Coupled analysis of a backfill hydration test (original) (raw)
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Analysis of the Expansive Clay Hydration under Low Hydraulic Gradient
Springer Proceedings in Physics, 2007
Engineered barriers made up of compacted expansive clays are a basic component of the systems for the isolation of high-level radioactive waste (HLW). The barrier is initially unsaturated and during hydration the hydraulic gradient varies from high values (i.e. higher than 50000) at the beginning of the wetting to very low values (practically zero when reaching stationary conditions). The values of bentonite permeability are frequently obtained at the laboratory under high hydraulic gradients (i.e. higher than 15000), necessary to induce a measurable flow. A drawback of this practice is that the applied hydraulic gradients are very far from those (expected) in actual conditions. So, the experimental results obtained might not be realistic in many cases, affecting the reliability of the numerical analysis. To advance in the knowledge of expansive clays behaviour this work presents an experimental and numerical study. The hydraulic permeability of FEBEX clay samples has been measured under low injection pressures. This work presents the modelling of infiltration laboratory tests designed to reproduce the thermo-hydro-mechanical conditions of large-scale tests. The evolution of the tests is analysed and discussed in detail using non-standard flow models which consider the presence of a threshold gradient and thermo-osmotic effects.
E3S Web of Conferences
A water infiltration test was numerically simulated using the finite element method. The modified Barcelona Basic Model and the double-structure water retention model was used for the numerical analysis. A methodology is presented for parameter identification and calibration purpose. The experimental results highlighted the porosity redistribution and hydration-induced heterogeneity along the hydration-path. The simulation results successfully captured the moisture migration in the soil sample. A comparison between the measured and predicted total stress values revealed the influence of interfacial friction between the soil sample and cell wall.
Hydro-mechanical analysis of expansive clays : constitutive and numerical modelling
Bentonite-based materials are being currently considered in several countries as a backfill component in the multi-barrier concept for deep geological disposal of radioactive waste. The bentonite barrier fulfils several important functions: i) high swelling capacity to fill gaps and compress the excavation damaged zone and ii) very low hydraulic conductivity and important retention capacity which retards significantly radionuclides transport. Small-scale testing in geotechnical laboratories and in-situ experiments in underground research laboratories (URL) have demonstrated that initial state, water supply conditions and volume constrictions are the main aspects affecting the behaviour of bentonites. In this context, the main objective of the present study is the numerical simulation of the hydro-mechanical behaviour of expansive clays. For this purpose, a constitutive model has been developed to characterise the bentonite-based materials. The modelling of these materials is a quite...
Disposal of hazardous waste like high-level radioactive waste (HLW) is made by confining it in canisters surrounded by dense, ductile "buffer" clay for placement in deposition holes at a few hundred meters depth in crystalline rock. The concept favored by authorities in Sweden, Finland and Canada implies that the holes are bored from blasted or bored tunnels to be tightly backfilled by stacking compacted blocks of clay in the center and filling the remaining space between the blocks and the rock with blown-in clay pellets. The problem with this is that water flowing in from the rock can cause piping and erosion of the pellet filling, which can turn it into mud and disturb the placement of canisters and buffer clay. The controlling parameter is the rate of inflow of water per inflow point, which is determined by the structure and hydraulic conductivity of the rock. The paper describes a simple model of the mechanisms in penetration of water into the pellet fill and provides a basis for estimating the required rate of backfilling for avoiding critical conditions.
Anais da Academia Brasileira de Ciências, 2010
The storage of high level radioactive waste is still an unresolved problem of the nuclear industry, being geological disposal the most favoured option and, naturally, the one requiring the strongest geo-mechanical input. Most conceptual designs for the deep geological disposal of nuclear waste envisage placing the canisters containing the waste in horizontal drifts or vertical boreholes. The empty space surrounding the canisters is filled by an engineered barrier often made up of compacted swelling clay. In the barrier and the near field, significant thermo-hydro-mechanical (THM) phenomena take place that interact in a complex way. A good understanding of THM issues is, therefore, necessary to ensure a correct performance of engineered barriers and seals. The conditions of the bentonite in an engineered barrier for high-level radioactive waste disposal are being simulated in a mock-up heating test at almost scale, at the premises of CIEMAT in Madrid. The evolution of the main Thermo-Hydro-Mechanical (THM) variables of this test are analysed in this paper by using a fully coupled THM formulation and the corresponding finite element code. Special emphasis has been placed on the study of the effect of thermo-osmotic flow in the hydration of the clay barrier at an advanced staged of the experiment.
Water retention properties of Boom clay: A comparison between different experimental techniques
Unsaturated Soils: Advances in Geo-Engineering, 2008
The water retention properties of Boom clay samples extracted at a depth of 223 m have been determined in ENPC Paris and UPC Barcelona using different experimental techniques. Boom clay is a stiff clay in which an underground laboratory devoted to carry out research in radioactive waste disposal has been excavated near the city of Mol (Belgium). The retention properties of Boom clay have been investigated for two reasons: i) in good quality samples, a high suction develops in the saturated sample during extraction and its value is correlated with the sampling depth; ii) possible desaturation due to gallery venting during the operational phase may occur in the clay. Various suction control and measurement techniques have been used: osmotic, vapour equilibrium, filter paper, high-range tensiometer and chilled-mirror dew-point psychrometer readings. Some volume changes have also been measured along the equalisation or measuring stages. The values obtained are discussed according to the techniques used. They are compared with previous data on compacted Boom clay samples. The air entry value is estimated close to 4-5 MPa and the shrinkage-swelling properties are also examined. The sample suction at saturation is compared to the in-situ stress state.
Numerical Simulation of Consolidation Behavior of Large Hydrating Fill Mass
International Journal of Concrete Structures and Materials, 2020
Underground mined-out voids need to be backfilled for the stability of the surrounding rock and also to increase ore extraction from adjacent pillars. One of the relatively newer means is cemented paste backfilling, which has been extensively adopted in underground mining operations around the world. During and after the placement of cemented paste backfill (CPB) into stopes, complex multiphysics (thermal, hydraulic, mechanical and chemical) processes take place in the large mass of CPB and could affect its consolidation behavior. An analysis of the consolidation process in CPB mass is essential for the assessment of CPB behavior and cost-effective designs in practice. In this paper, multiphysics simulation of the consolidation behavior of CPB mass is performed under different conditions, including the mixture recipe, and backfilling, drainage, surrounding rock and curing conditions. It is found that the in situ consolidation behavior of CPB structures is a function of the multiphys...
Transport in Porous Media, 2006
A fully coupled formulation combining reactive transport and an existing thermo-hydro-mechanical (THM) code is presented. 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 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 total analytical concentrations (including precipitated minerals) are adopted as basic transport variables and chemical equilibrium is achieved by minimizing Gibbs Free Energy. The formulation has been incorporated in a general purpose computer code capable of performing numerical analysis of engineering problems. A validation exercise concerning a laboratory experiment involving the heating and hydration of an expansive compacted clay is described.
Influence of water density on the water-retention curve of expansive clays
Géotechnique, 2012
There is increasing evidence from researchers on clay mineralogy that the density of water attached to clay minerals may be greater than 1·0 Mg/m3. This fact becomes especially evident in compacted highly expansive clays close to water saturation. This kind of material is being considered in the design of engineered barriers for radioactive waste disposal, because of its sealing and retention properties. From a geotechnical point of view, most of the analyses required to check the performance of the barrier are sensitive to the value considered for the water density. This is the case for the unsaturated flow calculations, which depend on the water-retention properties of the soil. This paper presents, first, a review of measurements performed at microscopic level. A description of the hydration process of expansive clay considering the micro and macrostructure is included. Then a method to obtain an average water density as a function of suction for sodium and calcium bentonites is ...