Constitutive modeling for compacted bentonite buffer materials as unsaturated and saturated porous media

Takayama, Yusuke  ; Tachibana, Shinya*; Iizuka, Atsushi*; Kawai, Katsuyuki*; Kobayashi, Ichizo*

Bentonite has remarkable swelling characteristics and low permeability that makes it a suitable buffer material in a purpose built repository for the geological disposal of radioactive waste. It is necessary to use reliable numerical simulation techniques to demonstrate that the repository is safe and mechanically stable for tens of thousands of years. Constitutive model that describes the mechanical behavior of bentonite is a key technique in such numerical simulations. The current paper proposes a constitutive model for bentonite, which treats bentonite as an unsaturated elasto-plastic material that changes its mechanical properties as it becomes saturated. In the proposed model, the swelling index and an expression formula for negative dilatancy are treated as functions of degree-of-saturation. Montmorillonite content is used as an input parameter in the proposed model and so is applicable to a variety of bentonite based materials of varying montmorillonite content. Experimental results from swelling volume and swelling pressure tests taken from the literature are shown to be satisfactorily predicted by the proposed model.