Abstract: Diffusion is the dominant solute transport process in high slump backfill with low hydraulic conductivity. However, the method to measure the diffusion coefficient is not available. Based on the theory of dynamic leaching test, a dialysis test method was adapted to measure the diffusion coefficient of NaCl diffusing from flowing-state backfills with different bentonite contents at different initial NaCl concentrations. The solution for solute transport in a finite cylindrical medium was used to calculate the diffusion coefficient. Test procedure and problems in data analysis were discussed. The results show that, as more data are used, the effective diffusion coefficient of NaCl decreases and the relative regressing error increases. Diffusion in backfill is obviously faster than that in compacted specimen. The total data sets are truncated until a relative error of less than 0.5% is achieved. This relative error quantitatively describes the dominant role of diffusion in solute transport process, and assures the validity of the data fitting using the pure-diffusion analytical solution. The measured diffusion coefficient decreases as the bentonite contents and initial NaCl concentrations increase. However, variations of the values are roughly small in the test range. The dialysis test is a feasible method for quick measurement of solute diffusion coefficient in cutoff wall backfill.

Key words: cutoff wall, backfill, high slump, soil-bentonite, effective diffusion coefficient, dialysis test