Thermo-Hydro-Mechanical coupled processes in clay

1. Thermo-Hydro-Mechanical coupled processes in clay Geert Volckaert

2. Overview • Introduction • T-H-M coupling in (un)saturated clay • Which phenomena are important in the different phases of waste disposal? • What is needed for the PRACLAY demonstration test? • Conclusion

3. Introduction • Which knowledge is needed to model the THM behaviour of a HLW disposal demonstration test in a plastic clay? • Study by J-D Barnichon • What are the main THM coupling processes? • What are the main parameters? • How can we measure them? • What is still needed for PRACLAY?

4. Introduction: why is the THM behaviour important? • long-term safety is probably not influenced by THM transient but this is not yet demonstrated • a demonstration test • should be more than digging a gallery and install the simulated waste and engineered barriers • should be a demonstration that: • a concept is feasible • we know how it will behave • we know why it behaves that way • this is very difficult because: • demonstration test = a combination of mechanical, hydraulic and thermal transient

5. THM coupling in (un)saturated clay: thermal processes • conservation of energy: heat generated by the waste • heat transport by: conduction, convection, vaporisation, (radiation) • conduction by: solid, pore water and gas • depends on solid composition, porosity and degree of saturation • convection mainly gas • vaporisation: latent heat of water vapour is high! • effective heat conduction depends on temperature • radiation is not important at low temperatures

6. THM coupling in (un)saturated clay:thermal processes • H T coupling • degree of saturation influences conductivity and vapour transport • M T coupling • porosity depends on mechanical stress • parameters • heat capacity and conductivity of solids, water and gas • latent heat of water vapour • variables • degree of saturation, deformation (porosity), water and gas flux, gas vapour content, temperature

7. Unsaturated clay clay particle water Sand grain

8. THM coupling in (un)saturated clay: hydraulic processes • conservation of mass • transport of gas and liquid • extended Darcy’s law • saturation dependent permeabilities • saturation depends on suction • suction = sum of capillary and electro-chemical forces by which water is drawn into an unsaturated clay • dependence on saturation is highly non-linear • diffusive transport of water vapour in gas phase

9. THM coupling in (un)saturated clay: hydraulic processes • T H coupling • T dependence of fluid and gas viscosity and density and vapour partial pressure • M H coupling • permeability depends on porosity and thus on deformation • parameters • permeabilities for water and gas, water retention curve • fluid and gas viscosity and density relations • variables • degree of saturation, deformation (porosity), water and gas flux, gas vapour content, temperature

10. THM coupling in (un)saturated clay: mechanical processes • equilibrium condition => balance of forces • hydraulic and mechanical processes cannot be separated => effective stress concept • effective stress = net contact stress between clay particles = total stress - water pressure • elastic models => deformation is reversible (linear or non-linear) with stress • elastoplastic models => reversible (elastic) behaviour is bound by a limit (yield criterion) beyond which irreversible deformation occurs (plasticity) • visco-elasto-plastic => time dependent deformation under constant stress

11. THM coupling in (un)saturated clay: mechanical processes • in an unsaturated clay water potential depends on suction => effective stress depends on suction • effect of suction on unsaturated clay: • increases stiffness (or decreases compressibility) • variations in suction leads to deformation • wetting paths on (swelling) clays lead to complex swelling/collapse behaviour depending on external stress • deformations as consequence of loading (suction or external stress) have reversible and irreversible component => definition of another limit state • several models to describe these processes

12. Unsaturated clay clay particle water Sand grain

13. THM coupling in (un)saturated clay: mechanical processes • Example of tests needed for the Basic Barcelona Model

14. N S THM coupling in (un)saturated clay: mechanical processes • An example of strong H-M coupling behaviour: excavation disturbance

15. An example of strong H-M coupling behaviour: excavation disturbance Almost immediate pressure drop of 0.2 MPa at 60 m! in response to fracturing at starting chambers r = 60 m r = 70 m

16. An example of strong H-M coupling behaviour: response to liner removal

17. THM coupling in (un)saturated clay: mechanical processes • Thermal coupling T HM • thermal dilation of solid, gas and liquid • temperature dependence of fluid viscosity and skeleton plasticity/viscosity • temperature dependence of reversible/irreversible limit states • temperature influence on formation of fissures

18. Saturated clay clay particle water Sand grain

19. What is important in the different phases of waste disposal? h.b.c = hydraulic boundary conditions

20. What is important in the different phases of waste disposal?

21. What is important in the different phases of waste disposal?

22. What is needed for the PRACLAY demonstration test disposal? Initial state characteristics

23. What is needed for the PRACLAY demonstration test disposal? Required parameters

24. What is needed for the PRACLAY demonstration test disposal? What needs to be measured during the test:

25. Conclusions • T and H processes are well known and parameters are available or relatively easily accessible • TH coupling is known, main difficulty is heat transport by vapour • HM behaviour of saturated Boom clay is rather well known but fracturing in EDZ is still poorly understood • HM behaviour of unsaturated swelling clay (bentonite) is rather well understood but parameters are difficult to measure • THM behaviour of (unsaturated) clays is very complex and experimentally very demanding

26. Conclusions • Coupling processes and how they can be avoided should be considered in the concept review • Coupling processes that are unavoidable should be studied including determination of parameters and models