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.
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 (creep).
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?
The main objective of this proposed PhD thesis is to improve our understanding of the long-term creep behavior of Boom Clay with a new experimental program including both laboratory and in situ tests in the underground research laboratory, HADES. A special focus will be put on the mechanistic understanding of the creep behavior and on the determination of related parameters in view of developing a relevant constitutive law.
The new laboratory experimental program will be established based on the comprehensive and critical analysis of the existing test results. An emphasis will be put on the microstructure changes, like modification of the porosity, in different conditions.
In parallel of the small scale laboratory tests, in situ long-term creep tests are planned in HADES. For this purpose, long-term dilatometer tests in both vertical and horizontal boreholes will be installed to investigate the creep behaviour under more realistic in situ conditions.
An elasto-visco-plastic constitutive model will be further developed based on the existing results and additional results from this study. It will mainly consist in refinement of existing models for Boom Clay. The developed model will be verified and validated with some existing results at laboratory scale and at large scale with in situ test results.
 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.