Keywords: consolidation, solute transport, clay, advection, dispersion, sorption.

Consolidation-induced transport of contaminants occurs when contaminated soil, such as a layer of dredged sediment, undergoes consolidation due to material self-weight or changes in boundary stress conditions. Transport processes are similar to those for rigid porous media and include advection, dispersion, and sorption/desorption. The difference is that consolidation involves transient advective flows that produce volumetric strains and, in turn, cause changes in soil porosity, compressibility, and hydraulic conductivity. The most advanced treatment is currently provided in the CST2 computational model [1]. CST2 was developed using a dual-Lagrangian framework that separately follows the motions of fluid and solid phases. The consolidation algorithm accounts for large strains with associated geometric and material nonlinearities. The transport algorithm accounts for advection, dispersion, and sorption, and includes variation of effective diffusion coefficient during consolidation and nonlinear nonequilibrium sorption effects. This paper presents the validation of CST2 using experimental data.

For the experimental program, diffusion and large strain consolidation tests were performed on composite specimens of kaolinite slurry consisting of an upper uncontaminated layer and a lower layer contaminated with potassium bromide (KBr). The diffusion test involved no advection and produced final distributions of Br^- and K⁺ that indicated considerable redistribution of solute mass over the 18 d diffusion period. The consolidation tests produced significant outflows of Br^- solute mass in response to pore water outflow from the specimens. Outflows of K⁺ were much less due to sorption in the initially uncontaminated layer. When compared to the diffusion test, transient advective flows for the consolidation tests had a significant effect on solute breakthrough and final distribution of solute mass within the specimens. Diffusion processes were also important as indicated by the large differences in transport for the two consolidation tests conducted using different load-increment-ratios. Thus, the results indicate that both diffusion and consolidation-induced advection made an important contribution to solute transport and mass outflow. This limited testing program has also indicated that neglecting to consider transient consolidation effects may lead to serious errors in transport analyses for soft contaminated clays undergoing large volume change.

Numerical simulations obtained using the CST2 model were in close agreement with experimental measurements for both solute breakthrough and final concentration profiles corresponding to several different loading and transport conditions. This suggests that the CST2 computational model is capable of simulating local flow and transport effects as well as overall solute mass outflows of both tracer and sorptive contaminant species.

1

J. Lee, P.J. Fox, "Model for Consolidation-Induced Solute Transport with Nonlinear and Nonequilibrium Sorption", International Journal of Geomechanics, in review.

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