The consolidation behavior and hydraulic conductivity 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 of 20, 40, 60, 75, and 89% by dry weight and sand-bentonite mixtures with dry backfill bentonite contents 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 slump, in accordance with standard field practice for SB vertical cutoff walls. The results indicate that both the compression index , and swell index increase essentially linearly with or . However, the increases in and with are approximately 19 and 5 times greater, respectively, than the corresponding increases in and with , because of a greater amount of higher plasticity fines (i.e., bentonite). The overall increases in with either or are relatively small compared with those for with either or . Also, measured values for the coefficient of consolidation, , agree well with those reported in the literature, and decreases with an increase in or because of the decrease in with an increase in or . Finally, substantial amounts of fines may be required in soil-bentonite backfills to achieve 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 .