Investigation of the long-term behaviour of Boom Clay

The Belgian National Agency for Radioactive Waste and enriched Fissile Material (ONDRAF/NIRAS) is responsible, since 1980, for the safe management of radioactive waste in Belgium. To study and to demonstrate the feasibility of geological disposal of radioactive waste, an underground research laboratory (URL) was built in the Boom Clay Formation in Mol, Belgium. During the repository construction and exploration, the performance of the natural barrier will change in the long-term, because of the convergence of galleries over time, the evolution of the interaction between Boom Clay and the gallery concrete lining, etc.
The present PhD work aims at better understanding the long-term behaviour of Boom Clay through oedometer creep tests, constant rate of strain (CRS) tests, and triaxial creep tests. Moreover, the microstructural mechanisms related to creep and strain rate effects are investigated through mercury intrusion porosimetry (MIP), helping better understand the time-dependent mechanical behaviour.
The compressive and expansive creep obtained from oedometer tests are strongly dependent on the stress level and the number of unloading/reloading cycles. From the compressibility and creep parameters determined for Boom Clay, it is found that the compressive creep is accelerated under high stress level during loading while the expansive creep reached its maximum at low stress level during unloading. The MIP results performed at different stages of the oedometer creep tests show a microstructure change from unimodal pattern during loading to a bimodal pattern during unloading, highlighting the rearrangement of the clay particles in dense and loose structure, respectively.
The CRS tests highlight the strain rate-dependent behaviour of the Boom Clay. This behaviour was deduced from the unique relationship between (𝜎𝑣′, 𝜀𝑣, 𝜀𝑣̇ ) through the compression curves at different strain rates and evidenced by the isotach concept in which the curves move upward with the increase of strain rate. The good agreement between the CRS tests and the IL oedometer creep tests indicates the capability of CRS in determining the compressibility parameters, and also the creep parameter (secondary deformation coefficient) based on the 𝜎_𝑝′ -𝜀_𝑣̇ relationship.
In addition, the axial strain (𝜀₁) and the radial strain (𝜀₃) measured during triaxial creep tests are significantly affected by the increases of the deviator stress (𝑞_{𝑐𝑟𝑒𝑒𝑝}), the increment of the stress level (∆𝑆𝐿), and the effective confining pressure. The volumetric strain (𝜀_𝑣) showed a transition in behaviour from contraction to dilatancy and vice versa at higher levels of constant 𝑞_{𝑐𝑟𝑒𝑒𝑝}. From triaxial tests on Boom Clay, a creep threshold between 20% and 40% of 𝑞𝑚𝑎𝑥 under effective confining stress (𝜎₃′) is determined; two creep phases are identified, a secondary creep phase under 𝜎₃′ = 4.5 𝑀𝑃𝑎 at 90% of 𝑞𝑚𝑎𝑥, and primary creep for all other 𝑞_{𝑐𝑟𝑒𝑒𝑝} levels; and a relationship between the axial creep strain rate (𝜀_1̇) and the current volume change behaviour is established.
Finally, a new elasto-viscoplastic (EVP) model, ACC-2 EVP, which is an extension of the ACC-2 elastoplastic (EP), is developed based on the nonstationary flow surface (NSFS) theory and the unique stressstrain-viscoplastic strain rate concept. The proposed EVP model requires three viscous parameters
𝛼, 𝜀_𝑣̇^𝑣𝑝 _{,𝑟𝑒𝑓}, 𝑎𝑛𝑑 𝑝_𝑐̅′0,𝑟𝑒𝑓, which can be easily determined from the linear relation between 𝜎_𝑦′ and 𝜀_𝑣̇^𝑣𝑝 obtained from a set of CRS tests. The ACC-2 EVP model is able to describe various viscoplastic behaviours, including rate effects and drained creep. Good agreement between simulations and measurements was obtained, showing the performance of the model