Consolidation Behavior of Polymerized Bentonite-Amended Backfills: Fid-Bau Portal

Consolidation Behavior of Polymerized Bentonite-Amended Backfills

Bohnhoff, Gretchen L. / Shackelford, Charles D.

Abstract

The potential for incompatibility between soil-bentonite (SB) backfills of vertical cutoff walls composed of conventional sodium bentonite (CSB) and contaminated groundwater has led to the evaluation of chemically modified bentonites for improved chemical resistance. In addition, the stress conditions and consolidation behavior represent important considerations with respect to the performance of SB vertical cutoff walls. Accordingly, the consolidation behavior of SB backfills amended with a polymerized bentonite known as bentonite polymer nanocomposite (BPN) were evaluated and compared with those for a backfill composed of CSB. The three backfills that were evaluated included 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). The 5BPN2 backfill displayed bisemilog linear stress-strain behavior and was more sensitive to stress conditions than either the 5CSB5 backfill or the 2BPN2 backfill, both of which displayed semilog linear stress-strain behavior. The relatively low $C_{c}$ of 0.015 for the 2BPN2 backfill reflected a relatively incompressible backfill, probably the result of the low bentonite content for this particular backfill and the dominance of the sand matrix in terms of compressibility, although the hydraulic conductivity, $k$ , of this backfill generally was less than $10^{- 9} m/s$ . Despite a lower bentonite content for the 5BPN2 backfill relative to the 5CSB5 backfill, the 5BPN2 backfill exhibited a lower $k$ as a result of the superior hydraulic properties of the BPN. The results illustrate that the consolidation behavior of chemically modified bentonites such as the BPN that are used in SB backfills to overcome incompatibility concerns may be somewhat different than that typically associated with more conventional sodium bentonite-based backfills.