Effect of Consolidation on Chloride Diffusion Coefficient in a Clayey Soil

Due to low porosity and slow water flow in clayey soils, the dominant pollutant transport mechanism may be molecular diffusion. The effect of consolidation of clayey soil on the chloride diffusion coefficient was investigated. The soil sample was collected from Nazloo region on North West of Urmia. After determination of soil physical parameters, the consolidation parameters were obtained. For determination of the chloride diffusion coefficient at different degrees of consolidation, when the cylindrical soil sample reached a certain degree of consolidation in the consolidation apparatus, the diffusion test was conducted and the diffusion coefficient was measured. This was conducted by the computer code POLLUTE by fitting the computed data on the observed concentration-time data. The results showed that by increasing the degree of consolidation and decreasing the soil void ratio from 0.4 to 0.35, in the range of stresses from 25 kPa to 400 kPa, the diffusion coefficient decreased from 5.94×10-10 m2/s to 5.36×10-10 m2/s. In the unloading process of consolidation from 400 kPa to 50 kPa the diffusion coefficient increased from 5.36×10-10 m2/s to 5.42×10-10 m2/s. The results of this study showed that the process of pollutant migration by molecular diffusion in the fine grained clayey soil of Nazloo was dependent on the degree of consolidation of the soil but the effect of consolidation on the decrease of soil diffusion coefficient is not significant. The good agreement between the observed and predicted data showed that the designed physical model, the adopted laboratory procedures and the theoretical model had good accuracy.