Statistical mechanics approaches to granular media: between micromechanics and macromechanics

The mechanical behavior of granular materials has been traditionally approached through two theoretical and computational frameworks: macromechanics and micromechanics. Macromechanics focuses on continuum based models. In consequence it is assumed that the matter in the granular material is homogeneous and continuously distributed over its volume so that the smallest element cut from the body possesses the same physical properties as the body. In particular, it has some equivalent mechanical properties, represented by complex and non-linear constitutive relationships. Engineering problems are usually solved using computational methods such as FEM or FDM. On the other hand, micromechanics is the analysis of heterogeneous materials on the level of their individual constituents. In granular materials, if the properties of particles are known, a micromechanical approach can lead to a predictive response of the whole heterogeneous material. Two classes of numerical techniques can be differentiated: computational micromechanics, which consists on applying continuum mechanics on each of the phases of a representative volume element and then solving numerically the equations, and atomistic methods (DEM), which consist on applying rigid body dynamics together with interaction potentials to the particles. Statistical mechanics approaches arise between micro and macromechanics. It tries to state which the expected macroscopic properties of a granular system are, by starting from a micromechanical analysis of the features of the particles and the interactions. The main objective of this paper is to introduce this approach

Estudio mediante modelización numérica del efecto del estado tensional inicial del terreno en la carga de rotura de un ensayo trapdoor

Uno de los fenómenos que tiene más relevancia en la estabilidad de excavaciones subterráneas es el denominado efecto arco. Su estudio se puede llevar a cabo mediante el ensayo ?trapdoor?, conocido principalmente a partir de su presentación por Terzaghi en la primera ICSMFE en 1936 [1], aunque ya con alguna referencia en el siglo XIX. En este trabajo se lleva a cabo un estudio del ensayo "trapdoor", mediante modelización numérica, con el objetivo de analizar el efecto que tiene sobre la formación del arco estable el estado tensional inicial del terreno. Para ello se ha realizado un análisis paramétrico con el programa de diferencias finitas FLAC de ITASCA. La modelización numérica posibilita este tipo de estudios al poder fijar y variar cómodamente todos los parámetros que definen el ensayo. Pero, además, permite un exhaustivo seguimiento del proceso de rotura, por lo que se facilita el análisis de los resultados obtenidos. En un ensayo ?trapdoor? la carga última es función de la geometría del problema y de las propiedades del material, particularmente de su dilatancia. No obstante, los resultados de este trabajo muestran que el estado tensional del terreno determina, conjuntamente con el resto de parámetros del material, el proceso de rotura. Así, para situaciones con una relación entre la tensión horizontal y vertical superior a 0.5, se forma un primer arco estable de dimensiones reducidas que hace que se presente un mínimo en la carga de rotura, lo cual no ocurre en caso contrario. En consecuencia, se justifica que el estado tensional inicial del terreno incide claramente en la presión necesaria para estabilizar una excavación subterránea

Equations of state in soil compression based on statistical mechanics

The authors have published a very interesting contribution in which they present a new and general relationship between void ratio and overburden pressure in the compression process of soils. The study is based on the law of interparticle energy distribution used in statistical mechanics and, in particular, the compression process of a soil is described according to its initial and final void ratios, and to a parameter /? which is related to the potential energy of a soil element. Such potential energy depends not only on the mass and elevation of soil particles, but also on the interactions between them. To account for such interactions, the authors introduce the concept of "imaginary particles", which allows them to use empirically-calibrated P values in the formulation without the need to consider interparticle interactions explicitly. In fact, the results presented by the authors show that such approach reproduces successfully the compression behaviour of a wide range of situations and soils

Statistical distributions obtained from the compression of monodisperse, soft and frictionless particles

The cyclic compression of several granular systems has been simulated with a molecular dynamics code. All the samples consisted of bidimensional, soft, frictionless and equal-sized particles that were initially arranged according to a squared lattice and were compressed by randomly generated irregular walls. The compression protocols can be described by some control variables (volume or external force acting on the walls) and by some dimensionless factors, that relate stiffness, density, diameter, damping ratio and water surface tension to the external forces, displacements and periods. Each protocol, that is associated to a dynamic process, results in an arrangement with its own macroscopic features: volume (or packing ratio), coordination number, and stress; and the differences between packings can be highly significant. The statistical distribution of the force-moment state of the particles (i.e. the equivalent average stress multiplied by the volume) is analyzed. In spite of the lack of a theoretical framework based on statistical mechanics specific for these protocols, it is shown how the obtained distributions of mean and relative deviatoric force-moment are. Then it is discussed on the nature of these distributions and on their relation to specific protocols

Chemometric study on the forensic discrimination of soil types using their infrared spectral characteristics

Soil has been utilized in criminal investigations for some time because of its prevalence and transferability. It is usually the physical characteristics that are studied, however the research carried out here aims to make use of the chemical profile of soil samples. The research we are presenting in this work used sieved (2mm) soil samples taken from the top soil layer (about 10cm) that were then analysed using mid infrared spectroscopy. The spectra obtained were pre-treated and then input into two chemometric classification tools: Nonlinear iterative partial least squares followed by linear discriminant analysis (NIPALS-LDA) and partial least squares discriminant analysis (PLS-DA). The models produced show that it is possible to discriminate between soil samples from different land use types and both approaches are comparable in performance. NIPALS-LDA performs much better than PLS-DA in classifying samples to locatio

Síntesis y caracterización de óxido de titanio usando química verde para su aplicación en celdas fotovoltaicas simuladas por SCAPS-ID

"En esta tesis de doctorado se trabajó con la síntesis de nanopartículas de óxido de titanio usando química verde mediante la técnica de sol-gel. Se utilizaron diferentes extractos etanólicos de plantas de cola de caballo (Equisetum Arvense), clavo (Syzygium aromaticum) y té verde (Camellia Sinensis). Se realizó un estudio morfológico sobre la influencia de los fitoquímicos en el proceso de obtención de las nanopartículas de óxido de Titanio. Se discuten las propiedades estructurales, morfológicas, composicionales y ópticas de las nanopartículas de TiO2 preparadas con extractos etanólicos mediante un tratamiento térmico a 550 °C. El resultado más satisfactorio se logró utilizando extractos de E. arvense debido a que se tuvo un rango de tamaño de nanopartículas de 20-50 nm. Las nanopartículas de TiO2 estaban presentes solo en la fase cristalina de anatasa, con una banda prohibida de 3,22 eV, según lo confirmado por análisis de difracción de rayos X y UV-vis, respectivamente. Posteriormente, con el parámetro obtenido del gap de energía se realizó simulaciones de celdas fotovoltaicas en un simulador llamado SCAPS-1D. Se diseñaron diferentes configuraciones de celdas fotovoltaicas de carácter superestrato, en el cual el TiO2 es la capa transportadora de electrones para cada una de celdas simuladas"

A genetic algorithm for slope stability analyses with concave slip surfaces using custom operators

Heuristic methods are popular tools to find critical slip surfaces in slope stability analyses. A new genetic algorithm (GA) is proposed in this work that has a standard structure but a novel encoding and generation of individuals with custom-designed operators for mutation and crossover that produce kinematically feasible slip surfaces with a high probability. In addition, new indices to assess the efficiency of operators in their search for the minimum factor of safety (FS) are proposed. The proposed GA is applied to traditional benchmark examples from the literature, as well as to a new practical example. Results show that the proposed GA is reliable, flexible and robust: it provides good minimum FS estimates that are not very sensitive to the number of nodes and that are very similar for different replication

The forensic analysis of soil by FTIR with multivariate analysis

Over the past few years more and more studies have been carried out in an attempt to utilize chemical profiles of soil using a wide variety of analytical methods. The value of soil as evidence rests with its prevalence at crime scenes and its transferability between the scene and the criminal. This can be of value for comparison if the scene of crime is known, but could also be so in the identification of a scene. The main basis for the comparison of sites to determine provenance is that soils vary from one place to another. The aim of this work is to find simple methods to identify soil provenance based on FTIR and multivariate analysi

Analytical bearing capacity of strip footing in weightless materials with power-law failure criteria

Sokolovskii’s method of characteristics is extended to provide analytical solutions for the ultimate load at the moment of plastic failure under plane-strain conditions of shallow strip foundations on weightless rigid-plastic media with a noncohesive power-law failure envelope. The formulation is made parametrically in terms of the instantaneous friction angle, and the key idea to obtain the bearing capacity is that information can be transmitted from the free surface (where external loads are known) to the contact plane of the foundation. The methodology can consider foundations adjacent to a slope, external surcharges at the free surface, and inclined loads (both on the slope and on the foundation). Sensitivity analyses illustrate the influence on bearing capacity of changes in the different geometrical parameters involved. An application example is presented and design plots are provided, and model predictions are compared with results of bearing capacity tests under low gravity

La visita a campo como metodología para el proceso de enseñanza-aprendizaje

Dentro de la enseñanza de la geotecnia los viajes a campo son una herramienta útil para superar las limitaciones asociadas a la enseñanza en el aula así como para promover el autoaprendizaje del alumno, el cual se enfrenta en primera persona a la información en estado bruto. Mediante esta comunicación compartimos la experiencia de la visita a las obras de construcción de los Túneles de Sorbas y El Almendral dentro del Máster de "Geología Aplicada a la Obra Civil y los Recursos Hídricos" ofertado por la Universidad de Granada, comentando, con un enfoque docente, la planificación de la actividad en función de los resultados de aprendizaje deseados. Fieldtrips are a good tool to overcome the inherent difficulties associated to teaching engineering geology at the classroom and to encourage student self-learning, when they face raw data. In this paper, we share our recent experience with the organization of a fieldtrip to two tunneling construction site (Sorbas Tunnel and El Almendral Tunnel) for the MSc program of “Applied Geology in Civil Engineering and Water Resources” offered by the University of Granada, discussing, with a educational point of view, the planning and learning outcomes