Impacts of presence of lead contamination in clayey soil-calcium bentonite cutoff wall backfills

Previous studies have shown that clayey soil admixed with Ca-bentonite can be considered as an alternative backfill for slurry-trench cutoff walls. However, the effects of contaminant exposure on the engineering properties of the clayey soil/Ca-bentonite backfills, such as compressibility and hydraulic conductivity, have not been investigated. This study presents several series of oedometer tests conducted to evaluate the compressibility and hydraulic conductivity of clayey soil/Ca-bentonite backfills exposed to different levels of lead (Pb) contamination. Kaolin is used as the base clayey soil, and it is amended with Ca-bentonite at three content levels (5, 10 and 15% by dry weight) to prepare the backfill samples. Pb is a representative metal contaminant commonly found at contaminated sites. The backfills are prepared by thoroughly mixing the kaolin-bentonite mixture with Pb concentrations of 60, 120 and 600 mmol/L. The results reveal that the level of Pb concentration and bentonite content are the critical factors that affect the compressibility of the backfill. The ratio of the compression index of the Pb-contaminated backfill to that of the clean backfill decreases with an increase in the Pb concentration and bentonite content. The void ratio at an effective vertical compression stress of 1 kPa is a useful characteristic parameter to correlate with the compression index for all the backfills tested here. The Pb-contaminated backfills exhibit a maximum fifty-fold increase in hydraulic conductivity when compared to clean backfills. The hydraulic conductivity of the clean backfills and those contaminated at low concentration of Pb (60 mmol/L) is generally less than the typical regulatory limit of 10− 9 m/s; whereas the hydraulic conductivity exceeds the regulatory limit when the Pb concentration is higher than 120 mmol/L. The ratio of hydraulic conductivity for Pb-contaminated backfills to that for clean backfill decreases with an increase in the effective vertical compression stress. This indicates that under relatively higher effective vertical compression stress conditions the detrimental effect of Pb on hydraulic conductivity can be insignificant. The hydraulic conductivity of the Pb-contaminated backfills decreases slightly when there is an increase in the bentonite content for a given Pb concentration, while the hydraulic conductivity ratio increases significantly with a rise in the bentonite content. A proposed empirical method is assessed to predict the hydraulic conductivity of Pb-contaminated clayey soil/Ca-bentonite backfills as well as clean ones subjected to vertical compressive stress lower than 1600 kPa. Using this method, the predicted hydraulic conductivity values fall in the range of 1/3 to 3 times those obtained from the oedometer tests.