Updated State-Of-The-Art on THM Behaviour of I) Buffer Clay Materials and of II) Host Clay Materials

Kód výsledku v IS VaVaI

<a href="https://www.isvavai.cz/riv?ss=detail&h=RIV%2F68407700%3A21110%2F24%3A00382559" target="_blank" >RIV/68407700:21110/24:00382559 - isvavai.cz</a>

Výsledek na webu

<a href="https://www.ejp-eurad.eu/publications/eurad-d72-updated-state-art-thm-behaviour-i-buffer-clay-materials-and-ii-host-clay" target="_blank" >https://www.ejp-eurad.eu/publications/eurad-d72-updated-state-art-thm-behaviour-i-buffer-clay-materials-and-ii-host-clay</a>

DOI - Digital Object Identifier

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Jazyk výsledku

angličtina

Název v původním jazyce

Updated State-Of-The-Art on THM Behaviour of I) Buffer Clay Materials and of II) Host Clay Materials

Popis výsledku v původním jazyce

on bentonite properties was tackled both by analysing the change of properties of preheated material and by determining the hydro-mechanical properties of the bentonite at high temperatures. To cover the first instance, bentonite was heated at 150°C in dry and wet conditions for different periods of time up to 2 years. Clay mineralogy was remarkably preserved. The slight changes observed in other properties were opposite depending on the heating conditions: if evaporation was allowed, decreases in cation exchange capacity, specific surface area, sorption coefficients and swelling pressure (only in high-density samples) were observed. It is presumed that the changes were caused not by the temperature itself, but by the strong drying induced by the elevated temperature. Bentonite was also subjected to hydration under thermal gradient in field and laboratory tests and then analysed. No structural modifications of the smectite were observed, but dissolution and precipitation of species occurred and were conditioned by the kind of bentonite and hydration water. These processes were accompanied by modification of the exchangeable cation complex. The determination of hydro-mechanical properties of expansive clay at elevated temperatures is challenging, because of the experimental and interpretation issues. The results were not conclusive concerning the impact of high temperature on swelling pressure, although in most cases a reduced swelling pressure was obtained when temperature increased, and the impact was more significant for the higher dry densities. The results may be affected by the experimental protocols, the use of bentonite or purified smectite, and the exchangeable cations. In any case, even at the highest temperatures the bentonite had the ability to fill voids and was able to develop large swelling pressures at high densities. Three thermo-hydro-mechanical models were developed or upgraded during the project to include thermal phenomena and dependencies, and they were applied to the simulation of new laboratory thermo-hydraulic tests in cells. The laboratory tests with host rock materials analysed two aspects: the impact of temperature on the short- and long-term behaviour of the clay host rock and the self-sealing processes. In the first case the focus was on the thermal pressurisation and the risk of damage if the effective vertical stress becomes tensile. Important hydro-mechanical couplings between peak pore water pressure, temperature, permeability and confining stress were identified. The triaxial tests showed that the thermal loading rates applied and the particular stress conditions during the tests significantly condition the behaviour observed (formation of microcracks, strength reduction). Nevertheless, the results confirmed that the claystone keeps its good mechanical and retention properties, even when heated at high temperature (up to 100°C). Provided that the clay content of the samples was high enough, self-sealing was an efficient mechanism whatever the experimental conditions, although temperature may have a slight delaying effect on the process. The benchmarks selected for the modelling activities comprised the modelling of generic cases of a high-level waste repository, two large-scale in situ tests experiments and triaxial laboratory tests. The benchmarks selected were the PRACLAY and ALC1605 heating tests, performed in the HADES (Belgium) and Bure (France) underground research laboratories, respectively. Consistent results were obtained on the generic cases by the different codes and teams, which increases the confidence one can have in these poro-elastic models and shows the robustness of the modelling approach used to design the repositories. Some models were improved to reproduce in-situ observations and predict the development of the excavated-damaged zone. The modelling of laboratory experiments showed the importance of a good understanding of the tests setup and of the boundary conditions. For both in-situ experiments, the teams successfully managed to reproduce the anisotropic response of the clay host rocks to excavation and heating. The evolutions of temperature and pore pressure were well modelled in the far-field with a poro-elastic approach, but more advanced models are needed to take into account the processes occurring around the tunnels (e.g., modification of hydraulic properties within the EDZ, creep).

Název v anglickém jazyce

Updated State-Of-The-Art on THM Behaviour of I) Buffer Clay Materials and of II) Host Clay Materials

Popis výsledku anglicky

on bentonite properties was tackled both by analysing the change of properties of preheated material and by determining the hydro-mechanical properties of the bentonite at high temperatures. To cover the first instance, bentonite was heated at 150°C in dry and wet conditions for different periods of time up to 2 years. Clay mineralogy was remarkably preserved. The slight changes observed in other properties were opposite depending on the heating conditions: if evaporation was allowed, decreases in cation exchange capacity, specific surface area, sorption coefficients and swelling pressure (only in high-density samples) were observed. It is presumed that the changes were caused not by the temperature itself, but by the strong drying induced by the elevated temperature. Bentonite was also subjected to hydration under thermal gradient in field and laboratory tests and then analysed. No structural modifications of the smectite were observed, but dissolution and precipitation of species occurred and were conditioned by the kind of bentonite and hydration water. These processes were accompanied by modification of the exchangeable cation complex. The determination of hydro-mechanical properties of expansive clay at elevated temperatures is challenging, because of the experimental and interpretation issues. The results were not conclusive concerning the impact of high temperature on swelling pressure, although in most cases a reduced swelling pressure was obtained when temperature increased, and the impact was more significant for the higher dry densities. The results may be affected by the experimental protocols, the use of bentonite or purified smectite, and the exchangeable cations. In any case, even at the highest temperatures the bentonite had the ability to fill voids and was able to develop large swelling pressures at high densities. Three thermo-hydro-mechanical models were developed or upgraded during the project to include thermal phenomena and dependencies, and they were applied to the simulation of new laboratory thermo-hydraulic tests in cells. The laboratory tests with host rock materials analysed two aspects: the impact of temperature on the short- and long-term behaviour of the clay host rock and the self-sealing processes. In the first case the focus was on the thermal pressurisation and the risk of damage if the effective vertical stress becomes tensile. Important hydro-mechanical couplings between peak pore water pressure, temperature, permeability and confining stress were identified. The triaxial tests showed that the thermal loading rates applied and the particular stress conditions during the tests significantly condition the behaviour observed (formation of microcracks, strength reduction). Nevertheless, the results confirmed that the claystone keeps its good mechanical and retention properties, even when heated at high temperature (up to 100°C). Provided that the clay content of the samples was high enough, self-sealing was an efficient mechanism whatever the experimental conditions, although temperature may have a slight delaying effect on the process. The benchmarks selected for the modelling activities comprised the modelling of generic cases of a high-level waste repository, two large-scale in situ tests experiments and triaxial laboratory tests. The benchmarks selected were the PRACLAY and ALC1605 heating tests, performed in the HADES (Belgium) and Bure (France) underground research laboratories, respectively. Consistent results were obtained on the generic cases by the different codes and teams, which increases the confidence one can have in these poro-elastic models and shows the robustness of the modelling approach used to design the repositories. Some models were improved to reproduce in-situ observations and predict the development of the excavated-damaged zone. The modelling of laboratory experiments showed the importance of a good understanding of the tests setup and of the boundary conditions. For both in-situ experiments, the teams successfully managed to reproduce the anisotropic response of the clay host rocks to excavation and heating. The evolutions of temperature and pore pressure were well modelled in the far-field with a poro-elastic approach, but more advanced models are needed to take into account the processes occurring around the tunnels (e.g., modification of hydraulic properties within the EDZ, creep).

Druh

O - Ostatní výsledky

CEP obor

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OECD FORD obor

20101 - Civil engineering

Projekt

—

Návaznosti

R - Projekt Ramcoveho programu EK

Rok uplatnění

2024

Kód důvěrnosti údajů

S - Úplné a pravdivé údaje o projektu nepodléhají ochraně podle zvláštních právních předpisů