage of capillary menisci at contacts. In order to upscale the interparticular model, triaxial loading paths are simulated on a granular assembly and the results interpreted through the Mohr-Coulomb criterion. The micro-mechanical approach is validated with a capillary cohesion induced at the macroscopic scale. It is shown that interparticular menisci contribute to the soil resistance by increasing normal forces at contacts. In addition, more than the capillary pressure level or the degree of saturation, our findings highlight the importance of the density number of liquid bonds on the overall behaviour of the material

Une description générale de la rupture dans les sols et son implication dans la stabilité des ouvrages hydrauliques

National audienceLa rupture des sols est classiquement décrite par le critère de Mohr-Coulomb correspondant à la condition limite de plasticité. Toutefois, les matériaux tels que les sols (présentant une règle d'écoulement non associée) peuvent être sujets à des modes de rupture pour des états de contrainte situés strictement à l'intérieur du critère de Mohr-Coulomb. Cela signifie que la vérification de la stabilité d'un ouvrage en sol s'appuyant uniquement sur un critère de rupture de type Mohr-Coulomb (soit sur la plastification du sol) ne permet pas, en général, de se prévenir de l'ensemble des modes de rupture pouvant se développer au sein du matériau constitutif. Dans cette communication, nous présentons un cadre unique permettant à la fois la description de la rupture plastique (survenant sur le critère de Mohr-Coulomb), et des ruptures se développant à l'intérieur du critère de Mohr-Coulomb. Nous indiquons comment détecter à l'aide du travail du second ordre les états de contrainte à partir desquels les ruptures sont susceptibles de se développer ; et nous donnons les conditions nécessaires et suffisantes (portant à la fois sur la direction de chargement et sur le mode de contrôle du volume de sol considéré) au développement effectif de ces ruptures. Enfin, l'implication de cette description généralisée de la rupture sur l'analyse de la stabilité des ouvrages hydrauliques en terre est abordée. L'ensemble de la discussion est appuyée sur des résultats d'essais de laboratoire et de simulations numériques

Real-scale investigation of the kinematic response of a rockfall protection embankment

This paper addresses the response of rockfall protection embankments when exposed to a rock impact. For this purpose, real-scale impact experiments were conducted with impact energies ranging from 200 to 2200 kJ. The structure was composed of a 4m high cellular wall leaned against a levee. The wall was a double-layer sandwich made from gabion cages filled with either stones or a sand–schreddedtyre mixture. For the first time, sensors were placed in different locations within the structure to measure real-time accelerations and displacements. The test conditions, measurement methods and results are presented in detail. The structure’s response is discussed in a descriptive and phenomenological approach and compared with previous real-scale experiments on other types of embankments

P-positive definite matrices and stability of non conservative systems

International audienceThe bifurcation problem of constrained non-conservative systems with non symmetric stiffness matrices is investigated. It leads to study the subset $D_{p,n}$ of $ℳn(ℝ)$ of the so called $p$-positive definite matrices ($1 ≤ p ≤ n$). The main result ($D_{1,n} ⊂ D_{p,n}$) is proved, the reciprocal result is investigated and the consequences on the stability of elastic nonconservative systems are highlighted

Pig liver carnitine palmitoyltransferase. Chimera studies show that both the N- and C-terminal regions of the enzyme are important for the unusual high malonyl-CoA sensitivity

Pig and rat liver carnitine palmitoyltransferase I (L-CPTI) share common K(m) values for palmitoyl-CoA and carnitine. However, they differ widely in their sensitivity to malonyl-CoA inhibition. Thus, pig l-CPTI has an IC(50) for malonyl-CoA of 141 nm, while that of rat L-CPTI is 2 microm. Using chimeras between rat L-CPTI and pig L-CPTI, we show that the entire C-terminal region behaves as a single domain, which dictates the overall malonyl-CoA sensitivity of this enzyme. The degree of malonyl-CoA sensitivity is determined by the structure adopted by this domain. Using deletion mutation analysis, we show that malonyl-CoA sensitivity also depends on the interaction of this single domain with the first 18 N-terminal amino acid residues. We conclude that pig and rat L-CPTI have different malonyl-CoA sensitivity, because the first 18 N-terminal amino acid residues interact differently with the C-terminal domain. This is the first study that describes how interactions between the C- and N-terminal regions can determine the malonyl-CoA sensitivity of L-CPTI enzymes using active C-terminal chimeras

A Discrete Approach to Describe the Elastic-plastic Behaviour of Snow

Snow is composed of small ice particles and, therefore, behaves as a granular material with a variety of sizes and shapes. Understanding the mechanical behaviour of snow is important in areas such as natural hazards e.g. snow avalanches, or traction characteristics of tires with a snow covered road. Therefore, the objective of the current contribution is to present an advanced discrete approach to predict the elastic-plastic behaviour of snow. For this purpose, snow is described by a finite number of discrete ice grains similar to the Discrete-Element Method (DEM) with ice bonds as a link between individual grains and including a creep law for ice. Bonds may rupture under excessive load or may be formed during a contact between ice grains. Hence, the integral behaviour of snow is represented by the combined properties of impact and bonds depending on strain rate, density and temperature. The collision model is based on the linear hysteretic model developed by Walton and Braun [], which accounts for the effect of plasticity. For this behaviour the impact between grains is distinguished into a loading and detaching phase represented by a loading and unloading stiffness constant. Additionally, friction behaviour into the tangential direction at the point of impact and dissipation i.e. coefficient of restitution is taken into account. A bond between two ice grains is represented by a cylinder that is allowed to undergo tension, shear, torsion about its axis and bending. These translational and rotational displacements of a bonding cylinder lead to appropriate strains that yield corresponding stresses with a constitutive model including Youngs modulus E and the Poisson ratio ν. The stresses acting on a bond are converted to forces and moments that determine in conjunction with Newtons 2nd law the kinematics of the ice grains and the overall mechanical behaviour of the ensemble

Transition and control in rotating-disk boundary-layer

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Stabilité des systèmes non conservatifs et critère du travail du second ordre

Non conservative systems leading to a non symmetric stiffness matrix are known to have paradoxical behaviors like destabilization by adding viscosity. The study of divergence instability of non conservative system under kinematics constraints has been reported recently under one kinematics constraint. We investigate here the case of two constraints, to highlight consequences on divergence instability and to illustrate the results on a three degree of freedom Ziegler system subjected to follower loading under two additional kinematics constraints

Monitoring Stray Natural Gas in Groundwater With Dissolved Nitrogen. An Example From Parker County, Texas

Concern that hydraulic fracturing and natural gas production contaminates groundwater requires techniques to attribute and estimate methane flux. Although dissolved alkane and noble gas chemistry may distinguish thermogenic and microbial methane, low solubility and concentration of methane in atmosphereâ equilibrated groundwater precludes the use of methane to differentiate locations affected by high and low flux of stray methane. We present a method to estimate stray gas infiltration into groundwater using dissolved nitrogen. Due to the high concentration of nitrogen in atmosphericâ recharged groundwater and low concentration in natural gas, dissolved nitrogen in groundwater is much less sensitive to change than dissolved methane and may differentiate groundwater affected high and low flux of stray natural gas. We report alkane and nitrogen chemistry from shallow groundwater wells and eight natural gas production wells in the Barnett Shale footprint to attribute methane and estimate mixing ratios of thermogenic natural gas to groundwater. Most groundwater wells have trace to nondetect concentrations of methane. A cluster of groundwater wells have greater than 10 mg/L dissolved methane concentrations with alkane chemistries similar to natural gas from the Barnett Shale and/or shallower Strawn Group suggesting that localized migration of natural gas occurred. Twoâ component mixing models constructed with dissolved nitrogen concentrations and isotope values identify three wells that were likely affected by a large influx of natural gas with gas:water mixing ratios approaching 1:5. Most groundwater wells, even those with greater than 10â mg/L methane, have dissolved nitrogen chemistry typical of atmosphereâ equilibrated groundwater suggesting natural gas:water mixing ratios smaller than 1:20.Plain Language SummaryHydraulic fracturing, horizontal drilling, and associated natural gas production have dramatically changed the energy landscape across America over the past 10 years. Along with this renaissance in the energy sector has come public concern that hydraulic fracturing may contaminate groundwater. In this study we measure the chemistry of dissolved gas from shallow groundwater wells located above the Barnett Shale natural gas play, a tight gas reservoir located west of the Dallasâ Fort Worth Metroplex. We compare groundwater chemistry results to natural gas chemistry results from nearby production wells. Most groundwater wells have trace to nondetectible concentrations of methane, consistent with no measurable infiltration of natural gas into shallow groundwater. A cluster of groundwater wells have greater than 10 mg/L dissolved methane concentrations with alkane chemistries similar to natural gas. Using dissolved nitrogen and alkane concentrations and their stable isotope ratios in combination with chemical mixing models, we conclude that natural gas transported from the shallower Strawn Group affected these groundwater wells rather than natural gas from the deeper Barnett Shale, which is the target of hydraulic fracturing in this area. These results suggest that hydraulic fracturing has not affected shallow groundwater drinking sources in this area.Key PointsDissolved nitrogen in groundwater provides a means to differentiate highâ and lowâ flux infiltration of stray gasNitrogen concentrations and isotope values may attribute natural gas sourcesPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146362/1/wrcr23523.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146362/2/wrcr23523_am.pd