Compressibility and hydraulic conductivity of soil/Ca-bentonite backfills for vertical slurry cutoff wall

Soil-bentonite slurry-trench cutoff walls are used extensively as engineered barriers for groundwater contaminants containment. The compressibility and hydraulic conductivity of sand/Ca-bentonite and sand-clayey soil/Ca-bentonite backfills for slurry-trench cutoff wall are evaluated via a series of oedometer tests and falling-head hydraulic conductivity tests. The backfills are prepared by mixing base mixture with bentonite-water slurry containing 10% bentonite, and the water content is adjusted to achieve target slump for workability via slump test using miniature slump cone. The results indicate that a total Ca-bentonite content of 8.3% is required in soil/Ca-bentonite backfills to achieve typical regulatory limit (k-9 m/s). The compression index and hydraulic conductivity are insignificantly affected by the amount of clayey soil in backfill. In addition, the values of hydraulic conductivity measured using falling-head hydraulic conductivity test (kFH) are quite similar to these evaluated based on Terzaghi's one-dimensional consolidation theory (kT), and the ratio of kFH to kT is in the range of 1/3 to 3. Finally, the compressibility and hydraulic conductivity are assessed and predicted based on physical properties, including void ratio at effective vertical compression stress of 1 kPa (e1), void ratio of bentonite (eb), and liquid limit. ©, 2015, Shuili Xuebao/Journal of Hydraulic Engineering. All right reserved.