In the context of radioactive waste disposal, an underground research laboratory (URL) is a facility in which experiments are conducted to demonstrate the feasibility of constructing and operating a radioactive waste disposal facility within a geological formation. The Meuse/Haute-Marne URL is a site-specific facility planned to study the feasibility of a radioactive waste disposal in the Callovo-Oxfordian (COx) claystone. The thermo-hydro-mechanical (THM) behaviour of the host rock is significant for the design of the underground nuclear waste disposal facility and for its long-term safety. The French National Radioactive Waste Management Agency (Andra) has begun a research programme aiming to demonstrate the relevancy of the French high-level waste (HLW) concept. This paper presents the programme implemented from small-scale (small diameter) boreholes to full-scale demonstration experiments to study the THM effects of the thermal transient on the COx claystone and the strategy implemented in this new programme to demonstrate and optimise current disposal facility components for HLW. It shows that the French high-level waste concept is feasible and working in the COx claystone. It also exhibits that, as for other plastic clay or claystone, heating-induced pore pressure increases and that the THM behaviour is anisotropic

Lime treatment is a well-known technique to improve the mechanical response of clayey subgrades of road pavements or clayey soils used for embankment. Several studies show that lime treatment significantly modifies the physical and hydromechanical properties of compacted soils. Nevertheless, studies on the scale effect under climatic changes are scarce. Actually, wetting–drying cycles might significantly modify the microstructure of treated soils, giving rise to changes in hydromechanical properties. This modification could be dependent on the size of soil aggregates before lime treatment. In the present work, this scale effect was studied by investigating the stiffness of a compacted lime-treated clayey soil using bender elements. The studied soil was first air-dried and ground into a target maximum soil aggregates size (D_max). For each aggregate size, the soil was humidified to reach the target water contents w_i, then mixed with 3% of lime powder (mass of lime divided by mass of dried soil) prior to the static compaction at a dry density of 1·60 Mg/m³. Two initial water contents (w_i = 14 and 18%) and four maximum soil aggregates sizes (D_max = 0·4, 1·0, 2·0 and 5·0 mm) were considered. After the compaction, the soil specimen (50 mm in diameter and 50 mm high) was covered by plastic film in order to prevent soil moisture changes. The soil stiffness was then monitored at variable time intervals until reaching stabilisation. Afterwards, the soil specimen was subjected to full saturation followed by air-drying to come back to its initial water content. The results show that: (a) the soil stiffness after lime-treatment is significantly dependent on the aggregate size: the finer the aggregates the higher the soil stiffness; (b) the effect of initial water content on the stiffness is negligible; and (c) the wetting–drying cycles seem to slightly increase the soil stiffness in the case of lime-treated specimens and decrease the soil stiffness in the case of untreated specimens. Furthermore, when an intensive drying was applied reducing the soil water content lower than the initial level, the soil stiffness decreased drastically after the subsequent wetting.