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  • Gas flow in anisotropic claystone: modelling triaxial experiments

     Arnedo Gaute, Diego; Alonso Pérez de Agreda, Eduardo; Olivella Pastalle, Sebastian
    International journal for numerical and analytical methods in geomechanics
    Date of publication: 2013-10
    Journal article

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    Selected gas pulse tests on initially saturated claystone samples under isotropic confinement pressure are simulated using a 3D thermo-hydro-mechanical code. The constitutive model considers the hydro-mechanical anisotropy of argillaceous rocks. A cross-anisotropic linear elastic law is adopted for the mechanical behaviour. Elements for a proper modelling of gas flow along preferential paths include an embedded fracture permeability model. Rock permeability and its retention curve depend on strains through a fracture aperture. The hydraulic and mechanical behaviours have a common anisotropic structure. Small-scale heterogeneity is considered to enhance the initiation of flow through preferential paths, following the direction of the bedding planes. The numerical simulations were performed considering two different bedding orientations, parallel and normal to the imposed flow in the test. Simulations are in agreement with recorded upstream and downstream pressures in the tests. The evolution of fluid pressures, degree of saturation, element permeability and stress paths are presented for each case analysed. This information provides a good insight into the mechanisms of gas transport. Different flow patterns are obtained depending on bedding orientation, and the results provide an explanation for the results obtained in the tests.

  • Osmosis-induced water uptake by Eurobitum bituminized radioactive waste and pressure development in constant volume conditions

     Mariën, An; Mokni, Nadia; Valcke, Elie; Olivella Pastalle, Sebastian; Smets, Esteven; Li, Xiangling
    Journal of nuclear materials
    Date of publication: 2013-01
    Journal article

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    he chemo-hydro-mechanical (CHM) interaction between swelling Eurobitum radioactive bituminized waste (BW) and Boom Clay is investigated to assess the feasibility of geological disposal for the long-term management of this waste. These so-called compatibility studies include laboratory water uptake tests at the Belgian Nuclear Research Center SCK.CEN, and the development of a coupled CHM formulation for Eurobitum by the International Center for Numerical Methods and Engineering (CIMNE, Polytechnical University of Cataluna, Spain). In the water uptake tests, the osmosis-induced swelling, pressure increase and NaNO3 leaching of small cylindrical BW samples (diameter 38 mm, height 10 mm) is studied under constant total stress conditions and nearly constant volume conditions; the actual geological disposal conditions should be intermediate between these extremes...

  • Coupled phenomena induced by freezing in a granular material

     Casini, Francesca; Gens Sole, Antonio; Olivella Pastalle, Sebastian; Viggiani, Giulia M.B.
    International Symposium on Coupled Phenomena in Environmental Geotechnics
    Presentation's date: 2013-07
    Presentation of work at congresses

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    Artificial Ground Freezing (AGF) is a controllable process that can be used by engineers to stabilise temporarily the ground, provide structural support and/or exclude groundwater from an excavation until construction of the final lining provides permanent stability and water tightness. AGF is often carried out based on an observational approach, while a full understanding of the freezing process and of the generation of freezing-induced heave and settlements is still far from being achieved. In this work, the process of ground freezing is studied using a constitutive model that encompasses frozen and unfrozen behaviour within a unified effective-stress-based framework and employs a combination of ice pressure, liquid water pressure and total stress as state variables. The parameters of the constitutive model are calibrated against experimental data obtained from samples retrieved during construction of Napoli underground, in which AGF was extensively used to excavate in granular soils and weak fractured rock below the ground water table.

  • Reinventing civil engineering studies in Spain to face new challenges

     Lozano Galant, Jose Antono; Ruiz Ripoll, Lidia; Olivella Pastalle, Sebastian; Turmo Coderque, Jose
    EUCEET Association Conference
    Presentation's date: 2013-10-14
    Presentation of work at congresses

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  • Triaxial tests on frozen ground: formulation and modelling

     Casini, F.; Gens Sole, Antonio; Olivella Pastalle, Sebastian; Viggiani, Giulia M.B.
    International Conference on Computational Plasticity Fundamentals and Applications
    Presentation's date: 2013-09
    Presentation of work at congresses

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    Artificial Ground Freezing (AGF) is a controllable process that can be used by engineers to stabilise temporarily the ground, provide structural support and/or exclude groundwater from an excavation until construction of the final lining provides permanent stability and water tightness. In this work, the process of ground freezing is studied using a constitutive model that encompasses frozen and unfrozen behaviour within a unified effective-stress- based framework and employs a combination of ice pressure, liquid water pressure and total stress as state variables. The parameters of the constitutive model are calibrated against experimental data obtained from samples retrieved during construction of Napoli underground, in which AGF was extensively used to excavate in granular soils and weak fractured rock below the ground water table.

  • Development of a predictive framework for geothermal and geotechnical responses in cold regions experiencing climate change

     Nishimura, S.; Jardine, R.; Fenton, C.H.; Olivella Pastalle, Sebastian; Gens Sole, Antonio; Martin, C.J.
    International Conference on Soil Mechanics and Geotechnical Engineering
    Presentation's date: 2013-09
    Presentation of work at congresses

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    Cold regions, which are expected to suffer particularly severe future climate effects, will pose very challenging geotechnical conditions in the 21st century involving ground freezing and thawing. Given the uncertainty of future environmental changes and the vast expanses of the cold regions, it is appropriate to address problems such as pipeline or road construction with analytical methods that have multiple scales and layers. High- and middle-level predictive tools are described that integrate climatic predictions from AOGCMs and their down-scaling schemes, geological and topographical (DEM) information, remotely-sensed vegetation data and non-linear finite element analysis for soil freezing and thawing. These tools output broad scale predictions of geothermal responses, at a regional scale, that offer hazard zoning schemes related to permafrost thawing. A more intensive localscale predictive tool is then outlined that considers fully-coupled thermo-hydro-mechanical processes occurring at the soil-element level and outputs detailed predictions for temperature changes, pore water behaviour, ground stresses and deformation in and around geotechnical structures. Applications of these tools to specific problems set in Eastern Siberia and pipeline heave tests are illustrated.

  • Modelling 3D mechanics interfaces with continuum elements

     Puig Damians, Ivan; Olivella Pastalle, Sebastian; Lloret Morancho, Antonio; Josa Garcia-tornel, Alejandro; Bathurst, Richard
    Workshop of CODE-BRIGHT Users
    Presentation's date: 2013-05-07
    Presentation of work at congresses

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    This document presents preliminary results of FEM-numerical analysis of soilreinforcement pullout tests. The numerical model has been developed with CODE_BRIGHT and assuming the interfaces as continuum materials. The results of the preliminary parametric analyses described herein provide useful information on the shear behavior modeling of soil-reinforcement strip interfaces under working stress conditions

  • A constitutive law for rock joints considering the effects of suction and roughness on strength parameters

     Zandarín Iragorre, Maria Teresa; Alonso Pérez de Agreda, Eduardo; Olivella Pastalle, Sebastian
    International journal of rock mechanics and mining sciences (1997)
    Date of publication: 2013-06
    Journal article

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    The paper describes a set of experiments aimed at investigating suction effects on the shear behaviour of rock discontinuities. The experimental investigation focused on the effects of suction on the mechanical behaviour of rock joints. Laboratory tests were performed in a direct shear cell equipped with suction control. Suction was imposed using a forced vapour convection circuit connected to the cell and controlled by an air pump. Artificial joints of Lilla claystone were prepared. Joint roughness of varying intensity was created by carving the surfaces in contact in such a manner that rock ridges of different tip angles were formed. These angles ranged from 0° (smooth joint) to 45° (very rough joint profile). The geometric profiles of the two surfaces in contact were initially positioned in a ¿matching¿ situation...

  • A chemo-mechanical constitutive model accounting for cation exchange in expansive clays

     Guimarães, Leonardo do N; Gens Sole, Antonio; Sánchez, M.; Olivella Pastalle, Sebastian
    Géotechnique
    Date of publication: 2013-03-01
    Journal article

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  • Liquid CO2 injection for geological storage in deep saline aquifers

     Vilarrasa Riaño, Victor; Silva Rojas, Orlando Enrique; Olivella Pastalle, Sebastian; Carrera Ramírez, Jesús
    International journal of greenhouse gas control
    Date of publication: 2013-05
    Journal article

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    CO2 will remain in supercritical (SC) state (i.e. p > 7.382 MPa and T > 31.04 °C) under the pressure (p) and temperature (T) conditions appropriate for geological storage. Thus, it is usually assumed that CO2 will reach the aquifer in SC conditions. However, inflowing CO2 does not need to be in thermal equilibrium with the aquifer. In fact, surface operations are simpler for liquid than for SC CO2, because CO2 is transported in liquid state. Yet, problems might arise because of thermal stresses induced by cold CO2 injection and because of phase changes in the injection tubing or in the formation. Here, we propose liquid CO2 injection and analyze its evolution and the thermo-hydro-mechanical response of the formation and the caprock. We find that injecting CO2 in liquid state is energetically more efficient than in SC state because liquid CO2 is denser than SC CO2, leading to a lower overpressure not only at the wellhead, but also in the reservoir because a smaller fluid volume is displaced. Cold CO2 injection cools down the formation around the injection well. Further away, CO2 equilibrates thermally with the medium in an abrupt front. The liquid CO2 region close to the injection well advances far behind the SC CO2 interface. While the SC CO2 region is dominated by gravity override, the liquid CO2 region displays a steeper front because viscous forces dominate (liquid CO2 is not only denser, but also more viscous than SC CO2). The temperature decrease close to the injection well induces a stress reduction due to thermal contraction of the media. This can lead to shear slip of pre-existing fractures in the aquifer for large temperature contrasts in stiff rocks, which could enhance injectivity. In contrast, the mechanical stability of the caprock is improved in stress regimes where the maximum principal stress is the vertical.

  • Thermo-hydraulic behaviour of the vadose zone in sulphide tailings at Iberian Pyrite Belt: waste characterization, monitoring and modelling

     Blanco, Alejandro; Lloret Morancho, Antonio; Carrera Ramírez, Jesús; Olivella Pastalle, Sebastian
    Engineering geology
    Date of publication: 2013-10
    Journal article

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    Unsaturated conditions favour the oxidation of sulphide minerals from mine wastes, which results in the release of contaminant products into groundwater. An abandoned high-sulphide impoundment in the Iberia Pyritic Belt, wherein tailings have undergone oxidation for more than 28. years, was investigated for hydrological purposes. The objective was to understand the interactions between those mining tailings and the atmosphere under natural semiarid conditions (wet and dry seasons, short intensive rain events and strong daily temperature differences during the dry season).After the deposition, the sequence of waste textures that results from the sedimentation process is strongly dependent on the distance to discharge point. The spatial continuity of the sedimentation layers was studied by means of small scale dynamic penetration tests.The thermo-hydraulic characterization of the waste includes the determination of the water retention curve, saturated and unsaturated permeability, pore size distribution and thermal properties for the different textures.Atmospheric and waste physical measures, from 2002 to 2006, were performed using different techniques. The important changes in the salinity of the waste avoided the use of a single calibration for the electromagnetic sensors; a valid alternative was the evaluation of the water content from thermal conductivity estimations.Finally, a numerical model using the HYDRUS-1D software is presented. Model results reasonably fit the in-situ measures of soil moisture, soil temperature, soil water potential and soil heat flux in the vadose zone of tailings impoundment. Moreover, they provide information for energy and water balance determinations.

  • Rapid drawdown in embankment dams

     Pinyol Puigmarti, Nuria Merce; Alonso Pérez de Agreda, Eduardo; Olivella Pastalle, Sebastian
    Date of publication: 2012
    Book chapter

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  • Rapid drawdown and landslides

     Pinyol Puigmarti, Nuria Merce; Alonso Pérez de Agreda, Eduardo; Olivella Pastalle, Sebastian
    International Symposium on Landslides
    Presentation's date: 2012-06
    Presentation of work at congresses

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    Predicting the pore pressure distribution in a slope after rapid drawdown conditions requires the solution of a coupled flow‐deformation analysis in a saturated‐unsaturated porous media. A fully coupled finite element code (Code_Bright), able to handle in a consistent manner the drawdown conditions, is used to simulate the pore water pressure measured in the upstream slope of an earth dam (Glen Shira Dam, Scotland) subjected to a controlled rapid drawdown event. A comparison of some calculation alternatives is then given and compared with field pressure records. The paper describes also the analysis performed to interpret a recent case of a major landslide triggered by a rapid drawdown in a reservoir. A key aspect of the case is the correct characterization of permeability of representative soil profiles. This is achieved by combining laboratory test results and a back analysis of pore water pressure time records during a period of reservoir level fluctuations.

  • THM analysis of a large-scale heating test incorporating material fabric changes

     Sánchez, Marcelo; Gens Sole, Antonio; Olivella Pastalle, Sebastian
    International journal for numerical and analytical methods in geomechanics
    Date of publication: 2012-03
    Journal article

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  • Environmental effects on earth embankments

     Olivella Pastalle, Sebastian; Alonso Pérez de Agreda, Eduardo
    Date of publication: 2012
    Book chapter

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  • Thermo-Hydro-Mechanical Impacts of Carbon Dioxide (CO2) Injection in Deep Saline Aquifers.  Open access

     Vilarrasa Riaño, Victor
    Defense's date: 2012-07-20
    Universitat Politècnica de Catalunya
    Theses

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    Los procesos termo-hidro-mecánicos relacionados con el almacenamiento geológico de carbono deben ser entendidos y cuantificados para demostrar a la opinión pública de que la inyección de dióxido de carbono (CO2) es segura. Esta Tesis tiene como objetivo mejorar dicho conocimiento mediante el desarrollo de métodos para: (1) evaluar la evolución tanto de la geometría de la pluma de CO2 como de la presión de los fluidos; (2) definir un ensayo de campo que permita caracterizar la presión de inyección máxima sostenible y los parámetros hidromecánicos de las rocas sello y almacén; y (3) proponer un nuevo concepto de inyección que es energéticamente eficiente y que mejora la estabilidad de la roca sello en la mayoría de escenarios geológicos debido a efectos termo-mecánicos. modelo viscoplástico. Las simulaciones ilustran que, dependiendo de las condiciones de contorno, el momento más desfavorable ocurre al inicio de la inyección. Sin embargo, si los contornos son poco permeables, la presión de fluido continúa aumentando en todo el acuífero, lo que podría llegar a comprometer la estabilidad de la roca sello a largo plazo. Para evaluar dichos problemas, proponemos un ensayo de caracterización hidromecánica a escala de campo para estimar las propiedades hidromecánicas de las rocas sello y almacén. Obtenemos curvas para la sobrepresión y el desplazamiento vertical en función del término de la deformación volumétrica obtenido del análisis adimensional de las ecuaciones hidromecánicas. Ajustando las medidas de campo a estas curvas se pueden estimar los valores del módulo de Young y el coeficiente de Poisson del acuífero y del sello. Los resultados indican que la microsismicidad inducida tiene más probabilidades de ocurrir en el acuífero que en el sello. El inicio de la microsismicidad en el sello marca la presión de inyección máxima sostenible para asegurar un almacenamiento permanente de CO2 seguro. Finalmente, analizamos la evolución termodinámica del CO2 y la respuesta termohidro- mecánica de las rocas sello y almacén a la inyección de CO2 líquido (frío). Encontramos que inyectar CO2 en estado líquido es energéticamente más eficiente porque al ser más denso que el CO2 supercrítico, requiere menor presión en cabeza de pozo para una presión dad en el acuífero. De hecho, esta presión también es menor en el almacén porque se desplaza un volumen menor de fluido. La disminución de temperatura en el entorno del pozo induce una reducción de tensiones debido a la contracción térmica del medio. Esto puede producir deslizamiento de fracturas existentes en acuíferos formados por rocas rígidas bajo contrastes de temperatura grandes, lo que podría incrementar la inyectividad de la roca almacén. Por otro lado, la estabilidad mecánica de la roca sello mejora cuando la tensión principal máxima es la vertical. Primero, investigamos numérica y analíticamente los efectos de la variabilidad de la densidad y viscosidad del CO2 en la posición de la interfaz entre la fase rica en CO2 y la salmuera de la formación. Introducimos una corrección para tener en cuenta dicha variabilidad en las soluciones analíticas actuales. Encontramos que el error producido en la posición de la interfaz al despreciar la compresibilidad del CO2 es relativamente pequeño cuando dominan las fuerzas viscosas. Sin embargo, puede ser significativo cuando dominan las fuerzas de gravedad, lo que ocurre para tiempos y/o distancias largas de inyección. Segundo, desarrollamos una solución semianalítica para la evolución de la geometría de la pluma de CO2 y la presión de fluido, teniendo en cuenta tanto la compresibilidad del CO2 como los efectos de flotación dentro del pozo. Formulamos el problema en términos de un potencial de CO2 que facilita la solución en capas horizontales, en las que hemos discretizado el acuífero. El CO2 avanza inicialmente por la porción superior del acuífero. Pero a medida que aumenta la presión de CO2, la pluma crece no solo lateralmente, sino también hacia abajo, aunque no tiene porqué llegar a ocupar todo el espesor del acuífero. Tanto la interfaz CO2-salmuera como la presión de fluido muestran una buena comparación con las simulaciones numéricas. En tercer lugar, estudiamos posibles mecanismos de rotura, que podrían llegar a producir fugas de CO2, en un sistema acuífero-sello con simetría radial, utilizando un

    Els processos termo-hidro-mecànics relacionats amb l’emmagatzematge geològic de carboni han de ser entesos i quantificats per tal de demostrar a l’opinió pública de que la injecció de diòxid de carboni (CO2) és segura. Aquesta Tesi té com a objectiu millorar aquest coneixement mitjançant el desenvolupament de mètodes per a: (1) avaluar l'evolució tant de la geometria del plomall de CO2 com de la pressió dels fluids; (2) definir un assaig de camp que permeti caracteritzar la pressió d'injecció màxima sostenible i els paràmetres hidromecànics de les roques segell i magatzem; i (3) proposar un nou concepte d'injecció que és energèticament eficient i que millora l'estabilitat de la roca segell en la majoria d’escenaris geològics a causa d'efectes termo-mecànics. Primer, investiguem numèricament i analítica els efectes de la variabilitat de la densitat i viscositat del CO2 en la posició de la interfície entre la fase rica en CO2 i la salmorra de la formació. Introduïm una correcció per tal de tenir en compte aquesta variabilitat en les solucions analítiques actuals. Trobem que l'error produït en la posició de la interfície en menysprear la compressibilitat del CO2 és relativament petit quan dominen les forces viscoses. Malgrat això, l’error pot ser significatiu quan dominen les forces de gravetat, la qual cosa té lloc per a temps i/o distàncies llargues d'injecció. Segon, desenvolupem una solució semianalítica per a l'evolució de la geometria del plomall de CO2 i la pressió de fluid, tenint en compte tant la compressibilitat del CO2 com els efectes de flotació dins del pou. Formulem el problema en termes d'un potencial de CO2 que facilita la solució en capes horitzontals, en les quals hem discretitzat l'aqüífer. El CO2 avança inicialment per la porció superior de l'aqüífer. Però a mesura que augmenta la pressió de CO2, el plomall de CO2 no només creix lateralment, sinó que també ho fa cap avall, encara que no té perquè arribar a ocupar tot el gruix de l'aqüífer. Tant la interfície CO2-salmorra com la pressió de fluid mostren una bona comparació amb les simulacions numèriques. En tercer lloc, estudiem possibles mecanismes de trencament, que podrien arribar a produir fugues de CO2, en un sistema aqüífer-segell amb simetria radial, utilitzant un model viscoplàstic. Les simulacions il·lustren que, depenent de les condicions de contorn, el moment més desfavorable té lloc a l'inici de la injecció. Tot i això, si els contorns són poc permeables, la pressió de fluid continua augmentant en tot l'aqüífer, la qual cosa podria arribar a comprometre l'estabilitat de la roca segell a llarg termini. Per a avaluar aquests problemes, proposem un assaig de caracterització hidromecànica a escala de camp per a estimar les propietats hidromecàniques de les roques segell i magatzem. Obtenim corbes per a la sobrepressió i el desplaçament vertical en funció del terme de la deformació volumètrica obtingut de l'anàlisi adimensional de les equacions hidromecàniques. Ajustant les mesures de camp a aquestes corbes es poden estimar els valors del mòdul de Young i el coeficient de Poisson de l'aqüífer i del segell. Els resultats indiquen que la microsismicitat induïda té més probabilitats d'ocórrer en l'aqüífer que en el segell. L'inici de la microsismicitat en el segell marca la pressió d'injecció màxima sostenible per tal d’assegurar un emmagatzematge permanent de CO2 segur. Finalment, analitzem l'evolució termodinàmica del CO2 i la resposta termo-hidromecànica de les roques segell i magatzem a la injecció de CO2 líquid (fred). Trobem que injectar CO2 en estat líquid és energèticament més eficient perquè al ser més dens que el CO2 supercrític, requereix una pressió menor al cap de pou per a una pressió donada a l’aqüífer. De fet, aquesta pressió també és menor a l’aqüífer perquè es desplaça un volum menor de fluid. La disminució de temperatura a l'entorn del pou indueix una reducció de tensions a causa de la contracció tèrmica del medi. Això pot produir lliscament de fractures existents en aqüífers formats per roques rígides sota contrastos de temperatura grans, la qual cosa podria incrementar la injectivitat de la roca magatzem. D’altra banda, l'estabilitat mecànica de la roca segell millora quan la tensió principal màxima és la vertical.

    Coupled thermo-hydro-mechanical (THM) effects related to geologic carbon storage should be understood and quantified in order to convince the public that carbon dioxide (CO2) injection is safe. This Thesis aims to improve such understanding by developing methods to: evaluate the CO2 plume geometry and fluid pressure evolution; define a field test to characterize the maximum sustainable injection pressure and the hydromechanical (HM) properties of the aquifer and the caprock; and propose an energy efficient injection concept that improves the caprock mechanical stability in most geological settings due to thermo-mechanical effects. First, we investigate numerically and analytically the effect of CO2 density and viscosity variability on the position of the interface between the CO2-rich phase and the formation brine. We introduce a correction to account for this variability in current analytical solutions. We find that the error in the interface position caused by neglecting CO2 compressibility is relatively small when viscous forces dominate. However, it can become significant when gravity forces dominate, which is likely to occur at late times and/or far from the injection well. Second, we develop a semianalytical solution for the CO2 plume geometry and fluid pressure evolution, accounting for CO2 compressibility and buoyancy effects in the injection well. We formulate the problem in terms of a CO2 potential that facilitates solution in horizontal layers, in which we discretize the aquifer. We find that when a prescribed CO2 mass flow rate is injected, CO2 advances initially through the top portion of the aquifer. As CO2 pressure builds up, CO2 advances not only laterally, but also vertically downwards. However, the CO2 plume does not necessarily occupy the whole thickness of the aquifer. Both CO2 plume position and fluid pressure compare well with numerical simulations. Third, we study potential failure mechanisms, which could lead to CO2 leakage, in an axysimmetric horizontal aquifercaprock system, using a viscoplastic approach. Simulations illustrate that, depending on boundary conditions, the least favorable situation may occur at the beginning of injection. However, in the presence of low-permeability boundaries, fluid pressure continues to rise in the whole aquifer, which may compromise the caprock integrity in the long-term. Next, we propose a HM characterization test to estimate the HM properties of the aquifer and caprock at the field scale. We obtain curves for overpressure and vertical displacement as a function of the volumetric strain term obtained from a dimensional analysis of the HM equations. We can then estimate the values of the Young¿s modulus and the Poisson ratio of the aquifer and the caprock by introducing field measurements in these plots. Results indicate that induced microseismicity is more likely to occur in the aquifer than in the caprock. The onset of microseismicity in the caprock can be used to define the maximum sustainable injection pressure to ensure a safe permanent CO2 storage. Finally, we analyze the thermodynamic evolution of CO2 and the THM response of the formation and the caprock to liquid (cold) CO2 injection. We find that injecting CO2 in liquid state is energetically more efficient than in supercritical state because liquid CO2 is denser than supercritical CO2. Thus, the pressure required at the wellhead is much lower for liquid than for gas or supercritical injection. In fact, the overpressure required at the aquifer is also smaller because a smaller fluid volume is displaced. The temperature decrease close to the injection well induces a stress reduction due to thermal contraction of the media. This can lead to shear slip of pre-existing fractures in the aquifer for large temperature contrasts in stiff rocks, which could enhance injectivity. In contrast, the mechanical stability of the caprock is improved in stress regimes where the maximum principal stress is the vertical.

  • Numerical modeling of delayed and progressive failure in stiff clays with two-stage softening behaviour  Open access

     Zhan, Zhifeng
    Defense's date: 2012-06-19
    Universitat Politècnica de Catalunya
    Theses

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    Based on the laboratory, field and numerical results, the post-rupture strength defined firstly by Burland (1990) is verified and introduced in this thesis to describe the two-stage post-peak strain-softening characteristics of stiff clays. The first softening stage is induced by the loss of interbonding between particles, i.e. the cohesive component of strength at relatively small displacement and hence termed as cementation loss; while the second is due to the gradual realignment, i.e. reorientation of clay particles at large displacements and can be called as gradual frictional resistance loss. A generalised constitutive model is then established to simulate the two-stage softening characteristics of stiff clays by modifying the Mohr-Coulomb model. This model is formulated and then implemented into an explicit finite difference program FLAC. The new model is applied to simulate laboratory tests such as triaxial compression tests and direct shear box test to depict the new model. The numerical results demonstrate the capability and efficiency of the modified softening model to model the two-stage strain-softening behaviour of stiff clays. A series of analyses of delayed progressive failure of cut slopes in stiff clays have been performed using two-stage softening model incorporating post-rupture strength. The numerical results reproduce well the progressive failure process, position of failure surface and failure time, which proves further the validity of the new model. Meanwhile, parametric analyses are also carried out to demonstrate the general influence of post-rupture strength. The results demonstrate that the slope stability with the adoption of two-stage softening model is reduced compared with that using one-stage softening model due to the quicker cohesion reduction with deviatoric plastic strain in the first softening stage of two-stage softening model. Both post-rupture strength concept and two-stage softening model are applied to the modelling of a famous case¿ Aznalcóllar dam failure under both inhomogeneous and homogeneous hypotheses. The simulations reproduced well the failure of Aznalcóllar dam including the location and shape of the slip surface, the progressive failure course and the development of pore water pressure in terms of the developments of shear strain rate, shear strain increment, displacement, velocity and strength parameter softening. The mechanism of Aznalcóllar dam failure is deemed to be progressive failure mainly due to the softening of Guadalquivir blue clay. The developments of average stress ratio, average residual factor, average brittleness, average stress path, the distribution of shear stress and mobilised strength parameters along the slip surface confirm further the mechanism of progressive failure of Aznalcóllar dam with these values to be intermediate between peak and residual values during the failure course. The post-rupture state could be thought as the average one at initial failure. At final failure, most part of the slip surface is at residual state, especially along the horizontal part. The Aznalcóllar dam failure is sensitive to the softening rate. Larger rates will induce earlier failure and no failure will occur with slow softening rates. Only an appropriate setting of softening rates can cause failure at final phase under both inhomogeneous and homogeneous hypotheses. Finally, the post-rupture concept is introduced to derive analytical solutions to limit pressure, the stress, strain, and displacement fields for the cylindrical cavity expansion in stiff overconsolidated clay. The results of computational examples and the similarity between numerical solution and analytical one verify the reasonableness of the analytical solution to cavity expansion in stiff clays with two-stage softening characteristics.

    Basado en los datos de laboratorio y de campo y los resultados numéricos, la resistencia post-ruptura definida por Burland (1990) se verifica y introduce para describir las dos etapas de ablandamiento post-ruptura de las arcillas rígidas. La primera etapa del ablandamiento está inducida por la pérdida de inter-conexión entre las partículas, o de la componente cohesiva de la resistencia a desplazamientos relativamente pequeños; la segunda etapa es debida al realineamiento gradual o reorientación de las partículas de arcilla a grandes desplazamientos y se puede llamar como pérdida gradual de la resistencia friccional. Se establece un modelo constitutivo general para simular las características de ablandamiento en dos etapas de las arcillas rígidas, modificando el modelo Mohr-Coulomb. La formulación de este modelo es ilustrada detalladamente y luego implementada en un programa explícito de diferencia finitas FLAC. Se aplica el nuevo modelo para simular los ensayos de laboratorio como ensayos de compresión triaxial y ensayos de corte directo. Los resultados numéricos demuestran la capacidad y eficiencia del modelo modificado de ablandamiento para reproducir el comportamiento de ablandamiento por deformación en dos etapas de las arcillas rígidas. Se han realizado una serie de análisis de la rotura progresiva diferida de taludes en arcillas rígidas, utilizando el nuevo modelo de ablandamiento en dos etapas con incorporación de la resistencia de post-ruptura. Los resultados numéricos reproducen bastante bien el proceso progresivo de la rotura, la posición de la superficie de rotura y el tiempo de rotura, lo cual brinda validez adicional al nuevo modelo. Adicionalmente, se ha llevado a cabo un análisis paramétrico para demostrar la influencia general de la resistencia de post-ruptura. Los resultados demuestran que se reduce la estabilidad de los taludes con la adopción del modelo de ablandamiento de dos etapas en comparación con lo de solo una etapa, debido a la reducción más rápida de la cohesión con la deformación plástica en primera fase de ablandamiento del modelo de ablandamiento de dos etapas. El concepto de la resistencia post-ruptura y el modelo de ablandamiento en dos etapas son aplicados en la simulación numérica del famoso caso de la rotura de la presa de Aznalcóllar bajo las dos hipótesis no homogéneos y homogénea. Las simulaciones reproducen bien la rotura de la presa de Aznalcóllar incluyendo el proceso progresivo de la rotura, la ubicación y la forma de la superficie de deslizamiento y el desarrollo de la presión del agua en los poros, según el desarrollo del estado de la plasticidad, la velocidad de la deformación de corte, el incremento de la deformación de corte, el desplazamiento, la velocidad y parámetros de resistencia del ablandamiento. El desarrollo de la relación media de las tensiones, el factor residual, la fragilidad media, la trayectoria de tensiones media y la distribución de las tensiones de corte movilizadas y los parámetros de resistencia movilizados a lo largo de la superficie de deslizamiento, confirman el mecanismo de rotura progresiva de la presa de Aznalcóllar debida principalmente al ablandamiento de la masa de arcilla azul del Guadalquivir, con estos valores en el intermedio entre los valores pico y residual durante el proceso de la rotura. La rotura de la presa de Aznalcóllar es sensible a la velocidad de ablandamiento de la deformación de corte plástica. Finalmente, se introduce el concepto de la resistencia post-ruptura de las arcillas rígidas para derivar las soluciones analíticas para la presión límite, las tensiones, las deformaciones y campos de desplazamientos de la expansión de cavidades cilíndricas en arcillas rígidas. Los resultados de los ejemplos de cálculo y la similitud entre la solución numérica y analítica verifican la razonabilidad de la solución analítica a la expansión de la cavidad en arcillas rígidas con las características de ablandamiento de dos etapas.

  • FEBEXe project: progress report

     Gens Sole, Antonio; Olivella Pastalle, Sebastian
    Date: 2012-04-04
    Report

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  • Crack patterns in clayey soils: experiments and modeling

     Trabelsi, H; Jamei, M.; Zenzri, H.; Olivella Pastalle, Sebastian
    International journal for numerical and analytical methods in geomechanics
    Date of publication: 2012-08-10
    Journal article

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  • A study of the effects of suction and roughness on the mechanical behaviour of rock joints

     Zandarín Iragorre, Maria Teresa; Alonso Pérez de Agreda, Eduardo; Olivella Pastalle, Sebastian
    International Symposium on Computational Geomechanics
    Presentation's date: 2011-04
    Presentation of work at congresses

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    The experimental investigation focused on the effects of suction on the mechanical behaviour of rock joints. Available experimental data on the effect of moisture on joint behaviour is very scarce. Laboratory tests were performed in a direct shear cell equipped with suction control. Suction was imposed using a vapour forced convection circuit connected to the cell and controlled by an air pump. Artificial joints of Lilla claystone were prepared. Joint roughness of varying intensity was created by carving the surfaces in contact in such a manner that rock ridges of different tip angles were formed. Several tests were performed for different values of suction (20, 100 and 200 MPa) and for different values of vertical stress (30, 60 and 150 kPa). A constitutive model including the effects of suction and joints roughness is proposed to simulate the unsaturated behaviour of rock joints. The new constitutive law was incorporated into the FE computer code Code-Bright and experimental results were numerically simulated.

  • Coupled THMC analysis in porous media

     Gens Sole, Antonio; Guimarães, Leonardo do N; Olivella Pastalle, Sebastian
    Symposium on Mechanics and Physics of Porous Solids
    Presentation's date: 2011-04
    Presentation of work at congresses

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    A coupled thermo-hydro-mechanical-chemical (THMC) formulation is presented. The formulation combines a THM approach already existing and operational with the equations of reactive transport in a fully coupled manner. The reactive transport equations are set in the context of an unsaturated deformable porous medium with several chemical species dissolved in the liquid phase but also present in the solid phase. Chemical reactions considered in the formulation include: i) homogeneous reactions: aqueous complex formation, acid/base and oxidationlreduction, and ii) heterogeneous reactions: dissolution/precipitation of minerals and cation exchange. Local equilibrium is assumed for all the chemical reactions except for dissolutionlprecipitation of minerals where kinetics can also be considered. Two examples of application are described. The fust one addresses the issue of scale formation around boreholes when oil production is stimulated by waterflooding. In the second one, the THMC behaviour of a bentonite engineered barrier is analyzed in the context of a large scale in situ heating test.

  • Is the degree of saturation a good candidate for Bishop¿s X parameter?

     Pereira, J.M.; Coussy, Olivier; Alonso Pérez de Agreda, Eduardo; Vaunat, Jean; Olivella Pastalle, Sebastian
    International Conference on Unsaturated Soils
    Presentation's date: 2011
    Presentation of work at congresses

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    In unsaturated soil mechanics, the quest for an effective stress playing the same role as Terzaghi’s effective stress does for saturated soils has introduced a long standing debate, dating back to the 1960s. Several contributions have been proposed since the early work of Bishop. It is well recognized to date that a single constitutive stress is not sufficient by itself to catch the main features of the behaviour of unsaturated soils and it is often combined with matric suction. In this paper, focus is given to a largely used formulation for such a constitutive stress, based on the use of an averaged pore pressure. In particular, this paper discusses on thermodynamics bases the validity of the choice of the factor X weighting the fluid pressures contribution to the constitutive stress. This factor is usually assumed to be equal to the degree of saturation of water. In this work it is shown that the choice of this natural candidate implies restrictive assumptions on the plastic flow rule. As shown from experimental data obtained from a literature review, this choice may not be pertinent for certain classes of materials, particularly high plasticity clays.

  • Lechago dam: a case study

     Alonso Pérez de Agreda, Eduardo; Olivella Pastalle, Sebastian; Pinyol Puigmarti, Nuria Merce
    Laboratório Nacional de Engenharia Civil - International Conference on Dam Engineering
    Presentation's date: 2011
    Presentation of work at congresses

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    Lechago dam (Teruel, Spain) is a 40 m high zoned earth and rockfill dam founded on soft continental deltaic deposits. A relatively narrow central clay core is stabilized by wide rockfill shoulders. The dam was completed in January 2009 and impounding will probably start at the beginning of 2011. The dam was well instrumented and continuous records of stress development, pore water pressures and horizontal and vertical displacements are available for the construction period. The clay core material was investigated by means of tests performed on compacted specimens of Tertiary clays. Rockfill samples were excavated in outcrops of highly fractured Cambrian quartzitic shale. In the years to follow the dam design, an extensive testing program on compacted rockfill gravels was conducted under Relative Humidity control in large diameter oedometer and triaxial cells. The results of this research have been reported in a number of papers. A coupled FE model has been developed to analyze the dam behaviour during construction. Model predictions, essentially based on laboratory tests, are compared with measurements during construction.

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    CO2 injection in deep saline sloping aquifers through a vertical well  Open access

     Vilarrasa Riaño, Victor; Olivella Pastalle, Sebastian; Carrera Ramirez, Jesus
    Workshop of CODE-BRIGHT Users
    Presentation's date: 2011
    Presentation of work at congresses

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  • Evaluation of self-combustion risk in tire derived aggregate fills

     Arroyo Alvarez de Toledo, Marcos; San Martin, Ignacio; Olivella Pastalle, Sebastian; Saaltink, Maarten Willem
    Waste management
    Date of publication: 2011-09
    Journal article

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    Lightweight tire derived aggregate (TDA) fills are a proven recycling outlet for waste tires, requiring relatively low cost waste processing and being competitively priced against other lightweight fill alternatives. However its value has been marred as several TDA fills have self-combusted during the early applications of this technique. An empirical review of these cases led to prescriptive guidelines from the ASTM aimed at avoiding this problem. This approach has been successful in avoiding further incidents of self-combustion. However, at present there remains no rational method available to quantify selfcombustion risk in TDA fills. This means that it is not clear which aspects of the ASTM guidelines are essential and which are accessory. This hinders the practical use of TDA fills despite their inherent advantages as lightweight fill. Here a quantitative approach to self-combustion risk evaluation is developed and illustrated with a parametric analysis of an embankment case. This is later particularized to model a reported field self-combustion case. The approach is based on the available experimental observations and incorporates well-tested methodological (ISO corrosion evaluation) and theoretical tools (finite element analysis of coupled heat and mass flow). The results obtained offer clear insights into the critical aspects of the problem, allowing already some meaningful recommendations for guideline revision.

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    Modeling evaporation processes in a saline soil from saturation to oven dry conditions  Open access

     Gran Esforzado, Meritxell; Carrera Ramírez, Jesús; Olivella Pastalle, Sebastian; Saaltink, Maarten Willem
    Hydrology and Earth system sciences discussions
    Date of publication: 2011
    Journal article

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    Thermal, suction and osmotic gradients interact during evaporation from a salty soil. Vapor fluxes become the main water flow mechanism under very dry conditions. A coupled nonisothermal multiphase flow and a reactive transport model of a salty sand soil was developed to study such an intricate system. The model was calibrated with data from an evaporation experiment (volumetric water content, temperature and concentration). The retention curve and relative permeability functions were modified to simulate oven dry conditions. Experimental observations were satisfactorily reproduced, which suggests that the model can be used to assess the underlying processes. Results show that evaporation is controlled by heat, and limited by salinity and liquid and vapor fluxes. Below evaporation front vapor flows downwards controlled by temperature gradient and thus generates a dilution. Vapor diffusion and dilution are strongly influenced by heat boundary conditions. Gas diffusion plays a major role in the magnitude of vapor fluxes.

  • Dynamics of water vapor flux and water separation processes during evaporation from a salty dry soil

     GRAN, MERITXELL; MASSANA, JORDI; Carrera Ramírez, Jesús; Saaltink, Maarten Willem; Olivella Pastalle, Sebastian; Ayora Ibáñez, Carlos; Lloret Morancho, Antonio
    Journal of hydrology
    Date of publication: 2011-01
    Journal article

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    Evaporation from a salty soil generates salt accumulation near the surface with the subsequent deterioration of the soil quality. Salinization mechanisms are poorly understood despite their global impact. Vapor flux and solute transport were studied under evaporation conditions. Laboratory experiments consisted of open sand and silt columns initially saturated with epsomite (MgSO47H2O) or halite (NaCl) solutions. Salt precipitation occurred only above the evaporation front, which occupied a very narrow region. Vapor flowed both upwards and downwards from this front. The downward vapor flow condensed further down the column, diluting the solution. This gave rise to two areas: a high salinity area above the evaporation front, and a diluted solution area below it. The effects of thermal, suction and osmotic gradients on water fluxes were studied in order to better understand the underlying mechanisms of this phenomenon.

    Evaporation from a salty soil generates salt accumulation near the surface with the subsequent deterioration of the soil quality. Salinization mechanisms are poorly understood despite their global impact. Vapor flux and solute transport were studied under evaporation conditions. Laboratory experiments consisted of open sand and silt columns initially saturated with epsomite (MgSO4 7H2O) or halite (NaCl) solutions. Salt precipitation occurred only above the evaporation front, which occupied a very narrow region. Vapor flowed both upwards and downwards from this front. The downward vapor flow condensed further down the column, diluting the solution. This gave rise to two areas: a high salinity area above the evaporation front, and a diluted solution area below it. The effects of thermal, suction and osmotic gradients on water fluxes were studied in order to better understand the underlying mechanisms of this phenomenon.

  • Thermo-hydro-mechanical model of the Canister Retrieval Test

     Zandarín Iragorre, Maria Teresa; Gens Sole, Antonio; Olivella Pastalle, Sebastian; Alonso Pérez de Agreda, Eduardo
    Physics and chemistry of the Earth
    Date of publication: 2011
    Journal article

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  • Modelling the response of Lechago earth and rockfill dam

     Alonso Pérez de Agreda, Eduardo; Olivella Pastalle, Sebastian; Soriano, A.; Pinyol Puigmarti, Nuria Merce; Esteban, F.
    Géotechnique
    Date of publication: 2011-03
    Journal article

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    Lechago dam (Teruel, Spain) is a 40 m high zoned earth and rockfill dam sitting on soft continental deltaic deposits. A relatively narrow central clay core is stabilised by wide rockfill shoulders. The dam was well instrumented and continuous records of stress development, pore-water pressures and vertical displacements are available for the construction period. Compaction conditions were followed by means of laboratory and in situ control tests. Core clay material was investigated by means of tests performed on compacted specimens of tertiary clays. Rockfill samples were excavated in outcrops of highly fractured Cambrian quartzitic shale. A testing programme on compacted rockfill gravels was conducted under relative humidity control in a large-diameter oedometer and triaxial cells. A coupled finite-element model has been developed to analyse the tests performed and dam behaviour during construction. Model predictions, essentially based on laboratory tests, are compared with measurements during construction. The predicted response of the dam under an assumed programme of impounding is also given. In the future, once impounding occurs, it will be possible to compare these predictions with actual dam performance. The paper provides an integrated description of the dam design, construction and early behaviour. It presents a procedure to interpret available data (laboratory as well as in situ data) on compacted materials from the perspective of modern constitutive models. It also provides an evaluation of the capabilities of advanced numerical tools to reproduce the measured dam behaviour.

  • Porosity variations in saline media induced by temperature gradients: experimental evidences and modelling

     Olivella Pastalle, Sebastian; Castagna, S.; Alonso Pérez de Agreda, Eduardo; Lloret Morancho, Antonio
    Transport in porous media
    Date of publication: 2011-12
    Journal article

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    Porosity variations in saline media containing humidity are induced by temperature gradients. A temperature imposed on a porous salt sample prepared with some brine and closed to mass transfer leads to significant variations of porosity in few weeks. Modelling of the experiments permits to understand the processes involved.

  • MODELLING OF POST-CLOSURE BEHAVIOUR FOR MUNICIPAL SOLID WASTE LANDFILLS: SETTLEMENT AND SLOPE STABILITY

     Yu, Li
    Defense's date: 2011-07-08
    Department of Geotechnical Engineering and Geo-Sciences, Universitat Politècnica de Catalunya
    Theses

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  • Deformation and flow driven by osmotic processes in porous materials  Open access  awarded activity

     Mokni, Nadia
    Defense's date: 2011-02-22
    Department of Geotechnical Engineering and Geo-Sciences, Universitat Politècnica de Catalunya
    Theses

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    En el caso del almacenamiento de los residuos radioactivos los flujos osmóticos pueden ser relevantes y requieren un análisis en detalle. El residuo nuclear bituminizado (BW) será almacenado mediante contenedores en cavidades excavadas en la Boom Clay, que es una arcilla marina que presenta propiedades favorables para limitar y retrasar la migración de los contaminantes radioactivos. La interacción entre los dos materiales es un proceso acoplado químico-hidro-mecánico y depende de la respuesta hidromecánica de la Boom Clay y del BW. En condiciones de almacenamiento, el contacto del BW, que contienen cantidades importantes de NaNO3, con el agua subterránea induce la hidratación por gradientes osmóticos y el consiguiente hinchamiento, además de la difusión de la sal disuelta hacia la Boom Clay. Se pueden distinguir dos tipos de afecciones: la perturbación geomecánica causada por el hinchamiento del BW y el aumento de presión en el BW y cambio de las distribución de tensiones en la roca, y la perturbación físico química por la migración de grandes cantidades de sales. El objetivo de esta tesis es: (i) Mejorar la comprensión de los procesos que controlan la absorción de agua y el consecuente hinchamiento del BW que contengan sales (NaNO3), y (ii) Investigar los posibles efectos de la concentración de fluidos de los poros sobre el hinchamiento, la compresibilidad y comportamiento de corte de la Boom Clay. En primer lugar, se ha desarrollado una formulación para el análisis de la deformación inducida por la disolución de sales en medio poroso con contacto con agua. Las ecuaciones planteadas incluyen los flujos acoplados de agua y soluto. Se presenta también un trabajo teórico que ayuda a la comprensión del comportamiento mecánico del BW. Se considera este material como una mezcla de bitumen y cristales de NaNO3. Se ha desarrollado un modelo elasto-viscoplástico que describe el comportamiento de fluencia del BW considerando el comportamiento de fluencia de sus constituyentes. El modelo constitutivo elasto-viscoplástico ha sido implementado en el programa CODE_BRIGHT. Los resultados se han comparado con observaciones experimentales. Se ha estudiado el comportamiento a largo plazo del BW en contacto con agua al simular ensayos de hinchamiento por absorción de agua bajo condiciones confinadas. El análisis numérico ha demostrado ser capaz de proporcionar una representación satisfactoria de los principales patrones observados en su comportamiento. En lo que respecta al segundo objetivo de la tesis, se ha propuesto una formulación para el análisis de las deformaciones inducidas por procesos osmóticos en un medio poroso de doble estructura. Esta formulación distingue dentro del material un nivel micro-estructural y otro macro-estructural con cambios químicos que tienen un efecto significativo en la micro-estructura. Se han obtenido las ecuaciones básicas que describen los flujos acoplados de agua y solutos y el transporte de sus componentes a través de los macroporos así como las ecuaciones de balance de masa para agua y soluto en los macroporos y microporos. La formulación propuesta ha sido aplicada particularmente para analizar cualitativamente el efecto de la succión osmótica sobre el hinchamiento de los suelos arcillosos. Se han analizado los efectos a corto y largo plazo. Se ha investigado también la influencia del aumento de la concentración del fluido en los poros sobre las propiedades geotécnicas y el comportamiento de la Boom Clay no saturada. Se ha llevado a cabo un programa sistemático de investigación experimental, con control de succión osmótica y matricial, con el fin de investigar el efecto del incremento de la concentración del fluido de poros sobre la resistencia de corte y el cambio volumétrico bajo condiciones edométricas. Se ha observado, que bajo condiciones parcialmente saturadas, un cambio en la salinidad provoca una disminución en la compresibilidad y en la resistencia de corte del material.

    For deep storage of high-level nuclear waste osmotic flows can be significant and so require a careful analysis. In Belgium, The bituminized nuclear waste (BW) named Eurobitum contained in metallic drums will be placed inside a tunnel or a shaft excavated in the Boom Clay, which is 100 m thick marine clay presenting favourable properties to limit and delay the migration of the leached radionuclides over extended periods of time. In Geological disposal conditions, contact of the bituminized radioactive waste which contains high amounts of highly soluble salt (NaNO3) with groundwater will result in water uptake and swelling of the waste and in subsequent diffusion of the dissolved salt through the host clay formation. Basically, two types of disturbance can be distinguished: A geo-mechanical perturbation, caused by the swelling of the waste and the increase of the pressure in and around the waste and a physico-chemical perturbation by the release of large amounts of NaNO3 and other soluble salts. In this context the aim of this thesis is: (i) to improve the understanding of the processes controlling the water uptake and the subsequent swelling of bituminized waste containing soluble salts (NaNO3), and (ii) to investigate of the possible effects of the increase of pore fluid concentration on swelling, compressibility and shear behaviour of Boom Clay. A formulation has been proposed for the analysis of deformation induced by dissolution of salts in porous media in contact with water. The equations include the effect of coupled transport phenomena and the formulation has been included as an extension in the coupled THM program CODE_BRIGHT. A theoretical and experimental work aiming at understanding the mechanical behaviour of the Bituminized Waste has been presented.This material is considered for this purpose as a mixture of bitumen and crystals of NaNO3. An elasto-viscoplastic model has been developed that describes the creep behaviour of BW considering the constituents' creep behaviour. The elasto-viscoplastic constitutive model has been implemented into CODE_BRIGHT. The modelling results have been compared with the experimental data. The impact of osmotic forces on the swelling of the material has been investigated by simulating water uptake swelling tests under confined conditions and comparing the predictions with experimental results. The numerical analysis has proven to be able to furnish a satisfactory representation of the main observed patterns of the behaviour. In regard to the second objective of this thesis, a formulation has been proposed for the analysis of deformations induced by osmotic processes in double structure porous media. The formulation is based on the distinction within the material of a microstructural and a macrostructural levels with chemical changes having a significant effect on the microstructure. A macroscopic description of the system is provided. Then the basic equations describing coupled flows of water and solutes and the transport of its components through macropores and mass balance equations for water and solute in macro and micro pores have been obtained. The proposed formulation has been particularly applied to analyze qualitatively the effect of osmotic suction on swelling of clayey soils. Transient and long term effects have been analyzed. The influence of pore fluid concentration on the geotechnical properties and behavior of Boom Clay under partially saturated conditions has been investigated. A systematic experimental research program involving osmotic suction and matric suction controlled experiments has been carried to investigate the effect of the increase of pore fluid concentration on shear strength and on the volume change behaviour under odometer stress state conditions. It has been observed that under partially saturated conditions a change in salinity causes a decrease in compressibility and shear strength.

  • A bridge foundation analysis

     Puig Damians, Ivan; Olivella Pastalle, Sebastian; Josa Garcia-tornel, Alejandro
    Date: 2011-06-02
    Report

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    Geomechanical stability of the caprock during CO2 sequestration in deep saline aquifers  Open access

     Vilarrasa Riaño, Victor; Olivella Pastalle, Sebastian; Carrera Ramirez, Jesus
    Energy procedia
    Date of publication: 2011
    Journal article

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    Sequestration of carbon dioxide (CO2) in deep saline aquifers has emerged as a mitigation strategy for reducing greenhouse gas emissions to the atmosphere. The large amounts of supercritical CO2 that need to be injected into deep saline aquifers may cause large fluid pressure buildup. The resulting overpressure will produce changes in the effective stress field. This will deform the rock and may promote reactivation of sealed fractures or the creation of new ones in the caprock seal, which could lead to escape paths for CO2. To understand these coupled hydromechanical phenomena, we model an axisymmetric horizontal aquifer-caprock system. We study plastic strain propagation patterns using a viscoplastic approach. Simulations illustrate that plastic strain may propagate through the whole thickness of the caprock if horizontal stress is lower than vertical stress. In contrast, plastic strain concentrates in the contact between the aquifer and the caprock if horizontal stress is larger than vertical stress. Aquifers that present a low-permeability boundary experience an additional fluid pressure increase once the pressure buildup cone reaches the outer boundary. However, fluid pressure does not evolve uniformly in the aquifer. While it increases in the low-permeability boundary, it drops in the vicinity of the injection well because of the lower viscosity of CO2. Thus, caprock stability does not get worse in semi-closed aquifers compared to open aquifers. Overall, the caprock acts as a plate that bends because of pressure buildup, producing a horizontal extension of the upper part of the caprock. This implies a vertical compression of this zone, which may produce settlements instead of uplift in low-permeability (k≤10-18 m2) caprocks at early times of injection.

  • Deformation and flow driven by osmotic processes in porous materials: Application to Bituminised Waste Materials

     Mokni, Nadia; Olivella Pastalle, Sebastian; Valcke, Elie; Mariën, An; Smets, Esteven; Li, Xiangling
    Transport in porous media
    Date of publication: 2011-01
    Journal article

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  • Numerical analysis of the behaviour of embankments constructed with expansive soils

     Ortega, J.J.; Lloret Morancho, Antonio; Olivella Pastalle, Sebastian; Romero Morales, Enrique Edgar
    International Conference on Unsaturated Soils
    Presentation's date: 2010-09
    Presentation of work at congresses

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    The recent advances made in modern Unsaturated Soil Mechanics have permitted the development of new analysis methods for the prediction of the behaviour of embankments and other earth structures subject to changes in atmospheric conditions (precipitation, temperature, relative humidity) and under the action of traffic loading. One of the breakthroughs of greatest interest is the possibility to numerically model the processes of volume change in expansive soils when subjected to variations in humidity. In this paper, the analysis employed, based upon the Barcelona Basic Model constitutive model (Alonso et al., 1990), to study the viability of the embankments of the speed railway Montornès del Vallès-La Roca del Vallès (LAV Madrid-Zaragoza-Barcelona-French Frontier), is presented. A coupled flux-deformation analysis has been undertaken to model the water-infiltration process through the embankment, and the associated volume change deformations have been evaluated in terms of rainstorm duration.

  • Constitutive model developments for compacted unsaturated fine grained soils based on porosimetry

     Salgueiro, René; Olivella Pastalle, Sebastian; Suriol Castellvi, Jose
    International Conference on Unsaturated Soils
    Presentation's date: 2010-09-07
    Presentation of work at congresses

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    Modeling the mechanical behavior of compacted unsaturated soils against stress and suction changes is complex. One reason is that the structure mostly depends on the initial moisture content and compaction energy applied. Herein, it is proposed an alternative method to model this kind of soils based on its Pore Size Distribution (PSD), as a possible characterization of its structure. This latter can be obtained by means of the Mercury Intrusion Porosimetry test (MIP). The proposed model should permit the prediction of the volumetric deformations produced as a consequence of stress and suction changes. In order to do this, relationships have been developed to be applied to every pore size considered. The integration over all the sizes leads to the calculation of macroscopic values. Those relationships use fundamental physical principles. In order to determinate the stress-strain behavior, it has been considered that volumetric deformation moduli, both for changes in mean stress and suction, are function of pore size. Finally, the preliminary model equations have been applied to simulate suction controlled oedometer tests, including the subsequent sample’s collapse observed for different values of vertical stress. The investigated soil is a compacted silty clay for which the initial pore size distribution is known.

  • Influence of suction on the mechanical behaviour of rock joints under shear loading

     Zandarín Iragorre, Maria Teresa; Alonso Pérez de Agreda, Eduardo; Olivella Pastalle, Sebastian
    International Conference on Unsaturated Soils
    Presentation's date: 2010-09-08
    Presentation of work at congresses

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    A direct shear equipment was modified for testing rock joints with suction control. Modification includes a vapour forced convection circuit connected to the air chamber of shear cell for controlling relative humidity during testing. The main features of the device are described and test program procedures are presented. In addition the results of tests conducted under different suction, net normal stress and asperity roughness angle conditions are presented and analyzed. Finally, new expressions of resistance parameters of hyperbolic failure envelope are proposed incorporating the effect of suction and asperity roughness angle.

  • Gas migration in sand/bentonite mixtures through preferential paths

     Arnedo Gaute, Diego; Alonso Pérez de Agreda, Eduardo; Olivella Pastalle, Sebastian; Romero Morales, Enrique Edgar
    International Conference on Unsaturated Soils
    Presentation's date: 2010
    Presentation of work at congresses

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    Sand/bentonite mixtures are considered as buffer material in low and medium radioactive waste disposal schemes. While the bentonite provides low permeability and swelling capacity, the sand is intended to improve mechanical stability and to reduce the amount of bentonite required. One of the requirements of the buffer is to ensure the isolation of the gas-emitting waste. The pressurized gas may migrate towards the host geological media through several mechanisms (dissolution, two-phase flow, preferential paths). This paper is a contribution to this topic and presents experimental results as well as some modelling work. Laboratory experiments were performed on statically compacted 80/20 sand/bentonite mixture recovered from the in situ large scale GMT (Gas Migration Test) buffer. The sample was tested in a specifically designed Perspex test cell which allows the visualization of the boundary of the sample. Two needles inserted inside the sample are used to inject and recover gas. The formation of preferential paths following the compaction layers and the sample-wall interface were detected in the test. The test is analyzed by hydro-mechanical numerical simulations considering randomly distributed properties (porosity, permeability and hardening parameter) and the possibility of opening discontinuities in the permeability law are considered. Also a computational experiment inspired in the conditions prevailing on the in situ large scale GMT test, is presented.

  • Coupled hydromechanical modeling of CO2 sequestration in deep saline aquifers

     Vilarrasa Riaño, Victor; Bolster, Diogo; Olivella Pastalle, Sebastian; Carrera Ramírez, Jesús
    International journal of greenhouse gas control
    Date of publication: 2010-12
    Journal article

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    Some investigations about the tensile strength and the desiccation process of unsaturated clay  Open access

     Trabelsi, H; Jamei, M.; Guiras, H.; Hatem, H; Romero Morales, Enrique Edgar; Olivella Pastalle, Sebastian
    European physical journal. Special topics
    Date of publication: 2010-06-10
    Journal article

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    The current paper presents an investigation on the tensile strength of unsaturated clay, and a discussion about the significant differences between the behaviours of compacted soils and those of the completely remoulded soils. The adapted experimental device based on the shear box for the measurement of the tensile strength is also presented. In the tensile tests, stress-strain characteristics are measured using high precision transducers and electronic data acquisition. The equipment is capable of measuring tensile strengths of soil (about 10 MPa). Moreover, a new model to predict cracks is presented in 3D. This analytical model for the prediction of tensile strength, as a function of suction and porosity, is given.

  • Modelling thermo-hydro-mechano-chemical interactions for nuclear waste disposal

     Gens Sole, Antonio; Guimarães, Leonardo do N; Olivella Pastalle, Sebastian; Sánchez, M.
    Journal of rock mechanics and geotechnical engineering
    Date of publication: 2010
    Journal article

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  • A code for multiphase reactive transport modeling of concentrated solutions under extreme dry conditions  Open access

     Gamazo Rusnac, Pablo Andres
    Defense's date: 2010-12-20
    Department of Geotechnical Engineering and Geo-Sciences, Universitat Politècnica de Catalunya
    Theses

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    La modelació del transport reactiu multifàsic (TRMF) involucra la simulació del flux de fases fluïdes, el transport de compostos químics i energia, i les reaccions químiques. La majoria dels codis de transport reactiu desacoblen els càlculs de flux de fases del transport reactiu. Aquesta metodologia que s'utilitza per resoldre diversos problemes de TRMF resulta inadequada per adreçá problemes com ara l'evolució de piles de deixalles mineres o la salinització de sòls a on poden trobar-se solucions concentrades o condicions d'extrema sequedat. En aquests casos la quantitat d'aigua líquida pot ser tan petita que tant el vapor com la precipitació mineral poden afectar el seu balanç. Wissmeier i Barry (2008) han desenvolupat un codi que acobla el terme font químic i els càlculs de flux, però solament serveix per a casos on el transport està limitat a la fase líquida. Tot i això, sota aquestes condicions, el transport de gas pot ser important i l'activitat de l'aigua que controla la pressió de vapor es pot veure afectada tan per efectes capil·lars com ara de la salinitat. A més, certes paragènesis (les que produeixen punts invariants) poden fixar l'activitat de l'aigua, produint que la geoquímica controli la pressió de vapor, la qual és una variable clau per simular correctament el flux de gas. Per tant, una solució que acobli el flux de fases i el transport reactiu és necessària.El principal objectiu d'aquesta tesi és desenvolupar un codi de TRMF capaç de representar la influència de la geoquímica sobre els fluxes i el transport en solucions concentrades i condicions d'extrema sequedat. Com objectiu secundari s'estudien alguns casos sota aquestes condicions.En primer lloc, es discuteix l'acoblament de la salinitat i la pressió de vapor (i per tant l'evaporació), i es presenta un mètode per calcular l'evolució de sistemes altament salins. Es fa especial èmfasi en la valoració de punts invariants, on l'activitat de l'aigua és controlada pel conjunt de minerals precipitats. El mètode és contrastat contra un experiment d'evaporació d'una salmorra natural rica en MgSO4 i amb un model simplificat d'una llacuna salobre perenne. Els resultats obtinguts indiquen que la paragènesis mineral pot tenir una influència considerable sobre l'evolució de sistemes salobres en fixar la composició química durant un interval considerable de temps.En segon lloc, es discuteix l'evolució d'una solució concentrada en un mitjà porós. En aquestes condicions l'evaporació es veu afectada no solament per efectes de la salinitat sinó també per capil·lars, i per processos de transport. Es presenta una formulació composicional per TRMF que considera els efectes de l'acoblament entre processos geoquímics i hidrodinàmics, i es discuteixen alguns aspectes en relació a la seva solució numèrica. Els avantatges d'aquesta formulació són il·lustrades mitjançant la simulació de l'efecte de la deshidratació mineral en els processos hidrodinàmics en una columna de guix en condicions d'extrema sequedat. Els resultats indiquen que els efectes acoblats poden tenir una influència significativa en l'evolució de sistema.Finalment, presentem la implementació del codi. Es discuteix la importància de la flexibilitat en el desenvolupament de codis de transport reactiu mitjançant la programació orientada a objectes. Es presenten les principals classes que utilitza el codi i com interaccionen entre elles. El codi s'utilitza per modelar un experiment d'evaporació d'una columna de sorra saturada en una solució de MgSO4. La interacció entre processos hidrodinàmics i geoquímics és analitzada. Els resultats obtinguts mostren que l'aparició de punts invariants en l'extrem superior de la columna té un efecte apreciable en la valoració de la sortida de vapor i en la precipitació de sals en la columna. De fet, els punts invariants expliquen les fluctuacions espacials observades en la precipitació de sals.

    Multiphase Reactive Transport (MPRT) modelling involves simulating flow of fluid phases, transport of species and energy, and reactions between species within the same or different phases. Reactive transport codes decouple phase flow calculations from reactive transport. This approach has been successfully applied to a wide range of MPRT problems, but it may be unsuitable for problems like the chemical evolution of unsaturated tailings or the salinization of soils, where concentrated solutions or extremely dry conditions are be reached. The amount of liquid water in these cases can be so small that both vapor and water in hydrated minerals can be significant for the water balance. Wissmeier and Barry (2008) developed a code which couples chemical sink-sources and water flow, but only for cases where transport is limited to unsaturated liquid phase. However, under these extreme conditions gas transport becomes important and water activity, which controls vapor pressure, is affected by capillary and salinity effects. Moreover, certain mineral paragenesis (the ones that produce invariant points) fix water activity, causing the geochemistry to control vapor pressure, which is a key gas flow variable. Thus, a fully coupled solution of phase fluxes and reactive transport is required for these conditions.The main objective of this thesis is to develop a general MPRT code capable of representing the effect of geochemistry on flow and transport for concentrated solutions under extreme dry conditions. As a secondary objective, the behavior of some cases under such conditions is studied. Different aspects of this code and different cases are discussed through the chapters of this thesis. First, coupling of salinity and vapor pressure (and thus evaporation rate) is discussed, and a method for computing the evolution of high salinity systems is presented. Special emphasis is placed on the treatment of invariant points, that are sets of minerals that fix water activity. The method is applied to a natural MgSO4-rich brine evaporation experiment and to a simplified model of a perennial saline playa lake. The results indicate that mineral paragenesis can have a considerable influence on the evolution of shallow brine systems by fixing chemical composition for a significant portion of time. Second, the evolution of concentrated solution in porous media is discussed. In this scenario evaporation is affected not only by salinity but also by capillary effects. Also transport is considered. A generalized compositional formulation for MPRT, which considers coupling effects between geochemical and hydrodynamic process problems, is presented. Some aspects related to its numerical solution are discussed as well. The advantages of the formulation are illustrated by simulating the effect of mineral dehydration on the hydrodynamic processes in a gypsum column that reaches extremely dry conditions. The results indicate this significantly affects the evolution of the system.Finally, implementation of the code is presented. The importance of flexibility for reactive transport codes and the way how objects oriented programming can facilitate this feature is discussed. The code's main classes and their interactions are presented. The code is used to model a laboratory experiment where a sand column saturated with an MgSO4 solution is subject to evaporation. Extreme dry conditions and high salinity content are reached in this experiment. The interaction between hydrodynamic and geochemical processes on the model is analyzed. Model results shows that the occurrence of invariant points on the top of the domain can have an appreciable effect on the outlet of vapor from the column and on the distribution of salt precipitates along the column. In fact, invariant points explain spatial fluctuation of salt precipitates.

  • Thermo-Hydro-Mechanical analysis of joints. A theoretical and experimental study  Open access

     Zandarín Iragorre, Maria Teresa
    Defense's date: 2010-11-22
    Department of Geotechnical Engineering and Geo-Sciences, Universitat Politècnica de Catalunya
    Theses

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    The thesis presents numerical and experimental studies on joints.A formulation for the coupled analysis of thermo- hydro- mechanical problems in joints is presented. The work involves the establishment of equilibrium and mass and energy balance equations. Balance equations were formulated taking into account two phases: water and air. Once the joint element was implemented in Code_Bright computer code, it was then used to study some cases. The numerical simulation of hydraulic shear tests of roughness granite joints is presented. This allowed evaluating the coupling between permeability and the geometry of the opening of the joint. Then, a thermo-hydro-mechanical simulation of a large-scale nuclear waste repository was performed considering the joint element to model the interface between canister and bentonite. This example allowed evaluating the behaviour of the joint element when coupled to continuum elements. Then, other examples show how hydro-mechanical coupling affects the permeability and saturation of the joints and adjacent material. Finally, the gas flow through joints was also investigated.The experimental investigation focused on the effects of suction on the mechanical behaviour of rock joints. Laboratory tests were performed in a direct shear cell equipped with suction control. Suction was monitored using a vapour forced convection circuit controlled by an air pump that was connected to the shear cell. Once equilibrium conditions are achieved, the rock specimen is assumed to reach the Relative Humidity imposed by the flowing air at a given vapour concentration. Artificial joints of Lilla claystone were prepared. Joint roughness of varying intensity was created by carving the surfaces in contact in such a manner that rock ridges of different tip angles formed. These angles ranged from 0º (smooth joint) to 45º (very rough joint profile). The geometric profiles of the two surfaces in contact were initially positioned in a "matching" situation. Several tests were performed for different values of suction (200, 100 and 20 MPa) and for different values of vertical stress (30, 60 and 150 kPa).A constitutive model including the effects of suction and joint roughness is proposed to simulate the unsaturated behaviour of rock joints.The new constitutive law was incorporated in the code and experimental data were numerically simulated.

  • Modelación de problemas geotecnicos hidromecanicos utilizando el metodo del punto material  Open access

     Zabala, Francisco
    Defense's date: 2010-10-28
    Department of Geotechnical Engineering and Geo-Sciences, Universitat Politècnica de Catalunya
    Theses

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    En esta tesis se trata de la adaptación del Método del Punto Material al análisis de problemas geotécnicos incluyendo el comportamiento hidromecánico de materiales en condición saturada. El Método del Punto Material ó MPM (Sulsky et al. 1994, Sulsky et al. 1996) combina ventajas del método de los elementos finitos y de los métodos de partículas. Podría describirse como una extensión del método de los elementos finitos en el que se utiliza una malla para resolver las ecuaciones de gobierno, mientras que el continuo ó el material se representan mediante "puntos materiales" ó "partículas" langrangianas de masa fija. En el MPM la información que transportan las partículas: cantidad de movimiento, tensión, deformación etc. se "proyecta" en la malla para cada paso de la solución. Esta cubre íntegramente el dominio del problema y en ella se imponen las condiciones de contorno. Luego de resolver en la malla las formas discretas de las ecuaciones de gobierno, se actualiza la posición de las partículas y la información que transportan. Así, al final de cada paso, la malla puede descartarse porque toda la información la transportan las partículas. Se evitan al mismo tiempo la distorsión de la malla para grandes desplazamientos (mesh tangling) y los problemas de convección. El método se adapta especialmente a problemas dinámicos de grandes desplazamientos y deformaciones finitas e incorpora en forma natural un algoritmo de contacto sin deslizamiento que facilita su utilización en problemas de interacción entre sólidos.El análisis de problemas geotécnicos hidromecánicos utilizando el MPM requiere del desarrollo de algoritmos para resolver el acoplamiento entre el esqueleto de suelo y las fases agua y/o aire y de la regularización del problema de localización de deformaciones para evitar que las soluciones dependan en forma patológica de la malla, que en el caso del MPM es una malla "soporte". En este trabajo se ha desarrollado un algoritmo explícito, en el marco del Método del Punto Material, que resuelve el problema acoplado presión de poros-desplazamiento. Además se ha adaptado al MPM un procedimiento numérico para la introducción de discontinuidades fuertes en la mallasoporte (Wells et al. 2001, Wells et al. 2002). Se ha continuado con el desarrollo del código Geopart (Zabala et al, 2004) que utiliza una versión explícita del MPM con un algoritmo incrementalmente objetivo para deformaciones finitas que incluye: construcción, análisis acoplado presión-desplazamiento en tensiones efectivas y modelos constitutivos sencillos.En general el colapso de presas de materiales sueltos es inadmisible. Las presas utilizadas para almacenar agua se ubican en muchos casos, aguas arriba de ciudades importantes, en zonas donde la amenaza sísmica puede ser muy alta. Por otro lado el porcentaje mayor de los colapsos ocurridos en el mundo corresponde a los de presas de residuos mineros. En particular el colapso de la presa de residuos mineros de Aznalcóllar, (Alonso et al. 2006, Alonso et al. 2006, Gens et al. 2006) que se toma en esta tesis como caso de estudio, causó un daño ambiental muy importante. Este es un caso muy interesante por sus características de rotura progresiva, que ha sido generada por la construcción extendida durante muchos años del dique y la existencia de una arcilla frágil de muy baja permeabilidad en su fundación. La simulación y explicación del mecanismo de colapso observado en Aznalcollar no es trivial. Se ha modelado la construcción de la presa y el proceso de su falla progresiva hasta el colapso.Además se ha estudiado el comportamiento dinámico con grandes deformaciones de una presa de grava con pantalla de hormigón ubicada una zona donde la amenaza sísmica es muy elevada.

  • Landslides in reservoirs. A coupled thermo-hydromechanical approach  Open access  awarded activity

     Pinyol Puigmarti, Nuria Merce
    Defense's date: 2010-09-13
    Department of Geotechnical Engineering and Geo-Sciences, Universitat Politècnica de Catalunya
    Theses

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    La ocurrencia de deslizamientos es uno de los principales problemas presentes durante el diseño, construcción y explotación de presas y embalses. La inundación del pie de los taludes, así como el desembalse rápido, pueden provocar primeras roturas o reactivar antiguos deslizamientos, existentes con frecuencia en el entorno de los embalses. Esta Tesis se centra en el caso particular de los deslizamientos en las márgenes de embalses en los que una masa de terreno desliza sobre una superficie de corte bien definida sin que se observe una intensa degradación del material movilizado. En la primera parte de la Tesis se discute el desembalse rápido como un problema acoplado de flujo y deformación en condiciones saturadas/no saturadas. Un riesgo adicional aparece cuando el deslizamiento acelera e invade el embalse a gran velocidad, generando una ola. La discusión de los fenómenos responsables de la aceleración de los deslizamientos sigue actualmente activa. Probablemente la falta de casos bien documentados dificulta el avance en su conocimiento. En esta Tesis se considera como fenómeno responsable de la aceleración del deslizamiento el efecto del calor en la banda de corte, lo que induce la generación de presión de agua y, por consiguiente, la reducción de la resistencia friccional. Se definen las ecuaciones del problema (balance de masa y calor y ecuaciones constitutivas) en la banda de corte y se integran junto a la ecuación del movimiento. Con objeto de encontrar criterios prácticos para cuantificar el riesgo de aceleración debido al fenómeno analizado, se ha obtenido una solución analítica para el caso de deslizamientos planos bajo las hipótesis de incompresibilidad del agua, partículas sólidas y medio poroso. La comparación entre la solución analítica y la numérica (sin considerar las hipótesis impuestas en el desarrollo analítico), cuando se varían los parámetros más relevantes dentro de un rango razonable, indica que la solución analítica es suficientemente precisa para aplicaciones prácticas. El desarrollo termo-hidro-mecánico presentado se aplica al caso del deslizamiento de Vaiont. En primer lugar se explica la situación de estabilidad previa a la rotura introduciendo la resistencia de la roca movilizada. El análisis es coherente con la información disponible (geometrías del deslizamiento, resistencia residual, propiedades de los materiales y ensayos de laboratorio). Cuando el mecanismo de generación de presión de agua debido al calor resultante del trabajo friccional se incluye en el análisis dinámico del modelo del deslizamiento de Vaiont, se predice bien la alta velocidad observada. Se han realizado análisis de sensibilidad y de efecto escala tanto para el caso de deslizamiento plano como para la geometría de Vaiont. Se han encontrado tres parámetros relevantes para explicar la aceleración del movimiento: el espesor de la banda de corte, su permeabilidad y su rigidez. De hecho, la permeabilidad y el espesor de la banda de corte están directamente relacionados dado que ambos dependen de la granulometría del material. Los resultados obtenidos indican que la permeabilidad de la banda es el parámetro clave. Un umbral de permeabilidad establecido entre 10-8 y 10-10 m/s, marca la transición entre deslizamientos potencialmente acelerados (cuando la permeabilidad es baja) y deslizamientos seguros (permeabilidades mayores a 10-10 m/s). En el caso de deslizamientos de grandes dimensiones, combinaciones críticas de la permeabilidad y espesor de la banda de corte resultan en un incremento substancial de la temperatura. Los altos valores resultantes de los cálculos (cientos o miles de ºC) invalidan el análisis aquí presentado. Fenómenos como la evaporación o la fusión de la roca deberían ser incluidos en estos casos. En el último Capítulo se analiza el deslizamiento de Canelles ocurrido recientemente. El deslizamiento pone en riesgo el buen funcionamiento de la presa y del embalse. Algunos de los desarrollos descritos en la Tesis, como son la solución del desembalse rápido y el modelo termo-hidro-mecánico para el análisis de la aceleración de los deslizamientos se han aplicado al caso de Canelles. El capítulo describe la metodología adoptada, que puede ser aplicada en casos similares.

    Landsliding is an important problem when facing the design, construction and operation of dams and reservoirs. Impoundment of the slope toes as well as the rapid drawdown may trigger the movement of first-time landslides or reactivate ancient landslides often located in reservoir sites. This Thesis deals with the particular case of landslides around reservoirs defined as a mobilized mass that slides on a well-defined shearing surface without experiencing a major degradation. In the first part of the Thesis the mechanism of rapid drawdown is discussed as a fully coupled flow-deformation problem for saturated/unsaturated conditions. Additional risk appears when landslide accelerates and is able to enter the reservoir at high speed creating impulsive waves. The discussion on the different phenomena leading to the fast acceleration is today very active. Probably the lack of well-documented cases makes the advancement of knowledge difficult. The mechanism to explain the rapid acceleration of landslides favoured in this Thesis is based on thermal effects on the sliding surface that induce the generation of pore water pressure and therefore, the reduction of the frictional strength. The governing equations (mass and heat balance equations and constitutive equations) formulated in the shear band have been written and integrated together with the motion equation of the slide. With the aim of finding practical criteria to decide the actual risk of slide acceleration due the phenomena analysed, a closed-form solution has been obtained for the case of planar landslides under the hypothesis of incompressible water, solid particles and porous media. For a rational range of the most relevant parameters, comparison between analytical and numerical (relaxing the assumptions introduced in the analytical development) solutions shows a remarkable similarity and reveals that the closed-form solution is accurate enough for practical applications. The thermo-hydro-mechanical approach discussed is applied in the case of Vaiont landslide. The stability of this landslide before the failure is first discussed by means of a simple explanation introducing the internal strength of the mobilized rock. The analysis is consistent with the available data (slide geometry, residual strength, material properties and laboratory tests). When the self-feeding mechanism of pore pressure generation due to heat resulting from the frictional work is introduced in the dynamic analysis of the Vaiont model, the high velocity actually observed is predicted. Sensitivity and scale analysis have been performed for the case of a planar landslide and for the geometry of Vaiont. Three parameters have been found important to explain the acceleration of the motion: the thickness of the shearing band, its permeability and its stiffness. In fact, permeability and thickness can be related since both depend on the particle size distribution. Calculated results indicate that the permeability of the shear band is a key parameter. A threshold of permeability established around 10-8 to 10-10 m/s marks the transition for a potentially risky slide (when the permeability is lower) to a safe one (for higher values of permeability). For very large landslides, critical combinations of band permeability and band thickness result in a substantial increase in temperature. At high calculated values of temperature (hundreds or even thousands of ºC), the analysis presented is not applicable. Evaporation or advance constitutive equations including rock melting should be included. In a final chapter a recent large landslide located in Canelles reservoir is analyzed. The slide is regarded as a potential risk for the operation of the dam and the reservoir. Some of the developments made in the Thesis, namely the solution of rapid drawdown and the thermal coupled model for fast landsliding, are applied to Canelles. The chapter describes the methodology adopted which can be applied in similar cases.

  • THMC analysis of saturation and heating processes of an expansive clay barrier in radioactive waste isolation

     Guimarães, Leonardo do N; Gens Sole, Antonio; Sánchez, M.; Olivella Pastalle, Sebastian
    International Conference on Unsaturated Soils
    Presentation's date: 2010
    Presentation of work at congresses

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    A fully coupled formulation combining reactive transport and an existing thermo- hydro-mechanical (THM) code is briefly described. Special attention has been given to phenomena likely to be encountered in clay barriers used as part of containment systems of nuclear waste. The types of processes considered in the chemical formulation include hydrolysis, complex formation, oxidation/reduction reactions, acid/base reactions, precipitation/dissolution of minerals and cation exchange. Both kinetically controlled and equilibrium-controlled reactions have been incorporated. The formulation has been implemented in the numerical code CODE_BRIGHT. An application is presented concerning the performance of a large scale in situ heating test simulating high-level nuclear waste repository conditions.

  • Deformation of bitumen based porous material: experimental and numerical analysis

     Mokni, Nadia; Olivella Pastalle, Sebastian; Li, Xiangling; Smets, Esteven; Valcke, Elie; Mariën, An
    Journal of nuclear materials
    Date of publication: 2010-09
    Journal article

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  • Osmosis-induced swelling of Eurobitum bituminized radioactive waste in constant total stress conditions

     Valcke, Elie; Mariën, An; Smets, Esteven; Mokni, Nadia; Olivella Pastalle, Sebastian; Sillen, X.
    Journal of nuclear materials
    Date of publication: 2010-11
    Journal article

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  • A microstructurally based effective stress for unsaturated soils

     Alonso Pérez de Agreda, Eduardo; Pereira, J.M.; Vaunat, Jean; Olivella Pastalle, Sebastian
    Géotechnique
    Date of publication: 2010-06
    Journal article

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