Consolidation and Hydraulic Conductivity of Nine Model Soil-Bentonite Backfills

The consolidation behavior and hydraulic conductivity $\left(k\right)$ of nine backfill mixtures modeling those used for soil-bentonite (SB) vertical cutoff walls are evaluated. The backfills include natural clay-sand mixtures with backfill fines contents $\left(F_B\right)$ of 20, 40, 60, 75, and 89% by dry weight and sand-bentonite mixtures with dry backfill bentonite contents $\left(B_B\right)$ of 2, 3, 4, and 5%. Test specimens of the backfills are mixed with a sufficient amount of 5% bentonite-water slurry to provide a $100\text{-}\mathrm{mm}$ slump, in accordance with standard field practice for SB vertical cutoff walls. The results indicate that both the compression index $C_c$ , and swell index $C_s$ increase essentially linearly with $F_B$ or $B_B$ . However, the increases in $C_c$ and $C_s$ with $B_B$ are approximately 19 and 5 times greater, respectively, than the corresponding increases in $C_c$ and $C_s$ with $F_B$ , because of a greater amount of higher plasticity fines (i.e., bentonite). The overall increases in $C_s$ with either $F_B$ or $B_B$ are relatively small $(⩽9\%)$ compared with those for $C_c$ with either $F_B$ or $B_B$ . Also, measured values for the coefficient of consolidation, $c_v$ , agree well with those reported in the literature, and $c_v$ decreases with an increase in $F_B$ or $B_B$ because of the decrease in $k$ with an increase in $F_B$ or $B_B$ . Finally, substantial amounts of fines $(⩾40\%)$ may be required in soil-bentonite backfills to achieve $k⩽{10}^{-9}\mathrm{m}∕\mathrm{s}$ when the backfill contains only low-plasticity fines. Alternatively, when backfill consists of a clean (i.e., little or no fines), coarse-grained material, a significant amount of dry bentonite (5% in this study) may be required to achieve $k⩽{10}^{-9}\mathrm{m}∕\mathrm{s}$ .