Generalized Plasticity and Cyclic Pressuremeter Test Modelling

A constitutive model based on generalized elastoplasticity (Zienkiewicz et al., 1985) is used for the modelling of monotonic and cyclic pressuremeter tests in a clay. The permeability of the material is taken into account for the modelling of the excess pore water pressure generation during the test (combination of pore pressure build up and dissipation). It is shown how this type of model can simply represent the main features observed during a cyclic pressuremeter test in a clay, particularly the accumulation of excess pore water pressure during the cycles of loading, and the importance of dissipation on the excess pore water pressure build up

Résistance d'un milieu granulaire renforcé par injection de ciment

International audienceThe present Note describes an experimental study devoted to the strength of a sand reinforced by cement grouting. Through grouting, the granular medium gains a cohesion without significant change of the friction angle. The most significant experimental feature is that the cohesion is proportional to the volume fraction of cement in the grouted material. This result is interpreted within the framework of a periodic homogenization applied to yield design.On présente une étude expérimentale consacrée aux capacités de résistance d'un sable injecté par un coulis de ciment. L'effet principal de l'injection est un gain de cohésion sans modification significative de l'angle de frottement. La principale observation expérimentale réside dans le fait que la cohésion est proportionnelle à la fraction volumique de ciment réalisée dans le matériau injecté. On interprète ce résultat dans le cadre de l'homogénéisation périodique en Calcul à la Rupture

Flux emergence and coronal eruption

Our aim is to study the photospheric flux distribution of a twisted flux tube that emerges from the solar interior. We also report on the eruption of a new flux rope when the emerging tube rises into a pre-existing magnetic field in the corona. To study the evolution, we use 3D numerical simulations by solving the time-dependent and resistive MHD equations. We qualitatively compare our numerical results with MDI magnetograms of emerging flux at the solar surface. We find that the photospheric magnetic flux distribution consists of two regions of opposite polarities and elongated magnetic tails on the two sides of the polarity inversion line (PIL), depending on the azimuthal nature of the emerging field lines and the initial field strength of the rising tube. Their shape is progressively deformed due to plasma motions towards the PIL. Our results are in qualitative agreement with observational studies of magnetic flux emergence in active regions (ARs). Moreover, if the initial twist of the emerging tube is small, the photospheric magnetic field develops an undulating shape and does not possess tails. In all cases, we find that a new flux rope is formed above the original axis of the emerging tube that may erupt into the corona, depending on the strength of the ambient field.Comment: 5 pages, 3 figures, accepted for publication in A&

Behavior of Soil-structure interfaces under cyclic loading for large numbers of cycles: Application to piles

Pile foundations are usually subjected to cyclic loading which can be either environmental or industrial. Loading and unloading sequences of the pile cause very significant variation in the behavior of the pile-soil system and generate degradation of the bearing capacity and an accumulation of irreversible displacement. The paper presents a study of the behavior of piles subjected to axial cyclic loading for large numbers of cycles using a physical modeling approach in a calibration chamber. To focus on the degradation of the friction in soil-pile interface, the experiments are carried out in two-way displacement–controlled tests. Firstly, The experimental set-up was presented, then the typical results of the evolution of skin friction under cyclic loading. The results indicate that the application of a large number of cycles to the pile, the skin friction decrease initially then increase continually up to the end of the cyclic sequence. It can be concluded that the phase of friction reinforcement is due to a partially constrained dilatancy phenomenon of the sand within the interface zone.Résumé :Généralement les pieux sont soumis à des sollicitations cycliques qui peuvent être de type environnemental ou industriel. Le chargement et le déchargement du pieu provoquent une évolution très significative du comportement du système pieu- sol et engendrent une dégradation de la capacité portante ainsi que l’accumulation des déplacements irréversibles. L’article présente une étude du comportement des pieux chargés axialement, par une modélisation physique en chambre d’étalonnage, sous l’effet d’un chargement cyclique à grand nombre de cycles. Les essais ont été réalisés à déplacement contrôlé de type alterné afin de maître en évidence le phénomène de dégradation du frottement à l’interface sol-pieu. On présente tout d’abord le dispositif expérimental utilisé, puis les résultats typiques sous l’effet du chargement cyclique en s’intéressant, plus particulièrement, à l’évolution du frottement au niveau de l’interface sol-pieu. L’analyse des résultats trouvés nous a permis de conclure qu’en appliquant un chargement cyclique à grand nombre de cycles sur le pieu, le frottement latéral subira au début une dégradation puis une augmentation qui continuera jusqu’à la fin de la séquence cyclique. Ce renforcement observé du frottement latéral est dû à la forte dilatance partiellement empêchée de la zone d’interface sol-pieu.Mots clés : Chambre d’étalonnage, pieu, sol, interface, frottement latéral, chargement cyclique, dégradation

Nonlinear force-free and potential field models of active-region and global coronal fields during the Whole Heliospheric Interval

Between 2008/3/24 and 2008/4/2, the three active regions NOAA active regions 10987, 10988 and 10989 were observed daily by the Synoptic Optical Long-term Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM) while they traversed the solar disk. We use these measurements and the nonlinear force-free magnetic field code XTRAPOL to reconstruct the coronal magnetic field for each active region and compare model field lines with images from the Solar Terrestrial RElations Observatory (STEREO) and Hinode X-ray Telescope (XRT) telescopes. Synoptic maps made from continuous, round-the-clock Global Oscillations Network Group (GONG) magnetograms provide information on the global photospheric field and potential-field source-surface models based on these maps describe the global coronal field during the Whole Heliospheric Interval (WHI) and its neighboring rotations. Features of the modeled global field, such as the coronal holes and streamer belt locations, are discussed in comparison with extreme ultra-violet and coronagraph observations from STEREO. The global field is found to be far from a minimum, dipolar state. From the nonlinear models we compute physical quantities for the active regions such as the photospheric magnetic and electric current fluxes, the free magnetic energy and the relative helicity for each region each day where observations permit. The interconnectivity of the three regions is addressed in the context of the potential-field source-surface model. Using local and global quantities derived from the models, we briefly discuss the different observed activity levels of the regions.Comment: Accepted for publication in the Solar Physics Whole Heliospheric Interval (WHI) topical issue. We had difficulty squeezing this paper into arXiv's 15 Mb limit. The full paper is available here ftp://gong2.nso.edu/dsds_user/petrie/PetrieCanouAmari.pd

Fine particles production in sand under triaxial compression

An experimental study, carried out in laboratory using a triaxial apparatus, proposes to evaluate the influence of grain size, grain shape and confining stress in fine particles production in sand specimen subjected to triaxial loading. 16 triaxial tests were conducted on specimens of an initial density index of 0.90 using the pluviation technique, with three different sands and under initial confining pressure of 100, 400, 800 and 1200 kPa. The experimental results show that the fine particles production is considerably affected by confining stresses, even at low values (100 kPa), and also by grain geometry. The experimental results show also that the fine particles production increase with the strain level during the shearing test

Re-flaring of a Post-Flare Loop System Driven by Flux Rope Emergence and Twisting

In this letter, we study in detail the evolution of the post-flare loops on 2005 January 15 that occurred between two consecutive solar eruption events, both of which generated a fast halo CME and a major flare. The post-flare loop system, formed after the first CME/flare eruption, evolved rapidly, as manifested by the unusual accelerating rise motion of the loops. Through nonlinear force-free field (NLFFF) models, we obtain the magnetic structure over the active region. It clearly shows that the flux rope below the loops also kept rising accompanied with increasing twist and length. Finally, the post-flare magnetic configuration evolved to a state that resulted in the second CME/flare eruption. This is an event in which the post-flare loops can re-flare in a short period of $\sim$16 hr following the first CME/flare eruption. The observed re-flaring at the same location is likely driven by the rapid evolution of the flux rope caused by the magnetic flux emergence and the rotation of the sunspot. This observation provides valuable information on CME/flare models and their prediction.Comment: 11 pages, 4 figures, 1 table, accepted for publication in ApJ Lette

Influence of the cyclic loading of very large number of cycles on the pile capacity

The paper presents a study of the bearing capacity of a pile model in calibration chamber, under a cyclic axial loading, up to very large number of cycles. The experimental study is aimed to the evolution of the bearing capacity of pile obtained after a cyclic loading up to 105 cycles. Firstly, we present briefly the experimental device and then the results of cyclic tests performed on the pile model in the calibration chamber.  Several parameters which have a significant influence on the cyclic behavior of piles have been investigated, such as the cyclic displacement amplitude and the density of the soil. The analysis of results indicated that the mechanism of densification of the soil around the pile leads to improve pile capacity