Evolution of Hydraulic Conductivity of Unsaturated Compacted Na-Bentonite under Confined Condition—Including the Microstructure Effects

Compacted bentonite is envisaged as engineering buffer/backfill material in geological disposal for high-level radioactive waste. In particular, Na-bentonite is characterised by lower hydraulic conductivity and higher swelling competence and cation exchange capacity, compared with other clays. A solid understanding of the hydraulic behaviour of compacted bentonite remains challenging because of the microstructure expansion of the pore system over the confined wetting path. This work proposed a novel theoretical method of pore system evolution of compacted bentonite based on its stacked microstructure, including the dynamic transfer from micro to macro porosity. Furthermore, the Kozeny–Carman equation was revised to evaluate the saturated hydraulic conductivity of compacted bentonite, taking into account microstructure effects on key hydraulic parameters such as porosity, specific surface area and tortuosity. The results show that the prediction of the revised Kozeny–Carman model falls within the acceptable range of experimental saturated hydraulic conductivity. A new constitutive relationship of relative hydraulic conductivity was also developed by considering both the pore network evolution and suction. The proposed constitutive relationship well reveals that unsaturated hydraulic conductivity undergoes a decrease controlled by microstructure evolution before an increase dominated by dropping the gradient of suction during the wetting path, leading to a U-shaped relationship. The predictive outcomes of the new constitutive relationship show an excellent match with laboratory observation of unsaturated hydraulic conductivity for GMZ and MX80 bentonite over the entire wetting path, while the traditional approach overestimates the hydraulic conductivity without consideration of the microstructure effect.

Hydraulic conductivity variation in compacted bentonite-fly ash mixes under constant volume and free swelling flow conditions

Compacted bentonite-sand (B-S) and bentonite-fly ash (B-FA) are established combinations for the construction of landfill liners. This study determined the upper and lower bounds of equilibrium hydraulic conductivity (keq) of amended bentonite under extended duration of flow. The keq for constant volume flow condition differed from free swelling condition by more than two orders of magnitude due to the difference in geomaterial interaction, microstructural changes, and mineralization. Considering constant volume and free swelling condition, B-FA mix with class F and class C fulfilled the hydraulic conductivity criterion up to 70 % and 30 % amendment, respectively. The higher keq observed for the B mixed with class C FA was attributed to the formation of porous calcium aluminium silicate hydrate gel and ettringite needle type minerals. The time taken to achieve equilibrium was inversely related to keq by a power relationship. The data from this study were used to propose empirical relationships for estimating keq (long-term) based on k obtained at 48 hours (short-term), plasticity and geomaterial type. The study reveals that FA can be used as an alternate for S as amendment material and keq based on free swelling condition should be used for designing the liner.