S. Olivella

Mechanisms of gas transport in clay barriers

Los ensayos de flujo de gas sobre muestras saturadas e impermeables indican la formacion de sendas preferenciales de circulacion del gas. El articulo describe un procedimiento para integrar esas discontinuidades en una formulacion general THM tal y como se desarrolla en el Programa de elementos finitos CODE_BRIGHT. Con esta tecnica se ha reproducido el comportamiento observado en muestras ensayadas. Se consigue reproducir los maximos observados en la presion de inyeccion y en el flujo de descarga cuando se abre por primera vez un camino preferencial. El articulo discute tambien el papel de la heterogeneidad a pequena escala en el flujo de gas. Se ha desarrollado un experimento computacional inspirado en la geometria, materiales de barrera y condiciones de contorno del ensayo a gran escala GMT, desarrollado en el Laboratorio de Grimsel. Todos los elementos de la discretizacion incluyen discontinuidades embebidas. El analisis muestra que la pequena variabilidad de propiedades de la barrera facilita el desarrollo de caminos preferenciales al flujo de gas.

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

Abstract A fully coupled thermo-hydro-mechano-chemical (THMC) formulation is described in this paper. Special attention is paid to phenomena likely to be encountered in clay barriers used as engineered barriers in the disposal of nuclear radioactive 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 kinetics- and equilibrium-controlled reactions are incorporated. The formulation is implemented in a numerical code. An application is presented concerning the performance of a large-scale in-situ heating test simulating high-level radioactive waste repository conditions.

Evaluation of self-combustion risk in tire derived aggregate fills

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 self-combustion 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.

Modelling fluid flow in Opalinus Clay excavation damage zone. A semi-analytical approach

Abstract The HG-A in situ test, located at the Mont Terri Underground Rock Laboratory (URL), was analysed as part of the FORGE project. This test investigated the behaviour of the excavation damage zone (EDZ) around a backfilled microtunnel by a series of water- and gas-injection tests. Prior to testing, the backfilled microtunnel was sealed with a hydraulic megapacker system. A key aspect was the investigation of crack opening and closure along the EDZ in response to water and gas injections in the context of radioactive waste disposal. In the model, the intrinsic permeability of the EDZ was assumed to depend on deformation, and additional simplifying assumptions were considered: axisymmetry about the tunnel axis; no gravity; soil slices orthogonal to the tunnel axis move independently and in plane strain; liquid and gas flows along the EDZ parallel to the tunnel axis; and undrained saturated conditions for the Opalinus Clay. As a result, the field equations were reduced to differential equations for liquid and gas pressures defined in a one-dimensional (1D) domain representing the EDZ. The main trends of the pressure evolution observed in the test section were reproduced. A 2D axisymmetric model confirmed the validity of the simplifying assumptions, except for small zones of the EDZ near the megapacker ends.

Experimental and Numerical Investigation of Porosity Variations in Saline Media Induced by Temperature Gradients

Preliminary experimental results and a numerical interpretation of porosity variations in saline media induced by temperature gradients are presented in this paper. The experimental results show a pattern of porosity variation with the same features as obtained theoretically in Olivella et al (1996a). An interpretation of the experimental results using CODE_BRIGHT (Olivella et al, 1996b) has been carried out. Since it is possible to reproduce the experimental results with the numerical approach it can be concluded that the mechanism for porosity changes induced by temperature gradients proposed in a preceding work effectively takes place.