Hydraulic Conductivity of Vertical Cutoff Walls

Vertical cutoff walls have been used to control the movement of contaminants and contaminated groundwater since the remediation of contaminated sites began. There are, however, significant hydraulic conductivity differences between soil-bentonite, cement-bentonite, plastic concrete, and in situ mixed cutoff walls. The results of laboratory and field studies were assessed to show the influence of material properties, confining stress, permeameter type, water table position, and state of stress, on the hydraulic conductivity of vertical cutoffs. The results of these studies show the range of hydraulic conductivity expected for each of the cutoff wall types. Increasing confining stress markedly decreases the hydraulic conductivity of soil-bentonite and has a measurable but reduced impact on stronger backfill materials. Studies on soil-bentonite cutoff walls show that the stress at depth is less than predicted using the effective weight of the overlying materials. This reduction in stress is a result of soil-bentonite materials “hanging-up” on the side walls of the trench. Thus, applying the effective stress calculated from the effective weight of the overlying backfill overestimates the stress to be used in the laboratory tests and results in unconservative measures of hydraulic conductivity. Field data also reveals that, with time, the hydraulic conductivity of soil-bentonite above the water table may increase substantially. Further, the hydraulic conductivity does not significantly decrease upon re-saturation.