Characterisation and thermo-hydro-mechanical behaviour of unsaturated Boom clay: an experimental study

Abstract

El objetivo principal de esta investigación se centra en el análisis de los efectos de la temperatura y la hidratación o secado de las arcillas en el comportamiento de cambio de volumen, permeabilidad y características de retención de agua, ante una variedad de trayectorias de succión, tensión neta y temperatura. Se han utilizado dos muestras de arcilla de Boom (Mol, Bélgica) artificialmente preparadas con una estructura dominante de agregados y exhibiendo pautas de comportamiento extremas de las arcillas parcialmente saturadas. Las muestras artificiales se han caracterizado extensamente con relación a los aspectos hidráulicos y mecánicos, tanto desde el punto de vista micro-estructural como a escala macro-estructural, donde se identifican dos zonas claras de dominio de la succión a nivel 'intra-agregado' o a nivel 'inter-agregado'. <br/><br/>Se han diseñado y construido nuevas células edométricas y un equipo triaxial con control de temperatura y succión. La célula triaxial permite registrar la evolución temporal de las deformaciones axiales (a través de LVDT locales), de las deformaciones radiales y de las isócronas de los perfiles laterales (a través de sensores láser), que experimenta la muestra en diferentes trayectorias mecánicas, de hidratación y de temperatura. En la técnica de sobre-presión de aire se ha mantenido una presión constante, aplicándola inicialmente en condiciones de fase de aire continua. La temperatura se limitó a un máximo de 80°C para evitar cambios de fase y químicos apreciables, así como para limitar los problemas asociados a la técnica de traslación de ejes. Se llevaron a cabo dos tipos de ensayos: las trayectorias en condiciones isotérmicas a dos temperaturas contrastantes y los ensayos en condiciones no isotérmicas. Los ensayos isotérmicos se enfocaron en trayectorias de humedecimiento y de secado bajo tensiones netas verticales y medias constantes, en ciclos de carga y descarga a succión constante, así como en ensayos isócoros a succión controlada. <br/><br/>Los ensayos sobre muestras altamente sobreconsolidadas revelaron un hinchamiento irreversible en las trayectorias de calentamiento a succión constante, debido a una reestructuración del esqueleto de la arcilla asociada a un mecanismo de adsorción de agua. Los resultados no isotermos mostraron valores similares ante cambios de volumen y contenidos de agua a los obtenidos bajo condiciones isotérmicas en trayectorias de humedecimiento a diferentes temperaturas y estados tensionales equivalentes. La existencia de deformaciones plásticas y de cambios irreversibles en el contenido de agua, asociados con las trayectorias principales de hidratación y de secado, es un aspecto relevante de los resultados, siendo más importante a temperaturas elevadas. Los resultados a succión controlada y tensión neta media constante revelaron una contracción irreversible durante el calentamiento de una muestra de arcilla normalmente consolidada. Asimismo se identifica una zona de máximo colapso en las trayectorias isotérmicas de humedecimiento bajo tensión neta vertical constante realizadas sobre las muestras de alta porosidad. Los ensayos isócoros sobre las muestras de baja porosidad mostraron un hinchamiento dominante, asociado a una presión de hinchamiento mayor que la registrada por las muestras de alta porosidad, donde se detecta un colapso macroestructural compensado por la expansión reversible de la estructura. Basándose en estas observaciones, se han propuesto hipótesis para explicar las pautas de comportamiento desde el punto de vista de los diferentes fenómenos micro y macroscópicos que afectan a las arcillas no saturadas. Se han planteado una serie de expresiones para las curvas de retención y permeabilidad al agua a diferentes temperaturas, porosidades y grados de saturación. Los cambios producidos por la acción mecánica, la succión y la temperatura se han interpretado bajo un concepto elastoplástico utilizando diferentes variables de estado tensionales y sus correspondientes variables conjugadas extensivas. In view of the increasing use of clay in high-temperature environments, mainly related to the conceptual design of engineered barriers for radioactive waste disposal in deep geological media, and to enrich the existing body of knowledge in the thermo-hydro-mechanical response, a systematic laboratory research programme has been designed and carried out to investigate these aspects. The main objective of this research is to analyse the effects of temperature and hydration or drying paths on clay volume change behaviour, water permeability and water retention characteristics, under a variety of matric suction, net stress and temperature paths. Two artificially prepared packings of Boom clay powder with a dominant aggregated fabric, covering a wide overconsolidation range, have been used in the testing programme. Clay powder and artificially prepared samples have been extensively characterised in relation to hydraulic and mechanical aspects, both at microstructural and macrostructural scale.<br/> <br/>New temperature and suction controlled oedometer cells and triaxial equipment have been designed, developed and carefully calibrated for investigating mechanical and thermal aspects of behaviour. The fully instrumented triaxial cell allows registering the time evolution of axial strains (via internal LVDTs), radial deformations and isochrones of lateral profiles (via electro-optical laser sensors) that experiences the sample under different mechanical, hydration and temperature paths. Air overpressure technique maintaining a constant air pressure has been followed during the tests, starting from continuous air phase conditions. Maximum temperature has been limited to 80°C for no appreciable phase and chemical change to occur, as well as to limit spurious problems affecting axis translation technique. Two main test types were carried out: isothermal tests at two different temperatures and non-isothermal paths. Isothermal paths were focused on wetting and drying cycles at constant net vertical or mean stress and loading-unloading cycles at constant suction, as well as suction controlled isochoric swelling/shrinkage pressure tests.<br/> <br/>Reversible and irreversible features of volume change behaviour (swelling, collapse and shrinkage, thermal dilatation and contraction), under oedometer and isotropic stress state conditions have been observed. Tests on heavily overconsolidated states revealed irreversible expansion upon drained heating at constant suction due to a structural rearrangement of clay skeleton, giving similar values in terms of volume and water content changes compared to isothermal wetting paths at different temperatures and equivalent stress states. The existence of important plastic deformations and irreversible water content changes associated with the first suction reduction and main drying path is a relevant feature of the data, which is more important at higher temperatures. Testing results at constant suction and net mean stress show that temperature increase on an open normally consolidated structure revealed irreversible contraction. A maximum collapse/contraction zone is also identified in the isothermal wetting paths at constant net vertical stress performed on the high-porosity fabrics. Isochoric tests revealed a dominating swelling for the low-porosity sample, where a higher swelling pressure is detected, and a controlling macrostructural collapse compensated by the reversible expansion of the structure for the high-porosity fabric. Based on these observations, hypotheses are proposed to explain features of behaviour in terms of microscopic and macroscopic phenomena affecting unsaturated clays. Expressions for retention curves and water permeability functions at different temperatures, as well as values of the different parameters describing porosity, temperature and degree of saturation dependence, have been proposed and determined for both fabrics under a variety of conditions. Mechanical, suction and thermal induced changes in terms of different stress state variables and their associated work conjugate variables were discussed within an elastoplastic point of view, and parameters characterising their reversible and irreversible features respect to drained and undrained thermal coefficients, as well as suction decrease, suction increase and loading-collapse yield loci, determined.

Effect of volumetric deformation on saturation of unsaturated soils under constant suction

“…Moreover, figure 2 (referencing data from [11]) illustrates the capability of the model to account for scenarios where saturation diminishes alongside a reduction in void ratio during soil compression. Figure 3 delineates the impact of volumetric deformation on the water retention behavior of Speswhite kaolin [19] and Municipal Boom clay [20]. A notable decrease in the initial void ratio was observed to elevate the air entry value, paralleled by an increase in saturation at a specific suction.…”

Thermo-hydro-mechanical coupled analysis in low permeability media

“…Concerning the effects of temperature over the behaviour of expansive clays, increasing laboratory information has been available since the last few years. The contributions are mainly addressed to the study of thermal effects in saturated conditions (Demars & Charles, 1982;Baldi et al, 1988;Hueckel & Baldi, 1990;Lignau et al, 1996;Tanaka et al 1997;Komine & Ogata, 1998;Delage et al 2000;Sultan et al, 2002;Cekerevack & Laloui, 2004; among others), while the information related to the effect of temperature under unsaturated conditions is more scarce but growing (Saix, 1991;Wiebe et al, 1998;Romero, 1999;. A similar tendency is observed in the development of the constitutive models, that is, most of them have been proposed to handle the non-isothermal behaviour of clay under saturated conditions (Hueckel & Baldi, 1990;Hueckel & Borsetto, 1990;Modaressi & Laloui, 1997;Cui et al 2000, Graham et al, 2001Laloui & Cekerevack, 2003).…”

“…During its lifetime the barrier will undergo processes of heating, induced by the heat-emitting waste, and of hydration, from water coming from the host rock (which is generally in a saturated state). This scenario has led to a growing interest in the knowledge of the behaviour of unsaturated expansive soils under a wide range of testing conditions; in particular the THM behaviour has received special attention (Wiebe et al, 1998;Romero, 1999;Villar, 2000;Sultan et al, 2002;Villar et al, 2003.b;. However, the interest on these materials is not limited to nuclear waste disposal applications, but they are present in other engineering problems such as shallow and deep foundations, slopes with stability problems, desiccation and formation of fissures in soils in arid regions, and clay based liners for waste isolation from the environment.…”

“…The fabric of expansive clays has been actively studied (Atabeck, 1991;Push, 1982;Romero 1999;Cui et al, 2002a; observing a marked double structure. In those tests a clear bimodal pores distribution, which is characteristic of expansive clays, can be observed (in Chapter IV, results of mercury intrusion porosimeter tests of the FEBEX bentonite are presented).…”

“…The swelling pressure test offers valuable information related to the behaviour of expansive soils. In this section the simulation of a test carried out by Romero (1999) an a sample of Boom clay pellets is presented. In these materials two structural levels can be clearly distinguished: the microstructure associated to the high density pellets (dry density 1.96 Mg/m 3 ); and the macrostructure that corresponds to the granular-like arrangement of these pellets (dry density 1.37 Mg/m 3 ) with macropores between the pellets.…”