FEM × DEM modelling of cohesive granular materials: Numerical homogenisation and multi-scale simulations

The article presents a multi-scale modelling approach of cohesive granular materials, its numerical implementation and its results. At microscopic level, Discrete Element Method (DEM) is used to model dense grains packing. At the macroscopic level, the numerical solution is obtained by a Finite Element Method (FEM). In order to bridge the micro- and macro-scales, the concept of Representative Elementary Volume (REV) is applied, in which the average REV stress and the consistent tangent operators are obtained in each macroscopic integration point as the results of DEM’s simulation. In this way, the numerical constitutive law is determined through the detailed modelling of the microstructure, taking into account the nature of granular materials. We first elaborate the principle of the computation homogenisation (FEM × DEM), then demonstrate the features of our multiscale computation in terms of a biaxial compression test. Macroscopic strain location is observed and discussed.     

边坡开挖破坏过程的离心模型试验研究

边坡开挖是工程中经常遇到的问题。在清华大学土工离心机上,开发了一种离心机运转过程中的边坡开挖模拟技术。进行了土坡开挖的离心模型试验,测量了开挖过程中边坡的位移场变化。结果表明,边坡的破坏过程是从坡体下部开始的剪切带逐渐向上发展贯通的过程,并经历了滑裂面贯通前的平缓变形和贯通后变形迅速增大两个阶段。坡顶裂缝最初均是由受拉引起的,但随着剪切带的不断向上发展,裂缝的扩展是由拉伸和剪切两种作用综合引起的。     

开挖条件下含水平砂土夹层边坡破坏模式研究

工程中经常遇到边坡中含有夹层的情况,对含夹层边坡的稳定性研究很有意义。目前的研究多着重于含夹层岩质边坡的稳定性,对土质边坡涉及较少,尚无系统的试验分析。开挖条件下含夹层土质边坡更容易发生破坏,对其破坏模式的正确认知有助于对其稳定性进行合理的安全评价。进行了含水平砂土夹层边坡开挖的离心模型试验,夹层位于边坡中部。通过与边界和试验条件相同的素土边坡进行对比,分析了中部夹砂层边坡开挖后的破坏模式。试验结果表明,中部砂土夹层的存在降低了边坡在开挖后的稳定性,且其下边界是边坡破坏的薄弱控制面     

离心场中风力电机桩基复杂加载测试设备的研制

桩基础是近海风力发电中经常采用的基础形式,承受着包括低频风循环荷载在内的复杂荷载组合。为了采用离心模型试验来研究风机桩基础特性,研制了离心场中风机桩基础复杂加载测试设备。该套设备除了可以施加水平向和竖向静力荷载外,还实现了离心场中长时间低频率的水平循环加载,并能够实现多种荷载同时或者先后施加。通过一系列测试试验,验证了该套设备的有效性,满足离心模型试验中对模拟风机桩基础的复杂荷载要求。初步试验结果表明,桩顶水平位移随着水平荷载循环周数的增加而增大,但增长速率逐渐减小,至一定周数后,桩顶位移趋于稳定,土体变形主要集中在桩周一定区域内     

水位下降条件下土钉加固土坡破坏特性研究

水位下降是导致滑坡的重要原因之一,而土钉是加固土坡的有效手段。进行离心模型试验,再现了水位下降时土钉加固土坡的变形和破坏过程,测量了土坡的位移变化。试验结果表明,水位下降条件下土钉加固土坡的破坏模式以绕钉破坏为主,滑裂面从坡顶逐渐向下发展至坡面;土钉加固土坡的破坏过程与变形局部化过程表现出显著的耦合变化;土钉加固机制主要表现为通过土钉与坡体的相互作用,减小土坡的变形和变形局部化程度,从而提高土坡的稳定性。增加土钉长度使得滑裂面向坡内部移动,显著减小土坡的变形及变形局部化程度,从而提高了土坡安全性。     

Reliable Procedure for Molecular Analysis of Airborne Microflora in Three Indoor Environments: An Office and Two Different Museum Contexts

Biological aerosols from air constitute a significant source of exposure to microorganisms in public places. Airborne microorganisms are involved in the development of certain respiratory symptoms, allergies, or infections among users and occupants. Various sampling instruments have commonly been used in aerobiology to collect bacteria and fungi suspended in the air. The objective of this study was to develop a reliable procedure for sampling in indoor public environments presenting different levels of occupancy, airborne bacteria and fungi to be subjected to molecular analysis (bacteria and fungi quantitative PCR, capillary electrophoresis single strand conformation polymorphism fingerprinting). Four different sampling devices were tested in situ in an office building (open‐plan type) and the sampling strategy chosen was tested in two museum contexts. In accordance with the drawbacks involved to our study (quantitative and qualitative aspects, cost, and overcrowding), cyclone device appeared to be most suitable. The results underline the effectiveness of this high‐volume aerosol sampling device for both qualitative and quantitative molecular analysis. Four in situ sampling collections were carried out in 1 day in the Louvre Museum to study quantitative and qualitative variations of airborne bacterial and fungal diversity. The quantitative results revealed a similar order of magnitude for the numbers of both bacteria and fungi. In the Louvre Museum, the samples yielded between 3.7 × 10⁴ and 4.1 × 10⁴ genome equivalent (GE) bacteria/m³ air and between 5.0 × 10⁴ and 5.9 × 10⁴ GE fungi/m³ air and in the Decorative Arts Museum between, 2.1 × 10⁴ and 2.5 × 10⁴ GE bacteria/m³ air and between 1.4 × 10⁴ and 1.7 × 10⁴ GE fungi/m³ air. The results also indicate that the dominant bacterial community displayed a stable structure over a short period of time whereas dominant eukaryotic airborne community appeared more variable.     

Structure and nuclear density distribution in the cheralite—CaTh(PO4)2: studies of its behaviour under high pressure (36?GPa)

The crystal structure of the cheralite—CaTh(PO₄)₂—has been revisited by neutron diffraction and its behaviour under high pressure investigated by X-ray diffraction up to 36?GPa. The neutron diffraction data at ambient pressure gave a more accurate determination of the Ca/Th cation position than previous XRD data, taking advantage that the neutron scattering lengths of calcium and thorium are of same order of magnitude. The nuclear density distribution was also determined using the maximum entropy method (MEM) confirming that the two cations are not located at the same position in the unit cell but are slightly displaced from one another along a specific direction in order to minimize the electrostatic repulsion with the surrounding phosphorus atoms. At high pressure, the compound did not show any phase transition or amorphization. From the evolution of the unit-cell volume as a function of the pressure, the zero-pressure bulk modulus B₀ and its pressure derivative B₀ ^′ have been determined by fitting the experimental compressibility curve to the Birch–Murnaghan equation of state. The results are B₀?=?140(2) GPa and B ₀ ^′ ?=?4.4(4) GPa.     

Present-day trends of vertical ground motion along the coast lines

Vertical ground motion (VGM) rates stand as crucial information, either for predicting the impact of the actual sea level rise along low-lying coasts or refining geodynamic problems. Because present day VGM rates have a magnitude smaller than 10 mm/yr, they remain challenging to quantify and often elusive. We focus on the quantification of global-scale VGM rates in order to identify global or regional trends. We computed VGM rates by combining tide gauges records and local satellite altimetry, which yield a new dataset of 634 VGM rates. We further compare this database to previous studies that use geodetic techniques and tide gauges records in order to evaluate the consistency of both our results and previous ones. The magnitudes differ by less than 5 mm/yr, and similar subsidence and uplift general tendencies appear. Even if the asset of our database stands in the greater number of sites, the combination of all studies, each with different pros and cons, yields a hybrid dataset that makes our attempt to extract VGM trends more robust than any other, independent study. Fennoscandia, the West coast of North America, and the eastern coast of Australia are uplifting, while the eastern coast of North America, the British Isles and Western Europe, the eastern Mediterranean Sea, Japan, and the western coast of Australia are subsiding. Glacial Isostatic Adjustment (GIA) is expected to provide a major contribution to the present-day signal. Aside from Fennoscandia, observed VGM often depart from the GIA model predictions of Peltier (2004). This either results from an underestimate of the model predictions or from the influence of other processes: indeed, the influence of the geodynamic setting appears in particular along the coasts of western North America or Japan, where the alternation of transform faults and subduction zones makes it possible to assign contrasted behaviours to the local geodynamic context. Local mechanisms like anthropogenic processes or sediment compaction, also contribute to VGM. This remains true for the critical cases of Venice, the Gulf of Mexico, the Ganges delta, and the Maldives, which are particularly exposed to the current sea level rise.     

Ontogenetic Habitat Shifts of the Atlantic Tomcod (<Emphasis Type="Italic">Microgadus tomcod</Emphasis>) Across an Estuarine Transition Zone

Tomcod (Microgadus tomcod) in the St. Lawrence estuarine transition zone (ETZ) undergo an ontogenetic habitat shift. Older age classes, characterised by a male-dominated sex ratio, disperse downstream over the summer months to occupy the colder more saline waters of the estuary. Significant differences in length and mass along the salinity gradient were observed in September with upstream fish of any given age class generally exhibiting greater growth. These differences were not seen in early summer. Benthic amphipod δ ³⁴S signatures were strongly correlated with salinity and served to demonstrate that tomcod δ ³⁴S signatures were not in isotopic equilibrium in the more saline waters of the ETZ. Seasonal distributional patterns, growth dynamics and isotopic disequilibrium all indicate that the observed habitat shift may occur on an annual basis, following winter aggregation in warmer, less saline waters. Tomcod located in the downstream parts of the ETZ, predominantly males, were significantly more sexually developed than upstream tomcod for a given age. On the other hand, greater growth early in life is insured by occupying warmer, upstream waters during the summer months.