A novel hydro-mechanical model for swelling of argillaceous material

The safe disposal of nuclear and radioactive waste is one of the most challenging tasks in current and future environmental geosciences. In so-called deep geotechnical repositories nuclear waste barrels are either directly embedded in argillaceous buffers located in deep bedrock formations or the buffer is placed at distance to seal the deposit tunnel cavity. Due to their swelling capacity and low hydraulic permeability bentonite- or clay-based materials are widely regarded as suitable buffer materials.

The design of these deep geotechnical repositories is not a simple task and its evaluation and improvement is still subject of current research. During the design, finite element models can be used to simulate the behavior of the buffer and the bedrock subjected to hydro-mechanical loading. In order to achieve realistic predictions, these models have to meet several requirements: coupled hydro-mechanical simulation techniques are needed to capture the dependence of the swelling process of the buffer (or the bedrock) on the amount of supplied mountain water. Further, the swelling induced changes in the hydraulic permeability, strain and stress should be addressed in the simulations. The swelling process, however, is a path-dependent process which should also be taken into account by the numerical model.

Although several models capable of predicting the swelling process already exist, a hydro-mechanical model, which incorporates the capability of modelling swelling of an initially fully saturated material depending on its loading history, still lacks.

 

The proposed constitutive model is aimed to be suitable for application in both the dry and the swollen state. The swelling process is activated by a change in volumetric water content. We therefore introduce a swelling water content which defines how much of the pore water contributes to the swelling of the porous medium. The swelling water is assumed to be attached to the material particles and cannot be reduced by mechanical processes. Thus, the swelling process is regarded irreversible. To incorporate the path dependency of the swelling process, the evolution of the swelling water content depends on the effective stress state. Irreversible changes of the material due to swelling, e.g. a reduction of the stiffness, are modelled by introducing a swelling degree, which allows the transformation of material properties from the dry material to those of the swollen material.

The experimental studies which form the basis of the proposed constitutive model include oedometric swelling tests, standard oedometer tests and measurements for the determination of the suction-saturation relation and the hydraulic permeability. All tests are carried out on reconstituted samples of opalinus clay.

The proposed hydro-mechanical model is implemented using the finite element method and validated by numerical simulations. First simulations are in good agreement with the experimental results.