La estabilidad del frente de excavación de un túnel en terrenos heterogéneos o con un criterio de rotura no-lineal

El frente de un túnel puede colapsar si la presión aplicada sobre el es inferior a un valor limite denominado presión “critica” o “de colapso”. En este trabajo se desarrolla y presenta un mecanismo de rotura rotacional generado punto a punto para el cálculo de la presión de colapso del frente de túneles excavados en terrenos estratificados o en materiales que siguen un criterio de rotura nolineal. La solución propuesta es una solución de contorno superior en el marco del Análisis Límite y supone una generalización del mecanismo de rotura mas reciente existente en la bibliografía. La presencia de un terreno estratificado o con un criterio de rotura no-lineal implica una variabilidad espacial de las propiedades resistentes. Debido a esto, se generaliza el mecanismo desarrollado por Mollon et al. (2011b) para suelos, de tal forma que se puedan considerar valores locales del ángulo de rozamiento y de la cohesión. Además, la estratificación del terreno permite una rotura parcial del frente, por lo que se implementa esta posibilidad en el mecanismo, siendo la primera solución que emplea un mecanismo de rotura que se ajusta a la estratigrafía del terreno. Por otro lado, la presencia de un material con un criterio de rotura no-lineal exige introducir en el modelo, como variable de estudio, el estado tensional en el frente, el cual se somete al mismo proceso de optimización que las variables geométricas del mecanismo. Se emplea un modelo numérico 3D para validar las predicciones del mecanismo de Análisis Limite, demostrando que proporciona, con un esfuerzo computacional significativamente reducido, buenas predicciones de la presión critica, del tipo de rotura (global o parcial) en terrenos estratificados y de la geometría de fallo. El mecanismo validado se utiliza para realizar diferentes estudios paramétricos sobre la influencia de la estratigrafía en la presión de colapso. Igualmente, se emplea para elaborar cuadros de diseño de la presión de colapso para túneles ejecutados con tuneladora en macizos rocosos de mala calidad y para analizar la influencia en la estabilidad del frente del método constructivo. Asimismo, se lleva a cabo un estudio de fiabilidad de la estabilidad del frente de un túnel excavado en un macizo rocoso altamente fracturado. A partir de el se analiza como afectan las diferentes hipótesis acerca de los tipos de distribución y de las estructuras de correlación a los resultados de fiabilidad. Se investiga también la sensibilidad de los índices de fiabilidad a los cambios en las variables aleatorias, identificando las mas relevantes para el diseño. Por ultimo, se lleva a cabo un estudio experimental mediante un modelo de laboratorio a escala reducida. El modelo representa medio túnel, lo cual permite registrar el movimiento del material mediante una técnica de correlación de imágenes fotográficas. El ensayo se realiza con una arena seca y se controla por deformaciones mediante un pistón que simula el frente. Los resultados obtenidos se comparan con las estimaciones de la solución de Análisis Límite, obteniéndose un ajuste razonable, de acuerdo a la literatura, tanto en la geometría de rotura como en la presión de colapso. A tunnel face may collapse if the applied support pressure is lower than a limit value called the ‘critical’ or ‘collapse’ pressure. In this work, an advanced rotational failure mechanism generated ‘‘point-by-point” is developed to compute the collapse pressure for tunnel faces in layered (or stratified) grounds or in materials that follow a non-linear failure criterion. The proposed solution is an upper bound solution in the framework of limit analysis which extends the most advanced face failure mechanism in the literature. The excavation of the tunnel in a layered ground or in materials with a non-linear failure criterion may lead to a spatial variability of the strength properties. Because of this, the rotational mechanism recently proposed by Mollon et al. (2011b) for Mohr-Coulomb soils is generalized so that it can consider local values of the friction angle and of the cohesion. For layered soils, the mechanism needs to be extended to consider the possibility for partial collapse. The proposed methodology is the first solution with a partial collapse mechanism that can fit to the stratification. Similarly, the use of a nonlinear failure criterion introduces the need to introduce new parameters in the optimization problem to consider the distribution of normal stresses along the failure surface. A 3D numerical model is employed to validate the predictions of the limit analysis mechanism, demonstrating that it provides, with a significantly reduced computational effort, good predictions of critical pressure, of the type of collapse (global or partial) in layered soils, and of its geometry. The mechanism is then employed to conduct parametric studies of the influence of several geometrical and mechanical parameters on face stability of tunnels in layered soils. Similarly, the methodology has been further employed to develop simple design charts that provide the face collapse pressure of tunnels driven by TBM in low quality rock masses and to study the influence of the construction method. Finally, a reliability analysis of the stability of a tunnel face driven in a highly fractured rock mass is performed. The objective is to analyze how different assumptions about distributions types and correlation structures affect the reliability results. In addition, the sensitivity of the reliability index to changes in the random variables is studied, identifying the most relevant variables for engineering design. Finally, an experimental study is carried out using a small-scale laboratory model. The problem is modeled in half, cutting through the tunnel axis vertically, so that displacements of soil particles can be recorded by a digital image correlation technique. The tests were performed with dry sand and displacements are controlled by a piston that supports the soil. The results of the model are compared with the predictions of the Limit Analysis mechanism. A reasonable agreement, according to literature, is obtained between the shapes of the failure surfaces and between the collapse pressures observed in the model tests and computed with the analytical solution.

Análisis mediante elementos finitos del comportamiento de losas de hormigón armado sometidas a explosiones

Se presenta en este trabajo una investigación sobre el comportamiento de losas de hormigón armado sometidas a explosiones y la simulación numérica de dicho fenómeno mediante el método de los elementos finitos. El trabajo aborda el estudio de la respuesta de dichos elementos estructurales por comparación entre los resultados obtenidos en ensayos reales a escala 1:1 y los cálculos realizados mediante modelos de ordenador. Este procedimiento permite verificar la idoneidad, o no, de estos últimos. Se expone en primer lugar el comportamiento mecánico de los modelos de material que son susceptibles de emplearse en la simulación de estructuras mediante el software empleado en la presente investigación, así como las diferentes formas de aplicar cargas explosivas en estructuras modeladas mediante el método de los Elementos Finitos, razonándose en ambos casos la elección llevada a cabo. Posteriormente, se describen los ensayos experimentales disponibles, que tuvieron lugar en las instalaciones del Laboratorio de Balística de Efectos, perteneciente al Instituto Tecnológico de la Marañosa (ITM), de Madrid, para estudiar el comportamiento de losas de hormigón armado a escala 1:1 sometidas a explosiones reales. Se ha propuesto un método de interpretación del nivel de daño en las losas mediante el martillo de Schmidt, que posteriormente permitirá comparar resultados con los modelos de ordenador realizados. Asimismo, se propone un método analítico para la determinación del tamaño óptimo de la malla en las simulaciones realizadas, basado en la distribución de la energía interna del sistema. Es conocido que el comportamiento de los modelos pueden verse fuertemente influenciados por el mallado empleado. Según el mallado sea “grosero” o “fino” el fallo puede no alcanzarse o hacerlo de forma prematura, o excesiva, respectivamente. Es más, algunos modelos de material contemplan una “regularización” del tamaño de la malla, pero en la presente investigación se evidencia que dicho procedimiento tiene un rango de validez limitado, incluso se determina un entorno óptimo de valores. Finalmente, se han elaborado los modelos numéricos con el software comercial LS-DYNA, contemplando todos los aspectos reseñados en los párrafos anteriores, procediendo a realizar una comparación de los resultados obtenidos en las simulaciones con los procedidos en los ensayos reales a escala 1:1, observando que existe una muy buena correlación entre ambas situaciones que evidencian que el procedimiento propuesto en la investigación es de todo punto adecuado para la simulación de losas de hormigón armado sometidas a explosiones. ABSTRACT This doctoral thesis presents an investigation on the behavior of reinforced concrete slabs subjected to explosions along with the numerical simulation of this phenomenon by the finite elements method. The work involves the study of the response of these structural elements by comparing the results of field tests at full scale and the calculations performed by the computer model. This procedure allows to verify the appropriateness or not of the latter. Firstly, the mechanical behavior of the material models that are likely to be used in the modelling of structures is explained. In addition, different ways of choosing explosive charges when conducting finite element methods are analyzed and discussed. Secondly, several experimental tests, which took place at the Laboratorio de Balística de Efectos at the Instituto Tecnológico de la Marañosa (ITM), in Madrid, are described in order to study the behavior of these reinforced concrete slabs. A method for the description of the slab damage level by the Schmidt hammer is proposed, which will make possible to compare the modelling results extracted from the computation experiments. Furthermore, an analytical method for determining the optimal mesh size to be used in the simulations is proposed. It is well known that the behavior of the models can be strongly influenced by the mesh size used. According to this, when modifiying the meshing density the damaged cannot be reached or do it prematurely, or excessive, respectively. Moreover, some material models include a regularization of the mesh size, but the present investigation evidenced that this procedure has a limited range of validity, even an optimal environment values are determined. The method proposed is based on the distribution of the internal energy of the system. Finally, several expecific numerical models have been performed by using LS-DYNA commercial software, considering all the aspects listed in the preceding paragraphs. Comparisons of the results extracted from the simulations and full scale experiments were carried out, noting that there exists a very good correlation between both of them. This fact demonstrates that the proposed research procedure is highly suitable for the modelling of reinforced concrete slabs subjected to blast loading.

Dust tracking techniques applied to the STARDUST facility: First results

An important issue related to future nuclear fusion reactors fueled with deuterium and tritium is the creation of large amounts of dust due to several mechanisms (disruptions, ELMs and VDEs). The dust size expected in nuclear fusion experiments (such as ITER) is in the order of microns (between 0.1 and 1000 μm). Almost the total amount of this dust remains in the vacuum vessel (VV). This radiological dust can re-suspend in case of LOVA (loss of vacuum accident) and these phenomena can cause explosions and serious damages to the health of the operators and to the integrity of the device. The authors have developed a facility, STARDUST, in order to reproduce the thermo fluid-dynamic conditions comparable to those expected inside the VV of the next generation of experiments such as ITER in case of LOVA. The dust used inside the STARDUST facility presents particle sizes and physical characteristics comparable with those that created inside the VV of nuclear fusion experiments. In this facility an experimental campaign has been conducted with the purpose of tracking the dust re-suspended at low pressurization rates (comparable to those expected in case of LOVA in ITER and suggested by the General Safety and Security Report ITER-GSSR) using a fast camera with a frame rate from 1000 to 10,000 images per second. The velocity fields of the mobilized dust are derived from the imaging of a two-dimensional slice of the flow illuminated by optically adapted laser beam. The aim of this work is to demonstrate the possibility of dust tracking by means of image processing with the objective of determining the velocity field values of dust re-suspended during a LOVA.

A methodology to calibrate structural finite element models for reinforced concrete structures subject to blast loads

Civil buildings are not specifically designed to support blast loads, but it is important to take into account these potential scenarios because of their catastrophic effects, on persons and structures. A practical way to consider explosions on reinforced concrete structures is necessary. With this objective we propose a methodology to evaluate blast loads on large concrete buildings, using LS-DYNA code for calculation, with Lagrangian finite elements and explicit time integration. The methodology has three steps. First, individual structural elements of the building like columns and slabs are studied, using continuum 3D elements models subjected to blast loads. In these models reinforced concrete is represented with high precision, using advanced material models such as CSCM_CONCRETE model, and segregated rebars constrained within the continuum mesh. Regrettably this approach cannot be used for large structures because of its excessive computational cost. Second, models based on structural elements are developed, using shells and beam elements. In these models concrete is represented using CONCRETE_EC2 model and segregated rebars with offset formulation, being calibrated with continuum elements models from step one to obtain the same structural response: displacement, velocity, acceleration, damage and erosion. Third, models basedon structural elements are used to develop large models of complete buildings. They are used to study the global response of buildings subjected to blast loads and progressive collapse. This article carries out different techniques needed to calibrate properly the models based on structural elements, using shells and beam elements, in order to provide results of sufficient accuracy that can be used with moderate computational cost.

Evaluación de la seguridad al deslizamiento de una presa de fábrica en un cimiento con una familia de discontinuidades y con un criterio de rotura con ley de fluencia no asociada

En España existen del orden de 1,300 grandes presas, de las cuales un 20% fueron construidas antes de los años 60. El hecho de que existan actualmente una gran cantidad de presas antiguas aún en operación, ha producido un creciente interés en reevaluar su seguridad empleando herramientas nuevas o modificadas que incorporan modelos de fallo teóricos más completos, conceptos geotécnicos más complejos y nuevas técnicas de evaluación de la seguridad. Una manera muy común de abordar el análisis de estabilidad de presas de gravedad es, por ejemplo, considerar el deslizamiento a través de la interfase presa-cimiento empleando el criterio de rotura lineal de Mohr-Coulomb, en donde la cohesión y el ángulo de rozamiento son los parámetros que definen la resistencia al corte de la superficie de contacto. Sin embargo la influencia de aspectos como la presencia de planos de debilidad en el macizo rocoso de cimentación; la influencia de otros criterios de rotura para la junta y para el macizo rocoso (ej. el criterio de rotura de Hoek-Brown); las deformaciones volumétricas que ocurren durante la deformación plástica en el fallo del macizo rocoso (i.e., influencia de la dilatancia) no son usualmente consideradas durante el diseño original de la presa. En este contexto, en la presente tesis doctoral se propone una metodología analítica para el análisis de la estabilidad al deslizamiento de presas de hormigón, considerando un mecanismo de fallo en la cimentación caracterizado por la presencia de una familia de discontinuidades. En particular, se considera la posibilidad de que exista una junta sub-horizontal, preexistente y persistente en el macizo rocoso de la cimentación, con una superficie potencial de fallo que se extiende a través del macizo rocoso. El coeficiente de seguridad es entonces estimado usando una combinación de las resistencias a lo largo de los planos de rotura, cuyas resistencias son evaluadas empleando los criterios de rotura no lineales de Barton y Choubey (1977) y Barton y Bandis (1990), a lo largo del plano de deslizamiento de la junta; y el criterio de rotura de Hoek y Brown (1980) en su versión generalizada (Hoek et al. 2002), a lo largo del macizo rocoso. La metodología propuesta también considera la influencia del comportamiento del macizo rocoso cuando este sigue una ley de flujo no asociada con ángulo de dilatancia constante (Hoek y Brown 1997). La nueva metodología analítica propuesta es usada para evaluar las condiciones de estabilidad empleando dos modelos: un modelo determinista y un modelo probabilista, cuyos resultados son el valor del coeficiente de seguridad y la probabilidad de fallo al deslizamiento, respectivamente. El modelo determinista, implementado en MATLAB, es validado usando soluciones numéricas calculadas mediante el método de las diferencias finitas, empleando el código FLAC 6.0. El modelo propuesto proporciona resultados que son bastante similares a aquellos calculados con FLAC; sin embargo, los costos computacionales de la formulación propuesta son significativamente menores, facilitando el análisis de sensibilidad de la influencia de los diferentes parámetros de entrada sobre la seguridad de la presa, de cuyos resultados se obtienen los parámetros que más peso tienen en la estabilidad al deslizamiento de la estructura, manifestándose además la influencia de la ley de flujo en la rotura del macizo rocoso. La probabilidad de fallo es obtenida empleando el método de fiabilidad de primer orden (First Order Reliability Method; FORM), y los resultados de FORM son posteriormente validados mediante simulaciones de Monte Carlo. Los resultados obtenidos mediante ambas metodologías demuestran que, para el caso no asociado, los valores de probabilidad de fallo se ajustan de manera satisfactoria a los obtenidos mediante las simulaciones de Monte Carlo. Los resultados del caso asociado no son tan buenos, ya que producen resultados con errores del 0.7% al 66%, en los que no obstante se obtiene una buena concordancia cuando los casos se encuentran en, o cerca de, la situación de equilibrio límite. La eficiencia computacional es la principal ventaja que ofrece el método FORM para el análisis de la estabilidad de presas de hormigón, a diferencia de las simulaciones de Monte Carlo (que requiere de al menos 4 horas por cada ejecución) FORM requiere tan solo de 1 a 3 minutos en cada ejecución. There are 1,300 large dams in Spain, 20% of which were built before 1960. The fact that there are still many old dams in operation has produced an interest of reevaluate their safety using new or updated tools that incorporate state-of-the-art failure modes, geotechnical concepts and new safety assessment techniques. For instance, for gravity dams one common design approach considers the sliding through the dam-foundation interface, using a simple linear Mohr-Coulomb failure criterion with constant friction angle and cohesion parameters. But the influence of aspects such as the persistence of joint sets in the rock mass below the dam foundation; of the influence of others failure criteria proposed for rock joint and rock masses (e.g. the Hoek-Brown criterion); or the volumetric strains that occur during plastic failure of rock masses (i.e., the influence of dilatancy) are often no considered during the original dam design. In this context, an analytical methodology is proposed herein to assess the sliding stability of concrete dams, considering an extended failure mechanism in its rock foundation, which is characterized by the presence of an inclined, and impersistent joint set. In particular, the possibility of a preexisting sub-horizontal and impersistent joint set is considered, with a potential failure surface that could extend through the rock mass; the safety factor is therefore computed using a combination of strength along the rock joint (using the nonlinear Barton and Choubey (1977) and Barton and Bandis (1990) failure criteria) and along the rock mass (using the nonlinear failure criterion of Hoek and Brown (1980) in its generalized expression from Hoek et al. (2002)). The proposed methodology also considers the influence of a non-associative flow rule that has been incorporated using a (constant) dilation angle (Hoek and Brown 1997). The newly proposed analytical methodology is used to assess the dam stability conditions, employing for this purpose the deterministic and probabilistic models, resulting in the sliding safety factor and the probability of failure respectively. The deterministic model, implemented in MATLAB, is validated using numerical solution computed with the finite difference code FLAC 6.0. The proposed deterministic model provides results that are very similar to those computed with FLAC; however, since the new formulation can be implemented in a spreadsheet, the computational cost of the proposed model is significantly smaller, hence allowing to more easily conduct parametric analyses of the influence of the different input parameters on the dam’s safety. Once the model is validated, parametric analyses are conducting using the main parameters that describe the dam’s foundation. From this study, the impact of the more influential parameters on the sliding stability analysis is obtained and the error of considering the flow rule is assessed. The probability of failure is obtained employing the First Order Reliability Method (FORM). The probabilistic model is then validated using the Monte Carlo simulation method. Results obtained using both methodologies show good agreement for cases in which the rock mass has a nonassociate flow rule. For cases with an associated flow rule errors between 0.70% and 66% are obtained, so that the better adjustments are obtained for cases with, or close to, limit equilibrium conditions. The main advantage of FORM on sliding stability analyses of gravity dams is its computational efficiency, so that Monte Carlo simulations require at least 4 hours on each execution, whereas FORM requires only 1 to 3 minutes on each execution.

Contribución al conocimiento de los primeros geotécnicos del siglo XX

Leyendo distintos artículos en la Revista de Obras Públicas (Jiménez Salas, 1945) uno recuerda a las grandes figuras como Coulomb (1773), Poncelet (1840), Rankine (1856), Culmann (1866), Mohr (1871), Boussinesq (1876) y otros muchos, que construyeron la base de un conocimiento que poco a poco irían facilitando la complicada tarea que suponía la construcción. Pero sus avances eran aproximaciones que presentaban notables diferencias frente al comportamiento de la naturaleza. Esas discrepancias con la naturaleza llegó un momento que se hicieron demasiado patentes. Importantes asientos en la construcción de los modernos edificios, rotura de presas de materiales sueltos y grandes corrimientos de tierras, por ejemplo durante la construcción del canal de Panamá, llevaron a la Sociedad Americana de Ingenieros Civiles (ASCE) a crear un comité que analizase las prácticas de la construcción de la época. Hechos similares se producían en Europa, por ejemplo en desmontes para ferrocarriles, que en el caso de Suecia supusieron unas cuantiosas perdidas materiales y humanas. El ingeniero austriaco-americano Karl Terzaghi (1883) había podido comprobar, en su práctica profesional, la carencia de conocimientos para afrontar muchos de los retos que la naturaleza ofrecía. Inicialmente buscó la respuesta en la geología pero encontró que ésta carecía de la definición necesaria para la práctica de la ingeniería, por lo que se lanzó a una denodada tarea investigadora basada en el método experimental. Comenzó en 1917 con escasos medios, pero pronto llegó a desarrollar algunos ensayos que le permitieron establecer los primeros conceptos de una nueva ciencia, la Mecánica de Suelos. Ciencia que ve la luz en 1925 con la publicación de su libro Erdbaumechanik auf bodenphysikalischer Grundlage. Rápidamente otras figuras empezaron a hacer sus contribuciones científicas y de divulgación, como es el caso del ingeniero austriaco-americano Arthur Casagrande (1902), cuya iniciativa de organizar el primer Congreso Internacional de Mecánica de Suelos e Ingeniería de Cimentaciones proporcionó el altavoz que necesitaba esa nueva ciencia para su difusión. Al mismo tiempo, más figuras internacionales se fueron uniendo a este período de grandes avances e innovadores puntos de vista. Figuras como Alec Skempton (1914) en el Reino Unido, Ralph Peck (1912) en los Estados Unidos o Laurits Bjerrum (1918) en Noruega sobresalieron entre los grandes de la época. Esta tesis investiga las vidas de estos geotécnicos, artífices de múltiples avances científicos de la nueva ciencia denominada Mecánica de Suelos. Todas estas grandes figuras de la geotecnia fueron presidentes, en distintos periodos, de la Sociedad Internacional de Mecánica de Suelos e Ingeniería de Cimentaciones. Se deja constancia de ello en las biografías que han sido elaboradas a partir de fuentes de variada procedencia y de los datos cruzados encontrados sobre estos extraordinarios geotécnicos. Así, las biografías de Terzaghi, Casagrande, Skempton, Peck y Bjerrum contribuyen no solo a su conocimiento individual sino que constituyen conjuntamente un punto de vista privilegiado para la comprensión de los acontecimientos vividos por la Mecánica de Suelos en el segundo tercio del siglo XX, extendiéndose en algunos casos hasta los albores del siglo XXI. Las aportaciones científicas de estos geotécnicos encuentran también su lugar en la parte técnica de esta tesis, en la que sus contribuciones individuales iniciales que configuran los distintos capítulos conservan sus puntos de vista originales, lo que permite tener una visión de los principios de la Mecánica de Suelos desde su mismo origen. On reading several articles in the journal, Revista de Obras Públicas (Jiménez Salas, 1945), one recalls such leading figures as Coulomb (1773), Poncelet (1840), Rankine (1856), Culmann (1866), Mohr (1871) and Boussinesq (1876) among many others, who created the basis of scientific knowledge that would make the complicated task of construction progressively easier. However, their advances were approximations which suffered considerable discrepancies when faced with the behaviour of the forces of nature. There came a time when such discrepancies became all too evident. Substantial soil settlements when constructing modern buildings, embankment dam failures and grave landslides, during the construction of the Panama Canal for example, led the American Society of Civil Engineers (ASCE) to form a committee in order to analyse construction practices of the time. Similar incidents had taken place in Europe, for example with railway slides, which in the case of Sweden, had resulted in heavy losses in both materials and human lives. During the practice of his career, the Austrian-American engineer Karl Terzaghi (1883) had encountered the many challenges posed by the forces of nature and the lack of knowledge at his disposal with which to overcome them. Terzaghi first sought a solution in geology only to discover that this lacked the necessary accuracy for the practice of engineering. He therefore threw himself into tireless research based on the experimental method. He began in 1917 on limited means but soon managed to develop several tests, which would allow him to establish the basic fundamentals of a new science; Soil Mechanics, a science which first saw the light of day on the publication of Terzaghi’s book, Erdbaumechanik auf bodenphysikalischer Grundlage. Other figures were quick to make their own scientific contributions. Such was the case of Austrian-American engineer, Arthur Casagrande (1902), whose initiative to organize the first International Congress of Soil Mechanics and Foundation Engineering provided the springboard that this science needed. At the same time, other international figures were becoming involved in this period of great advances and innovative concepts. Figures including the likes of Alec Skempton (1914) in the United Kingdom, Ralph Peck (1912) in the United States, and Laurits Bjerrum (1918) in Norway stood out amongst the greatest of their time. This thesis investigates the lives of these geotechnical engineers to whom we are indebted for a great many scientific advances in this new science known as Soil Mechanics. Moreover, each of these eminent figures held the presidency of the International Society of Soil Mechanics and Foundation Engineering, record of which can be found in their biographies, drawn from diverse sources, and by crosschecking and referencing all the available information on these extraordinary geotechnical engineers. Thus, the biographies of Terzaghi, Casagrande, Skempton, Peck and Bjerrum not only serve to provide knowledge on the individual, but moreover, as a collective, they present us with an exceptional insight into the important developments which took place in Soil Mechanics in the second third of the 20th century, and indeed, in some cases, up to the dawn of the 21st. The scientific contributions of these geotechnical engineers also find their place in the technical part of this thesis in which the initial individual contributions which make up several chapters retain their original approaches allowing us a view of the principles of Soil Mechanics from its very beginnings.

Modelos de elementos finitos explícitos para explosiones en estructuras reticuladas de hormigón armado : aplicaciones al estudio del colapso de edificios

La frecuencia con la que se producen explosiones sobre edificios, ya sean accidentales o intencionadas, es reducida, pero sus efectos pueden ser catastróficos. Es deseable poder predecir de forma suficientemente precisa las consecuencias de estas acciones dinámicas sobre edificaciones civiles, entre las cuales las estructuras reticuladas de hormigón armado son una tipología habitual. En esta tesis doctoral se exploran distintas opciones prácticas para el modelado y cálculo numérico por ordenador de estructuras de hormigón armado sometidas a explosiones. Se emplean modelos numéricos de elementos finitos con integración explícita en el tiempo, que demuestran su capacidad efectiva para simular los fenómenos físicos y estructurales de dinámica rápida y altamente no lineales que suceden, pudiendo predecir los daños ocasionados tanto por la propia explosión como por el posible colapso progresivo de la estructura. El trabajo se ha llevado a cabo empleando el código comercial de elementos finitos LS-DYNA (Hallquist, 2006), desarrollando en el mismo distintos tipos de modelos de cálculo que se pueden clasificar en dos tipos principales: 1) modelos basados en elementos finitos de continuo, en los que se discretiza directamente el medio continuo mediante grados de libertad nodales de desplazamientos; 2) modelos basados en elementos finitos estructurales, mediante vigas y láminas, que incluyen hipótesis cinemáticas para elementos lineales o superficiales. Estos modelos se desarrollan y discuten a varios niveles distintos: 1) a nivel del comportamiento de los materiales, 2) a nivel de la respuesta de elementos estructurales tales como columnas, vigas o losas, y 3) a nivel de la respuesta de edificios completos o de partes significativas de los mismos. Se desarrollan modelos de elementos finitos de continuo 3D muy detallados que modelizan el hormigón en masa y el acero de armado de forma segregada. El hormigón se representa con un modelo constitutivo del hormigón CSCM (Murray et al., 2007), que tiene un comportamiento inelástico, con diferente respuesta a tracción y compresión, endurecimiento, daño por fisuración y compresión, y rotura. El acero se representa con un modelo constitutivo elastoplástico bilineal con rotura. Se modeliza la geometría precisa del hormigón mediante elementos finitos de continuo 3D y cada una de las barras de armado mediante elementos finitos tipo viga, con su posición exacta dentro de la masa de hormigón. La malla del modelo se construye mediante la superposición de los elementos de continuo de hormigón y los elementos tipo viga de las armaduras segregadas, que son obligadas a seguir la deformación del sólido en cada punto mediante un algoritmo de penalización, simulando así el comportamiento del hormigón armado. En este trabajo se denominarán a estos modelos simplificadamente como modelos de EF de continuo. Con estos modelos de EF de continuo se analiza la respuesta estructural de elementos constructivos (columnas, losas y pórticos) frente a acciones explosivas. Asimismo se han comparado con resultados experimentales, de ensayos sobre vigas y losas con distintas cargas de explosivo, verificándose una coincidencia aceptable y permitiendo una calibración de los parámetros de cálculo. Sin embargo estos modelos tan detallados no son recomendables para analizar edificios completos, ya que el elevado número de elementos finitos que serían necesarios eleva su coste computacional hasta hacerlos inviables para los recursos de cálculo actuales. Adicionalmente, se desarrollan modelos de elementos finitos estructurales (vigas y láminas) que, con un coste computacional reducido, son capaces de reproducir el comportamiento global de la estructura con una precisión similar. Se modelizan igualmente el hormigón en masa y el acero de armado de forma segregada. El hormigón se representa con el modelo constitutivo del hormigón EC2 (Hallquist et al., 2013), que también presenta un comportamiento inelástico, con diferente respuesta a tracción y compresión, endurecimiento, daño por fisuración y compresión, y rotura, y se usa en elementos finitos tipo lámina. El acero se representa de nuevo con un modelo constitutivo elastoplástico bilineal con rotura, usando elementos finitos tipo viga. Se modeliza una geometría equivalente del hormigón y del armado, y se tiene en cuenta la posición relativa del acero dentro de la masa de hormigón. Las mallas de ambos se unen mediante nodos comunes, produciendo una respuesta conjunta. En este trabajo se denominarán a estos modelos simplificadamente como modelos de EF estructurales. Con estos modelos de EF estructurales se simulan los mismos elementos constructivos que con los modelos de EF de continuo, y comparando sus respuestas estructurales frente a explosión se realiza la calibración de los primeros, de forma que se obtiene un comportamiento estructural similar con un coste computacional reducido. Se comprueba que estos mismos modelos, tanto los modelos de EF de continuo como los modelos de EF estructurales, son precisos también para el análisis del fenómeno de colapso progresivo en una estructura, y que se pueden utilizar para el estudio simultáneo de los daños de una explosión y el posterior colapso. Para ello se incluyen formulaciones que permiten considerar las fuerzas debidas al peso propio, sobrecargas y los contactos de unas partes de la estructura sobre otras. Se validan ambos modelos con un ensayo a escala real en el que un módulo con seis columnas y dos plantas colapsa al eliminar una de sus columnas. El coste computacional del modelo de EF de continuo para la simulación de este ensayo es mucho mayor que el del modelo de EF estructurales, lo cual hace inviable su aplicación en edificios completos, mientras que el modelo de EF estructurales presenta una respuesta global suficientemente precisa con un coste asumible. Por último se utilizan los modelos de EF estructurales para analizar explosiones sobre edificios de varias plantas, y se simulan dos escenarios con cargas explosivas para un edificio completo, con un coste computacional moderado. The frequency of explosions on buildings whether they are intended or accidental is small, but they can have catastrophic effects. Being able to predict in a accurate enough manner the consequences of these dynamic actions on civil buildings, among which frame-type reinforced concrete buildings are a frequent typology is desirable. In this doctoral thesis different practical options for the modeling and computer assisted numerical calculation of reinforced concrete structures submitted to explosions are explored. Numerical finite elements models with explicit time-based integration are employed, demonstrating their effective capacity in the simulation of the occurring fast dynamic and highly nonlinear physical and structural phenomena, allowing to predict the damage caused by the explosion itself as well as by the possible progressive collapse of the structure. The work has been carried out with the commercial finite elements code LS-DYNA (Hallquist, 2006), developing several types of calculation model classified in two main types: 1) Models based in continuum finite elements in which the continuous medium is discretized directly by means of nodal displacement degrees of freedom; 2) Models based on structural finite elements, with beams and shells, including kinematic hypothesis for linear and superficial elements. These models are developed and discussed at different levels: 1) material behaviour, 2) response of structural elements such as columns, beams and slabs, and 3) response of complete buildings or significative parts of them. Very detailed 3D continuum finite element models are developed, modeling mass concrete and reinforcement steel in a segregated manner. Concrete is represented with a constitutive concrete model CSCM (Murray et al., 2007), that has an inelastic behaviour, with different tension and compression response, hardening, cracking and compression damage and failure. The steel is represented with an elastic-plastic bilinear model with failure. The actual geometry of the concrete is modeled with 3D continuum finite elements and every and each of the reinforcing bars with beam-type finite elements, with their exact position in the concrete mass. The mesh of the model is generated by the superposition of the concrete continuum elements and the beam-type elements of the segregated reinforcement, which are made to follow the deformation of the solid in each point by means of a penalty algorithm, reproducing the behaviour of reinforced concrete. In this work these models will be called continuum FE models as a simplification. With these continuum FE models the response of construction elements (columns, slabs and frames) under explosive actions are analysed. They have also been compared with experimental results of tests on beams and slabs with various explosive charges, verifying an acceptable coincidence and allowing a calibration of the calculation parameters. These detailed models are however not advised for the analysis of complete buildings, as the high number of finite elements necessary raises its computational cost, making them unreliable for the current calculation resources. In addition to that, structural finite elements (beams and shells) models are developed, which, while having a reduced computational cost, are able to reproduce the global behaviour of the structure with a similar accuracy. Mass concrete and reinforcing steel are also modeled segregated. Concrete is represented with the concrete constitutive model EC2 (Hallquist et al., 2013), which also presents an inelastic behaviour, with a different tension and compression response, hardening, compression and cracking damage and failure, and is used in shell-type finite elements. Steel is represented once again with an elastic-plastic bilineal with failure constitutive model, using beam-type finite elements. An equivalent geometry of the concrete and the steel is modeled, considering the relative position of the steel inside the concrete mass. The meshes of both sets of elements are bound with common nodes, therefore producing a joint response. These models will be called structural FE models as a simplification. With these structural FE models the same construction elements as with the continuum FE models are simulated, and by comparing their response under explosive actions a calibration of the former is carried out, resulting in a similar response with a reduced computational cost. It is verified that both the continuum FE models and the structural FE models are also accurate for the analysis of the phenomenon of progressive collapse of a structure, and that they can be employed for the simultaneous study of an explosion damage and the resulting collapse. Both models are validated with an experimental full-scale test in which a six column, two floors module collapses after the removal of one of its columns. The computational cost of the continuum FE model for the simulation of this test is a lot higher than that of the structural FE model, making it non-viable for its application to full buildings, while the structural FE model presents a global response accurate enough with an admissible cost. Finally, structural FE models are used to analyze explosions on several story buildings, and two scenarios are simulated with explosive charges for a full building, with a moderate computational cost.

Análisis y reducción del riesgo de explosión por incendio en los almacenes de pirotecnia

Un incendio en el interior de un almacén de pirotecnia puede provocar la iniciación más o menos simultánea de los artículos existentes en su interior, con su consecuente onda de choque potenciada por la presión de los propios gases generados en su interior. La finalidad de esta tesis doctoral consiste en analizar los fenómenos que se desarrollan como consecuencia de un incendio interior en los almacenes de pirotecnia y establecer posibles medidas eficaces de prevención y protección con objeto de reducir el riesgo de explosión.. Inicialmente se llevó a cabo un análisis del riesgo de explosión, evaluando las consecuencias en caso de producirse un fuego interior. Los resultados fueron catastróficos. Finalmente, y con objeto de minimizar la posibilidad de producirse un incendio en el interior de un almacén, se evaluaron diferentes sistemas de detección y extinción automática de incendios para almacenes pirotécnicos, realizándose pruebas a escala real. En particular, la presente tesis consta de los siguientes trabajos: 1) Evaluación de la sobrepresión generada por la explosión de artificios pirotécnicos en el interior de almacenes, mediante pruebas de explosión de artículos pirotécnicos, con el fin de evaluar las posibles consecuencias en caso de materializarse una explosión, y disponer de información que permita conocer el grado de resistencia de las estructuras de los almacenes, así como el diseño adecuado de las mismas para soportar los efectos de tales explosiones. 2) Evaluación del efecto de un incendio en el interior de un almacén de pirotecnia mediante pruebas a escala real, con el fin de evaluar la posibilidad de la iniciación simultánea de los artículos pirotécnicos almacenados en su interior, y su consecuente explosión, así como las consecuencias en caso de materializarse la explosión. 3) Análisis de sistemas de detección y extinción automática de incendios para almacenes de pirotecnia, mediante la realización de pruebas a escala real para verificar la eficiencia y eficacia de tales sistemas y agentes para sofocar un incendio pirotécnico de forma tal que se evite la posible explosión asociada. 4) Evaluación de nuevos agentes extintores utilizados en los sistemas de extinción automática de incendios, que actualmente se están utilizando en el sector pirotécnico nacional, mediante la realización de pruebas a escala real. Los resultados obtenidos han permitido al Ministerio de Industria, Energía y Turismo actualizar la reglamentación vigente aplicable (Instrucción Técnica Complementaria número 17 del Reglamento de artículos pirotécnicos y cartuchería, aprobado por el Real Decreto 563/2010, de 7 de mayo). ABSTRACT A fire inside a pyrotechnics magazine can cause the more or less simultaneous initiation of the stored articles, with the wave of shock that is produced promoted by the pressure of the gases generated in its interior. The purpose of this PhD thesis is to analyze the phenomena that develop as a result of a fire inside the pyrotechnics stores and establish possible effective prevention and protection measures to reduce the risk of explosionAn analysis of the risk of explosion and fire consequences in a pyrotechnic magazine, testing to real scale, was initially carried out. The results were catastrophic. Finally, and in order to minimize the possibility of a fire inside a pyrotechnic magazine, different systems of detection and automatic extinction of fires in pyrotechnic magazines were evaluated, testing to real scale. Particularly, this thesis has the following parts: 1) Evaluation of the pressure generated by the pyrotechnic explosion in a magazine, with explosión tests of pyrotechnic articles, in order to evaluate the possible consequences in case of materialize an explosion, and provide information to know the resistance grade of the magazine structures, as well as the right design of those to withstand the effects of such explosions. 2) Evaluation of the effect of a fire inside a pyrotechnic magazine by full-scale tests, in order to evaluate the possibility of simultaneous initiation of the pyrotechnic articles stored inside, and the resulting explosion, as well as the consequences in case of materialize an explosion. 3) Analysis and evaluation of detection and automatic fire extinguishing in pyrotechnic magazines, by full-scale tests in order to verify the efficiency and effectiveness of such systems and agents to quell a pyrotechnic fire and avoid the posible mass explosion. 4) Evaluation of new extinguishing agent used in automatic fire extinguishers, which are currently being used in the national fireworks industry, by full-scale tests. The results obtained have allowed the Ministry of Industry, Energy and Tourism, update the current regulation (Complementary Technical Instruction 17 of the Regulation of Pyrotecnic Articles and Ammunition, approved by the Royal Decree 563/2010, of May 7).

Broken symmetry and strangeness of the semiconductor impurity band metal-insulator transition

The filamentary model of the metal-insulator transition in randomly doped semiconductor impurity bands is geometrically equivalent to similar models for continuous transitions in dilute antiferromagnets and even to the λ transition in liquid He, but the critical behaviors are different. The origin of these differences lies in two factors: quantum statistics and the presence of long range Coulomb forces on both sides of the transition in the electrical case. In the latter case, in addition to the main transition, there are two satellite transitions associated with disappearance of the filamentary structure in both insulating and metallic phases. These two satellite transitions were first identified by Fritzsche in 1958, and their physical origin is explained here in geometrical and topological terms that facilitate calculation of critical exponents.

Structure-based conformational preferences of amino acids

Proteins can be very tolerant to amino acid substitution, even within their core. Understanding the factors responsible for this behavior is of critical importance for protein engineering and design. Mutations in proteins have been quantified in terms of the changes in stability they induce. For example, guest residues in specific secondary structures have been used as probes of conformational preferences of amino acids, yielding propensity scales. Predicting these amino acid propensities would be a good test of any new potential energy functions used to mimic protein stability. We have recently developed a protein design procedure that optimizes whole sequences for a given target conformation based on the knowledge of the template backbone and on a semiempirical potential energy function. This energy function is purely physical, including steric interactions based on a Lennard-Jones potential, electrostatics based on a Coulomb potential, and hydrophobicity in the form of an environment free energy based on accessible surface area and interatomic contact areas. Sequences designed by this procedure for 10 different proteins were analyzed to extract conformational preferences for amino acids. The resulting structure-based propensity scales show significant agreements with experimental propensity scale values, both for α-helices and β-sheets. These results indicate that amino acid conformational preferences are a natural consequence of the potential energy we use. This confirms the accuracy of our potential and indicates that such preferences should not be added as a design criterion.

A “midinfrared” scenario for cuprate superconductivity

I conjecture that the mechanism of superconductivity in the cuprates is a saving, due to the improved screening resulting from Cooper pair formation, of the part of the Coulomb energy associated with long wavelengths and midinfrared frequencies. This scenario is shown to provide a plausible explanation of the trend of transition temperature with layering structure in the Ca-spaced compounds and to predict a spectacularly large decrease in the electron-energy-loss spectroscopy cross-section in the midinfrared region on transition to the superconducting state, as well as less spectacular but still surprisingly large changes in the optical behavior. Existing experimental results appear to be consistent with this picture.

The future of coral reefs

Coral reefs, with their millions of species, have changed profoundly because of the effects of people, and will continue to do so for the foreseeable future. Reefs are subject to many of the same processes that affect other human-dominated ecosystems, but some special features merit emphasis: (i) Many dominant reef builders spawn eggs and sperm into the water column, where fertilization occurs. They are thus particularly vulnerable to Allee effects, including potential extinction associated with chronic reproductive failure. (ii) The corals likely to be most resistant to the effects of habitat degradation are small, short-lived “weedy” corals that have limited dispersal capabilities at the larval stage. Habitat degradation, together with habitat fragmentation, will therefore lead to the establishment of genetically isolated clusters of inbreeding corals. (iii) Increases in average sea temperatures by as little as 1°C, a likely result of global climate change, can cause coral “bleaching” (the breakdown of coral–algal symbiosis), changes in symbiont communities, and coral death. (iv) The activities of people near reefs increase both fishing pressure and nutrient inputs. In general, these processes favor more rapidly growing competitors, often fleshy seaweeds, and may also result in explosions of predator populations. (v) Combinations of stress appear to be associated with threshold responses and ecological surprises, including devastating pathogen outbreaks. (vi) The fossil record suggests that corals as a group are more likely to suffer extinctions than some of the groups that associate with them, whose habitat requirements may be less stringent.

Supernovae, an accelerating universe and the cosmological constant

Observations of supernova explosions halfway back to the Big Bang give plausible evidence that the expansion of the universe has been accelerating since that epoch, approximately 8 billion years ago and suggest that energy associated with the vacuum itself may be responsible for the acceleration.

Cosmic microwave background theory

A long-standing goal of theorists has been to constrain cosmological parameters that define the structure formation theory from cosmic microwave background (CMB) anisotropy experiments and large-scale structure (LSS) observations. The status and future promise of this enterprise is described. Current band-powers in ℓ-space are consistent with a ΔT flat in frequency and broadly follow inflation-based expectations. That the levels are ∼(10−5)2 provides strong support for the gravitational instability theory, while the Far Infrared Absolute Spectrophotometer (FIRAS) constraints on energy injection rule out cosmic explosions as a dominant source of LSS. Band-powers at ℓ ≳ 100 suggest that the universe could not have re-ionized too early. To get the LSS of Cosmic Background Explorer (COBE)-normalized fluctuations right provides encouraging support that the initial fluctuation spectrum was not far off the scale invariant form that inflation models prefer: e.g., for tilted Λ cold dark matter sequences of fixed 13-Gyr age (with the Hubble constant H0 marginalized), ns = 1.17 ± 0.3 for Differential Microwave Radiometer (DMR) only; 1.15 ± 0.08 for DMR plus the SK95 experiment; 1.00 ± 0.04 for DMR plus all smaller angle experiments; 1.00 ± 0.05 when LSS constraints are included as well. The CMB alone currently gives weak constraints on Λ and moderate constraints on Ωtot, but theoretical forecasts of future long duration balloon and satellite experiments are shown which predict percent-level accuracy among a large fraction of the 10+ parameters characterizing the cosmic structure formation theory, at least if it is an inflation variant.