Hydraulic Conductivity of Polymerized Bentonite-Amended Backfills

The potential for chemical incompatibility between soil-bentonite (SB) backfills of vertical cutoff walls composed of conventional (chemically unmodified) sodium bentonite (CSB) and contaminated groundwater has led to evaluation of chemically modified bentonites for improved chemical resistance. Accordingly, the hydraulic conductivity to tap water (kw) and to CaCl2 solutions (kc) of SB backfills amended with a polymerized bentonite known as bentonite polymer nanocomposite (BPN) were measured and compared with those for a traditional backfill composed of CSB. The BPN was used both as a dry amendment and as the constituent in the bentonite slurry. Three backfills were evaluated, viz, clean silica sand amended with either 2 or 5% dry BPN and mixed with 2% BPN slurry (i.e., 2BPN2 and 5BPN2, respectively), and the same sand amended with 5% dry CSB and mixed with 5% CSB slurry (5CSB5). Based on permeation with 50 mM CaCl2, the ratio kc/kw varied in the order of 2BPN2<5CSB5<5BPN2, such that the 5BPN2 backfill performed the worst in terms of chemical resistance. However, because kw for the 5BPN2 backfill ranged from approximately two to four orders of magnitude lower than that of the other backfills, the final kc for the 5BPN2 backfill of 3.2×10−11 m/s was the lowest among the three backfills and was almost two orders of magnitude lower than that for the 5CSB5 backfill of 2.5×10−9 m/s, despite an overall lower total bentonite content for the 5BPN2 backfill (i.e., 5.5 versus 7.1%). Thus, for the permeant liquids and materials used in this study, the overall hydraulic performance of the backfill containing 5% dry BPN was significantly better than that of the backfill containing 5% dry CSB.