Consolidation and Hydraulic Conductivity of Nine Model Soil-Bentonite Backfills

The consolidation behavior and hydraulic conductivity (k) 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 (FB) of 20, 40, 60, 75, and 89% by dry weight and sand-bentonite mixtures with dry backfill bentonite contents (BB) 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-mm slump, in accordance with standard field practice for SB vertical cutoff walls. The results indicate that both the compression index Cc, and swell index Cs increase essentially linearly with FB or BB. However, the increases in Cc and Cs with BB are approximately 19 and 5 times greater, respectively, than the corresponding increases in Cc and Cs with FB, because of a greater amount of higher plasticity fines (i.e., bentonite). The overall increases in Cs with either FB or BB are relatively small (<=9%) compared with those for Cc with either FB or BB. Also, measured values for the coefficient of consolidation, cv, agree well with those reported in the literature, and cv decreases with an increase in FB or BB because of the decrease in k with an increase in FB or BB. Finally, substantial amounts of fines (40%) may be required in soil-bentonite backfills to achieve k<=10–9  m/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  m/s.