Insights into the Microstructural Evolution of a Sand–Bentonite Mixture with Low Bentonite Fraction due to Dehydration

A soil–bentonite vertical cutoff wall is commonly employed as an effective engineered barrier in contaminated sites to prevent the spread of chemical pollutants. Over time, the soil–bentonite backfills experience suction changes due to alternating rainfall and evaporation processes, as well as fluctuations in water levels, leading to a decrease in the inhibition capability and a settlement in the soil structure. To clarify this point, water retention tests and microscopic tests were conducted on a type of soil–bentonite backfill which was composed of river sand and a low content of bentonite (5%). Experimental results demonstrated that the water retention curve exhibited a typical anti-S-shaped pattern. By considering the air entry value ψAEV and the residual suction ψr as the dividing boundaries, three distinct zones could be closely identified from the water retention curve. The water loss and soil shrinkage of sand–bentonite backfills displayed similar development trends, with significant changes at the low-suction zone (ψAEV <s < ψr) and less noticeable changes at the high-suction zone (s > ψr). The pore structure of the backfill evolved from a single-porosity to a double-porosity structure with the increase of suction, and finally returned to a single-porosity structure while approaching to the dried state. This transformation in the pore structure was influenced by an aggregation of bentonite layers and the exposed pores between sand particles. A calculation method on the basis of water retention mechanisms was developed to quantitatively analyze the micropore evolution, and its accuracy was validated through a comparison with experimental data.