Experimental Study of Nonreactive Solute Transport in Fine-Grained Soils under Consolidation

This paper presents the results from a study on nonreactive solute transport in fine-grained soils under consolidation. The objective is to study the influence of consolidation on the migration of solute in saturated clays during the consolidation process. Bromide (Br^–) and chloride (Cl^–) were used as the tracers. Bromide was present only in the upper half of the 70 mm height specimen for the consolidation test, while chloride was homogeneous through the whole specimen because it was present in the kaolin powder used to prepare the soil sample. The consolidation tests were conducted under constant stress of 25 kPa with single drainage on the top boundary. Solution samples were collected for chemical analysis from the surrounding solution during the test and from the pore fluid of the specimen after it was sliced into four quarters and centrifuged. Analysis of the results shows that the effect of consolidation-induced advection on hydrodynamic dispersion is not significant. An equation was derived to estimate the advection velocity based on Terzaghi's one-dimensional consolidation theory. It was found that consolidation-induced advection velocity is very high (theoretically infinity) in the beginning of consolidation at the drainage surface, and decreases to zero when the consolidation is nearly concluded or near the non-flow boundary. Analysis of the Peclet numbers suggests that the consolidation-induced advection is the dominant transport mechanism only in a very short period of time after the start of the consolidation and in a very small region near the drainage surface. The temporal and spatial range that advection dominates is dependent on the consolidation stress, drainage length, porosity, hydraulic conductivity and molecular diffusion coefficient. Therefore the influence of consolidation on the nonreactive solute transport may not be significant except for providing an additional transport mechanism in addition to diffusion during the consolidation process.