Clay formations such as Boom Clay are being investigated world-wide as host medium for the disposal of radioactive waste. The long-term safety of a repository requires a thorough characterization and understanding of the fundamental mechanism of the long-term or time-dependent behavior of the host formation.
The construction of the disposal galleries induces different levels of variation in the in situ state of stresses and in pore water pressure, and generates different levels of deformation around the gallery. During the operational (which can last several dozen of years) and very long term post closure phase, the stress - deformation of the host formations will continue to evolve. This long term behavior of the clay formations is considered to be mainly controlled by two processes: water pressure dissipation through consolidation and the viscous behavior of the skeleton. This last one, namely creep, is characterized by a very slow deformation rate under a (quasi-)constant stress level. The creep deformation, under certain conditions and over long term, may be accelerated and may result in clay failure.
Characterization of these two processes controlling the long term behavior of clays is very challenging due to the inherent low permeability of the material, which imposes technical and time limitations for the experimental investigation. Most importantly, a lot of factors influence the creep behavior of clays: stress states, stress history, temperature evolution, pore water chemistry, etc.
The creep behavior of Boom Clay has been addressed in many studies, in terms of experimentation, constitutive modelling and numerical analysis (Le T.T. , 2008; Cui et al., 2009; Deng et al., 2012;Chen et al., 2011&2015; etc.). A set of evidence of creep behaviour has been acknowledged and collected. Nevertheless, it appears that there are still certain issues which still need to be further investigated as, for example:
How does the creep deformation affect the strength, in shear and in compression ?
Is-there a threshold for the creep-induced failure for Boom Clay?
Does the creep behaviour exhibits also anisotropy? In other words, does the structural anisotropy of Boom Clay affect the creep deformation?
Does the creep deformation influence the permeability of Boom Clay?
 Le TT (2008). Comportement thermo-hydro-mécanique de l’argile de Boom. PhD thesis, CERMES, Ecole Nationale des Ponts et Chaussées, Paris
Cui YJ, Le TT, Tang AM, Delage P, Li XL (2009). Investigating the time-dependent behaviour of Boom clay under thermo-mechanical loading. Géotechnique 59(4).
 Deng YF, Cui YJ, Tang AM, Li XL, Sillen X (2012). An experimental study on the secondary deformation of Boom clay. Applied Clay Science, 59–60: 19–25.
 Chen WZ, Yu HD, Shanpo Jia, Junjie Cao (2011). Long term hydro-mechanical behavior of Boom clay. IRSM , Contractual Final report.
 Chen WZ, Yu HD, Gong Z, Ma YS (2015). Thermo-hydro-mechanical behavior of Boom clay. IRSM, Contractual Final report.