Modelling the field behaviour of a granular expansive barrier

The large scale ‘‘Engineered Barrier’’ (EB) experiment, performed at the Mont Terri Underground Laboratory is described. A coupled hydromechanical model is then used to simulate the test performance. Constitutive parameters for the bentonite granular backfill are based on experimental work described in a companion paper. An elastoplastic model describes the granular fill, while the host rock is simulated by a damage model. Predictions of EDZ development around the tunnel are compared with some indirect measurements. Calculated evolutions of relative humidity and stresses within the buffer are compared with sensor records. Good agreement was found for the bentonite blocks supporting the canister. The granular expansive fill exhibit a more irregular behavior. Calculated displacements of the canister agree in absolute terms with actual measurements.Peer Reviewe

Hydro-mechanical behaviour of bentonite pellet mixtures

Granular mixtures made of high-density pellets of bentonite are being evaluated as an alternative buffer material for waste isolation. Ease of handling is an often-mentioned advantage. The paper described the experimental program performed to characterize the hydromechanical behaviour of compacted pellet’s mixtures used in the engineered barrier (EB) experiment. The material tested in the laboratory was based in the pellet’s mixtures actually used for the emplacement of the EB in situ experiment. Grain size distribution was adjusted to a maximum pellet size compatible with the specimen’s dimensions. Dry densities of statically compacted specimens varied in most of the cases in the range: 1.3–1.5 Mg/m3. Pellets had a very high dry density, close to 2 Mg/m3. The outstanding characteristic of these mixtures is its discontinuous porosity. Pore sizes of the compacted pellets vary around 10 nm. However the inter-pellet size of the pores is four to five orders of magnitude higher. This double porosity and the highly expansive nature of the pellets controlled all the hydraulic and mechanical properties of the mixture. Tests performed include infiltration tests using different water injection rates and mechanisms of water transfer (in liquid and vapour phases), suction controlled oedometer tests and swelling pressure tests. The interpretation of some of the tests performed required backanalysis procedures using a hydro-mechanical (HM) computer code. Material response was studied within the framework of the elastoplastic constitutive model proposed by Alonso et al. [Alonso, E.E., Gens, A., Josa, A., 1990. A constitutive model for partially saturated soils. Geotechnique 40 (3), 405–430] (Barcelona Basic Model, BBM). Parameters for the model were identified and also a set of hydraulic laws necessary to perform coupled HM analysis.Peer Reviewe

Small scale slope failure benchmark test: modelling and prediction

A benchmark was designed with the aim of evaluating the capabilities of current modelling techniques and computational codes to reproduce slope failures. Laboratory tests and three instrumented small scale experiments of slopes initially unsaturated and subjected to a controlled rainfall up to failure were performed. The objective of the benchmark was to predict one of the slope failures knowing the rest of the data. The paper presents the modelling strategy and the results obtained using the finite element code Code_Bright and the Barcelona Basic Model as the constitutive model for the unsaturated soils.Peer ReviewedPostprint (published version

Gb-SAR interferometry displacement measurements during dewatering in construction works : case of La Sagrera railway station in Barcelona, Spain

The final publication is available at Springer via http://dx.doi.org/10.1016/j.enggeo.2016.02.014Construction processes require monitoring to ensure safety and to control the new and existing structures. Traditional monitoring is based on land surveys and geotechnical instruments and only allows for point-like measurements. Ground-based synthetic aperture radar (GB-SAR) is a remote sensing radar installed in the ground that offers the possibility of acquiring measurements in 2D covering areas of up to a few square kilometers in a single acquisition. Because the GB-SAR technology measures phase shifts along the Line-of-Sight, it only allows for measurements in the longitudinal direction. Moreover, this technology requires coherence between subsequent acquisitions. These restrictions can be a limitation to the usage of GB-SAR for monitoring a construction process because in this context, the movements of soil and existing structures occur in any direction and at a very fast pace. This paper aims to test the GB-SAR suitability to measure movements during construction. To do so, an experiment was performed in the future railway station of La Sagrera, Barcelona (Spain), in which GB-SAR was used to accurately quantify wall displacements induced by dewatering and proved to be helpful to understand structural deformations and to identify vulnerable areas. The results were compared to traditional monitoring data and numerical models to confirm the reliability of the GB-SAR measurements.Postprint (published version

Caracterización hidromecánica de mezclas de pellets de bentonita. Estudio experimental y constitutivo.

Actualmente, la utilización de energía atómica representa un 16 % del total de la energía que se produce a nivel mundial. Uno de los mayores problemas que afronta su utilización, es encontrar una solución segura para almacenar los residuos resultantes del proceso de producción de energía.Una de las propuestas más estudiada y aceptada como solución a este problema, es la construcción de depósitos subterráneos donde mantener el residuo aislado durante el resto de su vida activa, ENRESA (2000), Reseal (2000). PRACLAY (1998). Estos depósitos se han diseñado siguiendo el criterio de multibarreras: conjunto de barreras independientes y redundantes que mantienen el residuo aislado. Los residuos se disponen, dentro de contenedores metálicos, en nichos horizontales o verticales, excavados en la roca. Para ellenar el espacio entre la roca y el contenedor metálico se busca un material que representa una verdadera barrera aislante, denominada, barrera de ingeniería. Estos materiales están constituidos mayormente por bentonita.El Proyecto EB, propone el estudio de un nuevo concepto de almacenamiento subterráneo para residuos radioactivos de larga actividad. El residuo se dispone dentro de contenedores metálicos alojados en nichos horizontales excavados en una roca arcillosa (Opalinus clay), y como material aislante, se utiliza una combinación de bloques y pellets de bentonita compactada. Ambos materiales se fabrican a partir de una misma bentonita, la bentonita Febex, ENRESA (2000).La presente tesis doctoral, está asociada a los trabajos experimentales de caracterización del comportamiento hidromecánico de la muestras de pellets, realizados en el marco del proyecto EB. Para poder estudiar los aspectos más relevantes de la respuesta del material es necesario combinar distintas técnicas experimentales para el control de la succión (Romero, 2001) y adecuar las distintas metodologías de ensayo.Los trabajos de tesis se realizaron en tres etapas fundamentales: Una primera etapa en la que se estudiaron las características estructurales de las mezclas de pellets de bentonita. Se prepararon muestras con distintas densidades secas y se realizaron ensayos de porosimetrías de mercurio (MIP) y ensayos de infiltración.Una vez finalizada esta primera etapa, se definió una metodología de trabajo y se llevaron adelante los distintos ensayos del programa experimental. En esta etapa se describen las distintas técnicas y equipos experimentales y se presentan los resultados obtenidos en los distintos ensayos divididos en tres grupos; ensayos de caracterización del comportamiento hidráulico, en segundo lugar ensayos de expansión, hinchamiento y compresibilidad con control de la succión y finalmente los ensayos realizados para estudiar la influencia del tipo de transferencia de agua y ritmo de mojado en el comportamiento del material.En la última etapa, se plantearon las bases conceptuales y leyes constitutivas de un modelo adecuado para materiales expansivos (Modelo BExM, Gens & Alonso (1992) y Alonso et al. (1999)). El modelo se implementó en un código numérico utilizando la técnica de diferencias finitas y aplicado al caso de las mezclas de pellets de bentonita. Se detalla la implementación de las distintas ecuaciones y se describe el planteo iterativo utilizado para su resolución. Adicionalmente, se sugiere una metodología para la deducción de los distintos parámetros del modelo, se indican las capacidades del modelo en distintos ejemplos y se comparan las predicciones del modelo con el comportamiento real observado.Al final de la tesis, se presentan los resultados experimentales obtenidos en un ensayo a mediana escala, llamados ensayos de Columna de Infiltración. El objetivo de estos ensayos es estudiar la respuesta del material en una escala media, en condiciones muy controladas y similares a las de su utilización como material, en una barrera de ingeniería.Postprint (published version

Modelling the field behaviour of a granular expansive barrier

The large scale ‘‘Engineered Barrier’’ (EB) experiment, performed at the Mont Terri Underground Laboratory is described. A coupled hydromechanical model is then used to simulate the test performance. Constitutive parameters for the bentonite granular backfill are based on experimental work described in a companion paper. An elastoplastic model describes the granular fill, while the host rock is simulated by a damage model. Predictions of EDZ development around the tunnel are compared with some indirect measurements. Calculated evolutions of relative humidity and stresses within the buffer are compared with sensor records. Good agreement was found for the bentonite blocks supporting the canister. The granular expansive fill exhibit a more irregular behavior. Calculated displacements of the canister agree in absolute terms with actual measurements.Peer Reviewe

Modelling the field behaviour of a granular expansive barrier

The large scale ‘‘Engineered Barrier’’ (EB) experiment, performed at the Mont Terri Underground Laboratory is described. A coupled hydromechanical model is then used to simulate the test performance. Constitutive parameters for the bentonite granular backfill are based on experimental work described in a companion paper. An elastoplastic model describes the granular fill, while the host rock is simulated by a damage model. Predictions of EDZ development around the tunnel are compared with some indirect measurements. Calculated evolutions of relative humidity and stresses within the buffer are compared with sensor records. Good agreement was found for the bentonite blocks supporting the canister. The granular expansive fill exhibit a more irregular behavior. Calculated displacements of the canister agree in absolute terms with actual measurements.Peer Reviewe

Pellet mixtures in isolation barriers

Granular mixtures made of high-density pellets of bentonite are being evaluated as an alternative buffer material for waste isolation. Ease of handing is an often-mentioned advantage. The paper describes the experimental program performed to characterize the hydromechanical behaviour of compacted pellet’s mixtures. Grain size distribution was adjusted to a maximum pellet size compatible with the specimen’s dimensions. Dry densities of statically compacted specimens varied in most of the cases in the range: 1.3 to 1.5 Mg/m3. Pellets had a very high dry density, close to 2 Mg/m3. To outstanding characteristics of these mixture is its discontinuous porosity. Pore sizes of the compacted pellets vary around 10 nm. However the inter-pellet size of the pores is four to five orders of magnitude higher. This double porosity and the highly expansive nature of the pellets controlled all the hydraulic and mechanical properties of the mixture. Test performed include infiltration tests using different water injection rates and mechanisms of water transfer (in liquid and vapour phases), suction controlled oedometer test and swelling pressure tests. The interpretation of some of the tests performed required backanalysis procedures using a Hydro-Mechanical(HM)computer code. Material response was studied within the framework of the elastoplastic constitutive model proposed by Alonso et al.(1990) (Barcelona Basic Model, BBM). Parameters for the model were identified and also a set of hydraulic laws necessary to perform coupled HM analysis. A large scale in situ test (the “EB” test in Mont Terri, Switzerland) is described and analyzed. Rock barrier parameters were adjusted on the basis of available test. The test excavation, barrier emplacement and forced hydration was simulated by means of the CODE_BRIGHT program. The comparison between measurement and computed results include data on relative humidity in the rock and the buffer, swelling pressures and displacement.Peer ReviewedPostprint (published version

Hydromechanical behaviour of compacted granular expansive mixtures: experimental and constitutive study

Compacted granular mixtures of high-density bentonite pellets have been evaluated as an alternative buffer material to fill the empty space around nuclear waste disposal canisters in horizontal drifts. Despite the obvious benefits of these compacted mixtures (the backfilling operation becomes easier and the gaps between the host rock and the buffer are minimised), there are several aspects of concern such as the effective blockage of the large inter-pellet pores due to granule swelling – this blockage improves the water permeability properties – and the tendency to develop initial collapses before reaching an adequate swelling pressure. Selected test results of a comprehensive laboratory experimental programme are presented to gain insight into the hydromechanical response of this multi-porosity compacted material. To improve the information on local transient behaviour, simulation-assisted techniques using a double-structure constitutive model are used. The paper presents a physically based one-dimensional model to simulate experimental results of different transient processes, such as the progressive loss of permeability during wetting and the occurrence of concurrent phenomena during fast flooding at constant stress (initial collapse of the granular arrangement and parallel expansion of granules)

Pellet mixtures in isolation barriers

Granular mixtures made of high-density pellets of bentonite are being evaluated as an alternative buffer material for waste isolation. Ease of handing is an often-mentioned advantage. The paper describes the experimental program performed to characterize the hydromechanical behaviour of compacted pellet’s mixtures. Grain size distribution was adjusted to a maximum pellet size compatible with the specimen’s dimensions. Dry densities of statically compacted specimens varied in most of the cases in the range: 1.3 to 1.5 Mg/m3. Pellets had a very high dry density, close to 2 Mg/m3. To outstanding characteristics of these mixture is its discontinuous porosity. Pore sizes of the compacted pellets vary around 10 nm. However the inter-pellet size of the pores is four to five orders of magnitude higher. This double porosity and the highly expansive nature of the pellets controlled all the hydraulic and mechanical properties of the mixture. Test performed include infiltration tests using different water injection rates and mechanisms of water transfer (in liquid and vapour phases), suction controlled oedometer test and swelling pressure tests. The interpretation of some of the tests performed required backanalysis procedures using a Hydro-Mechanical(HM)computer code. Material response was studied within the framework of the elastoplastic constitutive model proposed by Alonso et al.(1990) (Barcelona Basic Model, BBM). Parameters for the model were identified and also a set of hydraulic laws necessary to perform coupled HM analysis. A large scale in situ test (the “EB” test in Mont Terri, Switzerland) is described and analyzed. Rock barrier parameters were adjusted on the basis of available test. The test excavation, barrier emplacement and forced hydration was simulated by means of the CODE_BRIGHT program. The comparison between measurement and computed results include data on relative humidity in the rock and the buffer, swelling pressures and displacement.Peer Reviewe