Effects of phosphate dispersants on the liquid limit, sediment volume and apparent viscosity of clayey soil/calcium-bentonite slurry wall backfills

Abstract

Soil-bentonite slurry-trench wall, usually consisting of sandy soil and Na-bentonite, is used extensively as in-situ engineered barriers for contaminant containment. Clayey soil/Ca-bentonite may be considered as an alternative backfill when Na-bentonite is scarce at some sites. Adding trace amount of phosphate dispersant to clayey soil/Ca-bentonite backfills may be advantageous to maintain the deflocculated structure of bentonite, which is benefit to minimizing increases in hydraulic conductivity of the backfills when attacked by contaminants. However, studies on the application of phosphate dispersants to the clayey soil/Ca-bentonite backfills are very limited. A series of laboratory tests are conducted for the measurements of liquid limit, sediment volume and apparent viscosity of the phosphate dispersant-amended backfills. The phosphate dispersants tested are sodium hexametaphosphate, sodium tripolyphosphate and sodium pyrophosphate with contents ranging from 0 to 2%. The results demonstrate that the values of liquid limit, sediment volume and apparent viscosity of the backfills decrease sharply when the dispersant content is relatively low (≤0.1 to 0.5%), while they change slightly at relatively high dispersant content (> 0.1 to 0.5%). The type and content of the dispersant as well as Ca-bentonite content have significant effects on the liquid limit, sediment volume and apparent viscosity of the backfill. The sodium hexametaphosphate is shown to have higher dispersibility compared with the others in terms of greater reduction in apparent viscosity, and its optimum content is suggested in a range of 0.1% to 0.5%. A power function is proposed which well quantifies the relationship between the measured apparent viscosity and liquid limit.