The soil-bentonite (SB) cut-off wall has been widely considered a vertical barrier to effectively control the migration of pollutants in contaminated sites. Recently, active porous materials have been used as a promising candidate amendment for mitigating chemical degradation and improving the retardation capacity of the SB cut-off wall. In this study, the silty clay from a typical Cr(VI) contaminated site was selected as the body material of the SB cut-off wall, and zeolite and active carbon were used as the modifiers of the silty clay-bentonite backfills, respectively. The impact of the two modifiers on the engineering properties of the backfills was investigated through a series of slump tests, consolidation tests, hydraulic conductivity tests, and microstructure tests. The experimental results demonstrated that the slump curves closely exhibited a linear relationship between standard slump and moisture content. Meanwhile, bentonite could improve the optimum moisture content of the backfills, while the addition of the two modifiers yielded the opposite outcome. As the bentonite content increased, the compression index of the backfills significantly increased, while the hydraulic conductivity decreased. At a given bentonite content of 5%, the addition of zeolite or active carbon resulted in a reduction in the compression index and initial void ratio, while exhibiting minimal impact on the hydraulic conductivity. Scanning electron microscope (SEM) observations indicated that the silty clay-bentonite backfills became increasingly loose with increasing bentonite content, owing to the filling and expansion of dispersed bentonite layers. The amendment of zeolite or active carbon was able to decrease the backfill volume by promoting the agglomeration of layered bentonite. The findings of this study will be useful for the optimal selection of backfill materials and the performance evaluation of the cut-off wall.

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Keywords: Soil-bentonite (SB) backfill, Cut-off wall, Slump, Compressibility, Hydraulic conductivity