Modelling the behaviour of concrete when interacting with rock was one of the key points of Fatemeh Salehnia’s thesis. We know that the resistance of a vault especially depends on the pressure exerted on it and engineers take this resistance into account to meet the expectations of durability: the galleries must remain intact as long as possible to give future generations the opportunity to recuperate the wastes. It is therefore essential to model the behaviour of the concrete blocks in connection with the pressure exerted by the surrounding rock in the long term (several dozen or hundreds of years). As in the case of Bure, damaged zones and fractures appeared in the rock during excavation. Their structure was also studied. The result, as demonstrated by Fatemeh Salehnia, engenders a far more heterogeneous pressure on the support than it is usually the case. As the support is composed of concrete blocks – the voussoirs – its behaviour is particularly complex.  

Another major advance in the thesis was the fact that the concrete’s viscosity was taken into account. "This wasn’t at all planned when I began my research", Fatemeh Salehnia remembers. "But we had to consider it when we wanted to begin to do long-term simulations because we couldn’t explain the obtained answers". So what is the viscosity of concrete? To understand this notion, think of a shelf stacked with books. After a certain amount of time, even if you don’t add any extra weight, the shelf will acquire a permanent curve. And this deformation will be irreversible. The same is also true, to varying degrees, for all materials including concrete: it is just a question of time. Part of the simulations the researcher did, meant she had to take into account the phenomenon of concrete’s viscosity otherwise some of the answers in the long term would be incomprehensible. It is a phenomenon that must be taken into account if you want to make calculations over a long period of time.

Bentonite plugs

Back to France and the Bure site for the third thesis(3), defended by Anne-Catherine Dieudonné. This time, the focus is no longer on the host rock but on the plugs that could be used to seal the fractured galleries studied in the previous theses. These plugs are composed completely or partly of a clay known as bentonite, which swells when it absorbs water. All clays have this property but to highly varying degrees. Kaolinite swells very little or not at all; while smectite, the main component of bentonites, is a clay that reacts very strongly with water, hence the use of bentonite in applications such as the stabilisation of excavations or drilling for oil. "In the case in hand", Anne-Catherine Dieudonné explains, "we are using dry bentonite, which is extremely compacted and contains very little water. It will swell, through natural or artificial hydration, come into contact with the rock – there isn’t a concrete support structure in the French experiment –, exert pressure on this gallery and thus seal the fractures in the damaged zones and form a seal. This allows the galleries to be hydraulically sealed in just a few years." The technique has been known for a long time but it fells out of favour for a while. However, it recently became the subject of renewed interest requiring new studies, hence the thesis presented in Liège. The stake is important because when it comes to placing a plug such as this, it is necessary to know for how long it will be effective. Experiments showed that little was known of rehydration kinetics. The work of Anne-Catherine Dieudonné therefore consisted of trying to better understand the kinetics and how the material’s permeability evolves. A tricky job because bentonite has a bimodal distribution of porosity, with two classes of porosity. One on a very small scale: that of argillaceous particles, in layers of nanometric thickness; and the other on a larger scale: aggregates. It is the latter that mainly contributes to the material’s permeability. The thesis defended by Anne-Catherine Dieudonné shows that if water is used to make bentonite swell in a confined space, i.e. at a constant volume, the water obstructs the macroporosity because the bentonite can’t increase in volume. Thepermeability is therefore reduced and hydration occurs more and more slowly, as well as the sealing process. On the other hand, if hydration takes place at free volume, the bentonite can swell and permeability remains intact. The host rock is therefore sealed more quickly. The modelling of this phenomenon is very complex because, in reality, the two types of porosity come into play at the same time. There are indeed places where bentonite can swell freely and others where it can’t.

A second stake, also dealt with in the thesis, is to determine what pressure will ultimately be exerted by the plugs on the walls of the host rock: will it be homogenous or not? Enough to seal the cracks or not? Will this create new cracks if the pressure is too great? The answers to these questions will obviously determine the implemented solutions. Complex and refined, they will undoubtedly do a near perfect job of covering up the cracks but their cost and their complexity probably mean that they will never be deployed. On the other hand, if they are too simple and incomplete, they will be cheap and easier to apply but undoubtedly won’t provide a suf

(3) Hydromechanical behaviour of compacted bentonite: from micro-scale behaviour to macro-scale modelling , Dieudonné Anne-Catherine, University of Liège, 2016, doctoral thesis.