Investigation of the long-term behavior of Boom Clay

AWARKEH May

Promoter

Cui Yuyun, (CERMES-ENPC), yujun.cui@enpc.fr

SCK•CEN Mentor

Dizier Arnaud
arnaud.dizier@sckcen.be
+32 14 33 29 88

SCK•CEN Co-mentor

Yu Li
li.yu@sckcen.be
+32 14 33 32 37

Expert group

EURIDICE

PhD started

2018-10-01

Short project description

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?

  • etc.

references:

[1] Le TT (2008). Comportement thermo-hydro-mécanique de l’argile de Boom. PhD thesis, CERMES, Ecole Nationale des Ponts et Chaussées, Paris

[2]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). 

[3] 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.

[4] Chen WZ, Yu HD, Shanpo Jia, Junjie Cao (2011). Long term hydro-mechanical behavior of Boom clay. IRSM , Contractual Final report. 

[5] Chen WZ, Yu HD, Gong Z, Ma YS (2015). Thermo-hydro-mechanical behavior of Boom clay. IRSM, Contractual Final report.

Objective

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 PhD study will start by an in depth and critical analysis of the existing test results in order to identify and constrain the possible fundamental mechanisms involved in the creep behaviour and the most relevant influencing factors (stress path, stress level, etc.). To this end, an emphasis will be put on the microstructure changes, like modification of the porosity, in different conditions. Specific attention will be also paid to the analysis of the impact of the test procedures and test conditions on the test results.

Based on this comprehensive and critical analysis, a new specific test program will be established to further investigate the time-dependent behaviour of Boom-Clay through both oedometer and triaxial tests in laboratory.

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. The laboratory test results will be used to help interpret the field data. This experimental research plan will provide a very good opportunity to establish a link or upscaling characteristics between the lab and in-situ tests regarding the creep behavior of BC.

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. To this end, an in depth and critical analysis of the existing constitutive models for Boom Clay will be firstly performed. The developed model will be verified and validated with some existing results at laboratory scale and at large scale with in situ test results. Finally, this developed model will be applied to the construction of a repository to investigate the effect of the creep behavior on the long term stability of the gallery for example.