Measurement of hydraulic conductivity and diffusion coefficient of backfill for soil-bentonite cutoff wall under low consolidation pressure

Advection and diffusion are important mechanisms of contaminant transport through barriers. Whether flexible-wall permeameter and consolidated specimen must be used in the permeation or diffusion tests on soil-bentonite backfill under low consolidation pressure is still controversial. The soil-bentonite backfill is prepared according to the common construction procedure of cutoff walls. The hydraulic conductivity of the backfill is measured by a flexible-wall permeameter under effective consolidation pressures of 30, 50 and 100 kPa, respectively. The hydraulic conductivity and diffusion coefficient are also measured by rigid-wall column tests. Based on the theory of dynamic leaching tests, a dialysis method is proposed for quick measurement of the effective diffusion coefficient of the backfill. The results of flexible-wall tests show that the hydraulic conductivity of the backfill increases with the hydraulic gradient. There are initial hydraulic gradients ranging from 6.82 to 8 in the flexible-wall tests. The hydraulic conductivity decreases from 5.21×10-8 to 3.78×10-8 cm/s as the consolidation pressure increases from 30 to 100 kPa. Under the consolidation pressure of 10 kPa, the rigid-wall column tests give an initial hydraulic gradient of 5.67, a hydraulic conductivity of 7.14×10-8 cm/s, and an effective diffusion coefficient of 3.12×10-6 cm2/s. The backfill in the dialysis tests is not consolidated and the effective diffusion coefficient is 4.45×10-6 cm2/s. With a bentonite content of 6.02%, the hydraulic conductivity of the backfill decreases by 4 orders of magnitude, while the effective diffusion coefficient only decreases by about 50%, so diffusion will be the dominant contaminant transport process in soil-bentonite cutoff walls.