Membrane Behavior of Soil-Bentonite Slurry-Trench Cutoff Walls

Abstract

Investigations of membrane behavior in bentonite-based soil-bentonite (SB) backfills used in vertical cutoff walls for waste containment and site remediation have yielded encouraging results. Laboratory investigations using model soil-bentonite materials as well as samples obtained during slurry wall construction have been tested for membrane behavior. The results indicate that two model backfills, a natural clay with 89% fines and a mixture of clean sand with 5% by weight of dry sodium bentonite, act as semi-permeable membranes. The chemico-osmotic efficiency coefficients, ω, for the natural clay range from 0.018 to 0.024, whereas ω for the sand-bentonite backfill range from 0.118 to 0.166. The two SB backfills recovered from the field also exhibited membrane behavior, with ω ranging from 0.0119 to 0.0140 for a backfill recovered from a cutoff wall in New Jersey and from 0.0019 to 0.0172 for a backfill recovered from a cutoff wall in Delaware. Although the range of ω values for the field SB backfills was lower than that for the model SB backfills, modeling revealed that the measured membrane behavior for the field SB backfills would result in a reduction in contaminant transport by as much as 10% relative to that where membrane behavior is not considered.