A double-porosity formulation for the THM behaviour of bentonite-based materials
The thermo-hydro-mechanical (THM) behaviour of expansive clays has been extensively studied in the last decades due to the potential use of bentonites as components of engineered barrier systems (EBS) in deep geological repositories for high-level and long-lived radioactive wastes. Since the early p...
| Author: | |
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| Format: | Doctoral thesis |
| Status: | Published version |
| Publication date: | 2021 |
| Country: | España |
| Institution: | CBUC, CESCA |
| Repository: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/672141 |
| Online Access: | http://hdl.handle.net/10803/672141 https://dx.doi.org/10.5821/dissertation-2117-349570 |
| Access Level: | Open access |
| Keyword: | Àrees temàtiques de la UPC::Enginyeria civil 55 624 |
| _version_ | 1878736880050110464 |
|---|---|
| author | Vasconcellos, Ramon Barboza de |
| author_browse | Vasconcellos, Ramon Barboza de |
| author_facet | Vasconcellos, Ramon Barboza de |
| author_role | author |
| collection | TDR. Tesis Doctorales en Red |
| description | The thermo-hydro-mechanical (THM) behaviour of expansive clays has been extensively studied in the last decades due to the potential use of bentonites as components of engineered barrier systems (EBS) in deep geological repositories for high-level and long-lived radioactive wastes. Since the early post closure period, the EBS is subjected to changes in temperature, moisture content and stresses due to the coupled THM processes expected to occur in such an environment. The different structural levels found in unsaturated expansive clays requires the use of constitutive models that considers the explicit distinction of these pore-structure levels in their mathematical formulation in order to reproduce the development of the fabric of bentonite materials subjected to the complex THM paths taking place during the lifetime of a nuclear waste repository. A coupled THM formulation that represents the expansive soil as two overlapped but distinct structural media has been developed in the framework of classical and generalized plasticity theories. In such a double-structure approach, the THM behaviour of the expansive soil is characterized by constitutive laws formulated to account for the relevant processes affecting each porous medium and for the interaction mechanisms relating the deformation and the saturation states of the active clay particles to the structural arrangement of the clay aggregates and to the water potential in the larger interconnected pores. In addition, the mechanical response of the porous medium to any THM loading is intrinsically related to the compressibility of the clay minerals. The irreversible changes in the soil fabric are attributed to the loading-collapse (LC) mechanism and to the micro-macro structural coupling (ß-mechanism). Thermal effects are incorporated into the mathematical formulation of the double structure model, which has been implemented in a finite element code (CODE_BRIGHT) able to solve, in a fully coupled way, the system of partial differential equations arising from the governing equations (balance equations). An explicit and robust integration scheme with automatic sub-stepping and error control has been employed to update the stress tensor and the internal (history) variables. The capabilities of the implemented double-porosity model to predict the expected response of expansive clays under isothermal and non-isothermal scenarios have been checked by the performance of constitutive analyses following a number of prescribed THM paths under confined and unconfined conditions. In addition, sensitivity analyses have been carried out in order to verify the dependence of the local expansive response on the initial conditions and on the sequence of load application. Special attention has been placed on the role played by the pore-water mass transfer between the two pore-structure levels in the development of the swelling potential of the expansive porous medium. The performance of the model in reproducing the actual THM behaviour of laboratory-scale tests has also been examined by means of the modelling of the hydration of two heated columns made of granular bentonite materials, selected as potential buffer materials in the construction of engineered barriers. The comparison between the available experimental data and the model results has shown the ability of the current double-porosity formulation to simulate the main observed features of the THM behaviour of the expansive material when subjected to complex loading paths. |
| eu_rights_str_mv | openAccess |
| format | doctoralThesis |
| id | ES_96f45f54b1427784d7d5c55df609c64b |
| institution | CBUC, CESCA |
| instname_str | CBUC, CESCA |
| network_acronym_str | ES |
| network_name_str | España |
| oai_identifier_str | oai:www.tdx.cat:10803/672141 |
| publishDate | 2021 |
| publishDateSort | 2021 |
| publisher | Universitat Politècnica de Catalunya |
| publisherStr | Universitat Politècnica de Catalunya |
| reponame_str | TDR. Tesis Doctorales en Red |
| spelling | A double-porosity formulation for the THM behaviour of bentonite-based materials Vasconcellos, Ramon Barboza de Àrees temàtiques de la UPC::Enginyeria civil 55 624 The thermo-hydro-mechanical (THM) behaviour of expansive clays has been extensively studied in the last decades due to the potential use of bentonites as components of engineered barrier systems (EBS) in deep geological repositories for high-level and long-lived radioactive wastes. Since the early post closure period, the EBS is subjected to changes in temperature, moisture content and stresses due to the coupled THM processes expected to occur in such an environment. The different structural levels found in unsaturated expansive clays requires the use of constitutive models that considers the explicit distinction of these pore-structure levels in their mathematical formulation in order to reproduce the development of the fabric of bentonite materials subjected to the complex THM paths taking place during the lifetime of a nuclear waste repository. A coupled THM formulation that represents the expansive soil as two overlapped but distinct structural media has been developed in the framework of classical and generalized plasticity theories. In such a double-structure approach, the THM behaviour of the expansive soil is characterized by constitutive laws formulated to account for the relevant processes affecting each porous medium and for the interaction mechanisms relating the deformation and the saturation states of the active clay particles to the structural arrangement of the clay aggregates and to the water potential in the larger interconnected pores. In addition, the mechanical response of the porous medium to any THM loading is intrinsically related to the compressibility of the clay minerals. The irreversible changes in the soil fabric are attributed to the loading-collapse (LC) mechanism and to the micro-macro structural coupling (ß-mechanism). Thermal effects are incorporated into the mathematical formulation of the double structure model, which has been implemented in a finite element code (CODE_BRIGHT) able to solve, in a fully coupled way, the system of partial differential equations arising from the governing equations (balance equations). An explicit and robust integration scheme with automatic sub-stepping and error control has been employed to update the stress tensor and the internal (history) variables. The capabilities of the implemented double-porosity model to predict the expected response of expansive clays under isothermal and non-isothermal scenarios have been checked by the performance of constitutive analyses following a number of prescribed THM paths under confined and unconfined conditions. In addition, sensitivity analyses have been carried out in order to verify the dependence of the local expansive response on the initial conditions and on the sequence of load application. Special attention has been placed on the role played by the pore-water mass transfer between the two pore-structure levels in the development of the swelling potential of the expansive porous medium. The performance of the model in reproducing the actual THM behaviour of laboratory-scale tests has also been examined by means of the modelling of the hydration of two heated columns made of granular bentonite materials, selected as potential buffer materials in the construction of engineered barriers. The comparison between the available experimental data and the model results has shown the ability of the current double-porosity formulation to simulate the main observed features of the THM behaviour of the expansive material when subjected to complex loading paths. El comportamiento termo-hidro-mecánico (THM) de las arcillas expansivas ha sido ampliamente estudiado en las últimas décadas debido al uso potencial de bentonitas en barreras de ingeniería (EB) en depósitos geológicos profundos para desechos radiactivos de alta actividad y de larga vida. Desde la post-clausura del repositorio, la EB está sometida a cambios de temperatura, contenido de humedad y tensiones a causa de los procesos acoplados THM que se espera que ocurran en dicho entorno. Los diferentes niveles estructurales presentes en las arcillas expansivas no saturadas requieren el uso de modelos constitutivos que consideren en su formulación matemática la distinción explícita de los distintos tipos de poros a fin de reproducir la evolución estructural de los materiales bentoníticos bajo las complejas trayectorias de cargas THM que tienen lugar durante la vida de un repositorio de residuos nucleares. Se ha desarrollado una formulación THM acoplada, en el marco teórico de la plasticidad clásica y generalizada, que describe el suelo expansivo como dos medios estructurales superpuestos pero distintos. Este planteamiento de modelo de doble porosidad considera que el comportamiento THM del suelo expansivo es descrito por leyes constitutivas que tienen en cuenta los procesos relevantes en cada medio poroso y los mecanismos de interacción que relacionan la deformación y el estado de saturación de las partículas de arcilla activas con la disposición estructural de los agregados de arcilla y con el potencial de agua en los macro poros. Además, la respuesta mecánica del medio poroso bajo cualquier carga THM está intrínsecamente relacionada con la compresibilidad de los minerales de arcilla. Los cambios estructurales irreversibles se atribuyen al mecanismo de carga-colapso (LC) y al acoplamiento estructural micro-macro (mecanismo ß). Los efectos térmicos han sido incorporados a la formulación matemática del modelo de doble estructura, que ha sido implementada en un código de elementos finitos (CODE_BRIGHT) capaz de resolver, de forma totalmente acoplada, el sistema de ecuaciones diferenciales parciales que se originan de las ecuaciones de balance. Se ha empleado un esquema robusto de integración explícito con control automático del incremento de integración y del error para actualizar el tensor de tensiones y las variables de historia. La capacidad del modelo de doble porosidad para predecir la respuesta esperada de las arcillas expansivas en escenarios isotermos y no isotermos se ha comprobado mediante la realización de análisis constitutivos siguiendo trayectorias THM prescritas tanto a volumen constante cuanto en condiciones no confinadas. Además, se han realizado análisis de sensibilidad para verificar la dependencia de la respuesta expansiva de las condiciones iniciales y de la secuencia de aplicación de cargas. Se ha prestado especial atención al papel que desempeña la transferencia de agua entre los dos niveles de poros en el desarrollo del potencial de hinchamiento del medio poroso expansivo. También se ha examinado el desempeño del modelo en la reproducción del comportamiento THM en ensayos a escala de laboratorio mediante la simulación del calentamiento e hidratación de dos columnas de materiales granulares de bentonita seleccionados como potenciales materiales de sellado en la construcción de barreras de ingeniería. La comparación entre los datos experimentales disponibles y los resultados del modelo ha demostrado la capacidad de la presente formulación de doble porosidad de simular los principales aspectos observados en el comportamiento del material expansivo cuando está sometido a trayectorias THM complejas. Enginyeria del terreny DOCTORAT EN ENGINYERIA DEL TERRENY (Pla 2012) Universitat Politècnica de Catalunya http://hdl.handle.net/10803/672141 https://dx.doi.org/10.5821/dissertation-2117-349570 |
| spellingShingle | A double-porosity formulation for the THM behaviour of bentonite-based materials Vasconcellos, Ramon Barboza de Àrees temàtiques de la UPC::Enginyeria civil 55 624 |
| status_str | publishedVersion |
| title | A double-porosity formulation for the THM behaviour of bentonite-based materials |
| title_full | A double-porosity formulation for the THM behaviour of bentonite-based materials |
| title_fullStr | A double-porosity formulation for the THM behaviour of bentonite-based materials |
| title_full_unstemmed | A double-porosity formulation for the THM behaviour of bentonite-based materials |
| title_short | A double-porosity formulation for the THM behaviour of bentonite-based materials |
| title_sort | A double-porosity formulation for the THM behaviour of bentonite-based materials |
| topic | Àrees temàtiques de la UPC::Enginyeria civil 55 624 |
| topic_facet | Àrees temàtiques de la UPC::Enginyeria civil 55 624 |
| url | http://hdl.handle.net/10803/672141 https://dx.doi.org/10.5821/dissertation-2117-349570 |