Effect of aging on shear strength of compacted GMZ bentonite

The shear strength characteristics of compacted bentonite preferred as feasible buffer/backfill component is essential to the mechanical stability of the multi-barrier system in the context of nuclear waste disposal. This paper presents an investigation on the potential aging-induced changes in shear strength and microstructure of compacted GMZ bentonite at a dry density of 1.68 Mg/m³ and various water contents. For this purpose, direct shear and Mercury Intrusion Porosimetry (MIP) tests were correspondingly performed, as well as the soil-water retention tests. Statically compacted bentonite specimens were kept at constant volume and water content conditions for different periods of aging time up to 90 days, prior to experimental investigations. Results showed that the shear strength decreased with aging time, with a more noticeable trend during the first 30 days. Aging effect on shear strength was less pronounced for specimens at water content of 24%. Significant changes in microstructure after aging were observed, characterized by an increase in tiny pores (< 6 nm) and a decrease in inter-aggregate pores. The former phenomenon is related to the interlayer hydration as water redistribution is expected to occur, while the latter one is attributed to the exfoliation of clay particles that fill and reduce the inter-aggregate pores, which accounts for the dry density effect on soil-water retention curves (SWRC). Aging-induced changes in microstructure are governed by the water uptake from the non-interlayer space acting as reservoir into the interlayer pores, thereby promoting a further clay layer hydration and an arrangement of clay fabric. As such, both the effective degree of saturation (mainly considering the capillary water) and total suction decreased with aging, as well as the Bishop-type effective stress, thereby resulting in a decrease in the shear strength of compacted GMZ bentonite.