Coupled Large Deformation and Contaminant Transport in Low-Permeability Clay Layers: Role of Threshold Hydraulic Gradient

Natural low-permeability clay is usually used as underlying layers at dredged sludge disposal sites. Both laboratory and field measurements of clay settlement revealed the existence of a threshold hydraulic gradient for water flow, attributable to the strong influence of bound water electrostatically adsorbed on clay particle surfaces in low-permeability clays. However, few studies have been conducted to investigate the effect of the threshold hydraulic gradient on contaminant transport in such clay layers. This study develops and numerically solves a coupled one-dimensional model for large- and small-strain nonlinear consolidation and contaminant transport in a low-permeability clay layer, incorporating the threshold hydraulic gradient. The proposed numerical solution is benchmarked against results from the consolidation model, the coupled consolidation-transport model, and experimental tests on consolidation-induced contaminant transport. Based on the coupled model developed in this study, trichloroethylene (TCE) is used as an example to analyze its transport patterns in the low-permeability clay layer. The results showed that when the threshold hydraulic gradient is considered, low-permeability clay layers exhibit a moving flow front during consolidation. Threshold hydraulic gradient, volume of dredged sludge, initial contaminant concentration, and model geometric assumptions all influence the downward movement capacity of the flow front. During soil consolidation, the weaker the movement capability, the stronger the TCE transport capability. Additionally, the consolidation rate of dredged sludge has a negligible effect on the contaminant transport patterns within the clay layer. Therefore, the top boundary can be simplified as a permeable boundary when establishing theoretical models.