Analysis of particle contact using frustrated total internal reflection

Within the field of soil mechanics a continuum assumption is generally adopted in order to avoid the complications of modelling micro-mechanical behaviour. However, certain constitutive behaviour can only be explained by investigating particle level interactions. Numerical investigations, such as those using the Discrete Element Method (DEM) to model soil particles as clusters of spheres, have delivered a greater understanding of the micro-mechanical behaviour. One of the limiting factors in current DEM approaches is modelling of the particle–particle or particle–surface contact behaviour. Hence, an experimental methodology has been developed and used to study particle–surface contact behaviour. The experimental methodology involves loading particles onto a piece of sapphire glass and observing the resulting contact area. In order to distinguish between the contacted area and the rest of the particle, the principle of frustrated total internal reflection and evanescent waves was used which results in only objects in very close proximity to the glass being illuminated and visible. This methodology hence allows the number of contacts and the area of those contacts to be tracked during loading and over time. This paper presents the validation of the experimental methodology by comparing the observed contact behaviour of plastic beads against Hertzian contact theory. In addition, the results from tests on sand samples are presented which show a density of 0.40 and 0.80 contacts per D250 for coarse and fine grained sand respectively at an isotropic stress state which subsequently increases to 0.90 to 1.00 contacts per D250 at peak deviatoric stress. It was also found that the fine sand particle contacts carried a maximum load of approximately 0.27 N per contact whereas the coarser sand was able to carry substantially higher loads

Geocellular railway drainage systems: physical and numerical modelling

The importance of resilient railway infrastructure is paramount when considering the increased likelihood of extreme weather and flash flood events in coming years. One of the main causes of instability of railway tracks is excess water in the trackbed, particularly when it is at or above the interface of the ballast and subgrade. Conventional drainage systems are susceptible to clogging and deterioration. Resilient track drainage systems should therefore have sufficient capacity to allow water to dissipate quickly, but they should also be designed to ensure long-term operation with minimal or easily performed maintenance. This paper presents results from an investigation of a potential new railway drainage system using geocellular components. In the paper, the development of a large scale physical model is described which represents a full scale unit cell of a sleeper-to-sleeper track substructure. The physical model includes ballast and subgrade layers, under-track and lateral drainage systems, rainfall simulation, and instrumentation. Results demonstrate the relative hydraulic response of the drainage system with and without the geocellular components. The paper also describes the development of a numerical model of the track subgrade and drainage system, which was first calibrated and verified using experimental data from the physical model, then extended to study the effect of certain parameters on the hydraulic response of the railway track. Results indicate that the under-track geocellular drainage system offers potential benefits in terms of maintaining a lower water table level within the subgrade as well as in aiding the migration of fines out of the ballast

Experimental characterisation of clay-sand mixtures treated with lime

peer reviewedLime stabilisation is a process which chemically improves numerous characteristics of the soils, among which aspects, the mechanical properties increase are the most noticeable. This paper investigates the lime treatment of mixtures of sand and bentonite at different proportions. Three physical characteristics are measured at different curing times: the mechanical strength is deduced from unconfined compressive strength (UCS); the chemical reaction is evaluated based on the lime consumption (LC) and the electrical properties are obtained from electrical resistivity (ER). Results show that all three physical characteristics increase with curing time and that the optimal UCS appears at mixtures with low bentonite content (i.e. 10–15% bentonite). LC curves show that the kinetics of the lime reaction is also slower at these low bentonite contents. Finally, the ER curves show similar patterns for all mixtures and are closely related to the LC

Analytical solution of discontinuous heat extraction for sustainability and recovery aspects of borehole heat exchangers

peer reviewedExisting analytical solutions for thermal analysis of ground source heat pump (GSHP) systems evaluate temperature change in the carrier-fluid and the surrounding ground in the production period of a single borehole heat exchanger (BHE) only if a continuous heat load is assigned. In the present study, we modified the Green's function, which is the solution of heat conduction/advection/dispersion equation in porous media, for discontinuous heat extraction by analytically convoluting rectangular function or pulses in time domain both for single and multi-BHEs field. The adapted analytical models for discontinuous heat extraction are verified with numerical finite element code. The comparison results agree well with numerical results both for conduction and advection dominated heat transfer systems, and analytical solutions provide significantly shorter runtime compared to numerical simulations (approx. 1500 times shorter). Furthermore, we investigated the sustainability and recovery aspects of GSHP systems by using proposed analytical models under different hydro-geological conditions. According to the engineering guideline VDI 4640, a linear relationship between thermal conductivity of the ground and the sustainable heat extraction rate is demonstrated for multi-BHEs

A tomographic imagery segmentation methodology for three-phase geomaterials based on simultaneous region growing

peer reviewedX-Ray Computed Tomography (X-Ray CT) is a powerful non-destructive technique used in many domains to obtain the three-dimensional representation of objects, starting from the reconstitution of two-dimensional images of radiographic scanning. This technique is now able to analyze objects within a few microns resolution. Consequently, X-Ray micro-computed tomography (X-Ray μCT) opens perspectivesfor the analysis of the fabric of multi-phase geomaterials such as soils, concretes, rocks or ceramics. To be able to characterize the spatial distribution of the different phases in such complex and disordered materials, automated phase recognition has to be implemented through image segmentation. A crucial difficulty in segmenting images lies in the presence of noise in the obtained tomographic representation, making it difficult to assign a specific phase to each voxel (vx) of the image. In the present study, simultaneous region growing is used to reconstitute the three-dimensional segmented image of granular materials. First, based on a set of expected phases in the image, regions where specific phases are sure to be present are identified, leaving uncertain regions of the image unidentified. Subsequently, the identified regions are grown until growing phases meet each other with vanishing unidentified regions. The methodrequires a limited number of manual parameters that are easily determined. The developed method is illustrated based on three applications on granular materials, comparing the phase volume fractions obtained by segmentation with macroscopic data. It is demonstrated that the algorithm rapidly converges and fills the image after a few iterations

Multi-scale analysis of lime treated sand-bentonite mixtures

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Lime-treatment of sand improved by bentonite addition.

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A general framework for coupled hydro-mechanical modelling of rainfall-induced instability in unsaturated slopes with multivariate random fields

An accurate estimation of rainfall-induced instability of slopes for extremely nonhomogeneous materials such as lignite mine spoils is a major challenge. This paper investigates the stability of nonhomogeneous soil slopes with respect to slip surface development, size of sliding volume, and determination of safety factor. Specified dependent random variables are cross-correlated using a multivariate Gaussian copula, the use of which provides a faster and more accurate representation of the inter-dependent properties of randomly-distributed soil. A Monte-Carlo simulation is used to generate a series of multivariate random fields for slopes. These are then implemented in Abaqus and analysed under constant rainfall conditions using a fully coupled hydro-elasto-plastic model. The resulting stress, strain, pore pressure, and displacement data are further processed in MATLAB to evaluate critical slip surfaces and safety factors. Results indicate that the factor of safety in a homogenous case is overestimated compared to the nonhomogeneous condition, while the sliding volume is underestimated. Moreover, the factor of safety decreases as the rainfall simulation continues and the probability of failure increases to nearly 100% after 10 days of rainfall. The framework developed in this paper can provide guidance for conducting relatively inexpensive probabilistic analyses

Two-scale geomechanical characterization of sand-bentonite mixtures treated with lime

The use of lime for soil stabilization has greatly increased since the second half of the 20th century. A lot of research has been conducted to understand the mechanisms of stabilization. These mechanisms are caused by pozzolanic reactions between lime and clay minerals. However, it has not yet been possible to quantify the factors affecting the evolution of these reactions. The variety of soils and the disruptive elements do not allow comparing these soils with each other and giving any quantitative and generalized conclusions in terms of mechanical improvement. The goal of this study is to build a progressive understanding of this phenomenon by avoiding any disruptive elements and controlling most of the parameters. Consequently, the choice is made to study a synthetic soil by controlling its particle size distribution and composition. This soil is a mixture of sand and bentonite taken at different compositions. An analysis of the evolution of lime treatment on such model soils is carried out on two scales: the macroscopic scale and the microscopic scale. The macroscopic scale studies the evolution through unconfined compressive strength, lime consumption, electrical resistivity as well as complementary studies such as sonic and triaxial tests. Results from macroscopic tests show that sand takes an important part into soil stabilization, meaning that a soil containing a lot of clay does not necessary give the best long term mechanical characteristics. Tests that allow a much more detailed comprehension of stabilization are also presented. Microscopic evolution is studied through X-Ray Computed Tomography and Mercury Intrusion Porosimetry. A study on tomographic image treatment has also been carried out to segment the images from its different constituents.L'utilisation de la chaux pour stabiliser le sol s'est considérablement déve-lop-pée depuis la seconde moitié du 20e siècle. De nombreuses recherches ont vu le jour pour comprendre les mécanis-mes de cette stabilisation. Ces mé-ca-nis-mes sont causés par les réactions pouzzolaniques entre la chaux et les minéraux argileux. Cependant, il n'a pas encore été possible de quantifier totalement les facteurs influençant le déroulement de ces réactions. La diversité des sols et la présence d'éléments perturbateurs ne permettent pas de les comparer et d'en tirer des conclusions quantitatives et généralisables en termes d'amélioration des paramètres mécaniques. Cette étude envisage donc de construire une compréhension progressive du phénomène en ne prenant pas en compte les éléments perturbateurs et en contrôlant à priori un maximum de paramètres. Pour ce faire, il est proposé d'étudier un sol synthétique dont la granulométrie et la composition peuvent être contrôlées. Ce sol est un mélange de sable et de bentonite pris à différentes compositions. Une analyse sur l'évolution du traitement à la chaux est alors effectuée sur ces mélanges sur deux échelles :l'échelle macroscopique et l'échelle microscopique. L'échelle macroscopique envisage des essais de compression simple, de consommation de chaux, de résistivité électrique ainsi que des essais complémentaires tels que les essais soniques et triaxiaux. Les résultats macroscopiques montrent que le sable joue un rôle important dans la stabilisation, le sol présentant la fraction argileuse la plus importante n'ayant pas les meilleures caractéristiques mécaniques à long terme. Les tests permettant une compréhension plus fine de cette stabilisation sont également présentés. L'échelle microscopique est étudiée via la tomographie aux rayons-X et la porosimétrie au mercure. Une étude sur le traitement des images tomographiques est aussi mise en oeuvre pour segmenter de manière adéquate les images de ses différents constituants.Doctorat en Sciences de l'ingénieur et technologieinfo:eu-repo/semantics/nonPublishe

Two-scale geomechanical characterization of sand-bentonite mixtures treated with lime

The use of lime for soil stabilization has greatly increased since the second half of the 20th century. A lot of research has been conducted to understand the mechanisms of stabilization. These mechanisms are caused by pozzolanic reactions between lime and clay minerals. However, it has not yet been possible to quantify the factors affecting the evolution of these reactions. The variety of soils and the disruptive elements do not allow comparing these soils with each other and giving any quantitative and generalized conclusions in terms of mechanical improvement. The goal of this study is to build a progressive understanding of this phenomenon by avoiding any disruptive elements and controlling most of the parameters. Consequently, the choice is made to study a synthetic soil by controlling its particle size distribution and composition. This soil is a mixture of sand and bentonite taken at different compositions. An analysis of the evolution of lime treatment on such model soils is carried out on two scales: the macroscopic scale and the microscopic scale. The macroscopic scale studies the evolution through unconfined compressive strength, lime consumption, electrical resistivity as well as complementary studies such as sonic and triaxial tests. Results from macroscopic tests show that sand takes an important part into soil stabilization, meaning that a soil containing a lot of clay does not necessary give the best long term mechanical characteristics. Tests that allow a much more detailed comprehension of stabilization are also presented. Microscopic evolution is studied through X-Ray Computed Tomography and Mercury Intrusion Porosimetry. A study on tomographic image treatment has also been carried out to segment the images from its different constituents.L'utilisation de la chaux pour stabiliser le sol s'est considérablement déve-lop-pée depuis la seconde moitié du 20e siècle. De nombreuses recherches ont vu le jour pour comprendre les mécanis-mes de cette stabilisation. Ces mé-ca-nis-mes sont causés par les réactions pouzzolaniques entre la chaux et les minéraux argileux. Cependant, il n'a pas encore été possible de quantifier totalement les facteurs influençant le déroulement de ces réactions. La diversité des sols et la présence d'éléments perturbateurs ne permettent pas de les comparer et d'en tirer des conclusions quantitatives et généralisables en termes d'amélioration des paramètres mécaniques. Cette étude envisage donc de construire une compréhension progressive du phénomène en ne prenant pas en compte les éléments perturbateurs et en contrôlant à priori un maximum de paramètres. Pour ce faire, il est proposé d'étudier un sol synthétique dont la granulométrie et la composition peuvent être contrôlées. Ce sol est un mélange de sable et de bentonite pris à différentes compositions. Une analyse sur l'évolution du traitement à la chaux est alors effectuée sur ces mélanges sur deux échelles :l'échelle macroscopique et l'échelle microscopique. L'échelle macroscopique envisage des essais de compression simple, de consommation de chaux, de résistivité électrique ainsi que des essais complémentaires tels que les essais soniques et triaxiaux. Les résultats macroscopiques montrent que le sable joue un rôle important dans la stabilisation, le sol présentant la fraction argileuse la plus importante n'ayant pas les meilleures caractéristiques mécaniques à long terme. Les tests permettant une compréhension plus fine de cette stabilisation sont également présentés. L'échelle microscopique est étudiée via la tomographie aux rayons-X et la porosimétrie au mercure. Une étude sur le traitement des images tomographiques est aussi mise en oeuvre pour segmenter de manière adéquate les images de ses différents constituants.Doctorat en Sciences de l'ingénieur et technologieinfo:eu-repo/semantics/nonPublishe