Effects of surfactants and electrolyte solutions on the properties of soil

Semi-empirical modelling of hydraulic conductivity of clayey soils exposed to deionized and saline environments

Clay liners are widely used as porous membrane barriers to control solute transport and to prevent the leakage of leachate both in horizontal and vertical flow scenarios, such as the isolated base and ramps of sanitary landfills. Despite the primary importance of saturated hydraulic conductivity in a reliable simulation of fluid flow through clay barriers, there is no model to predict hydraulic conductivity of clayey soils permeated with saline aqueous solutions because most of the current models were developed for pure water. Therefore, the main motivation behind this study is to derive semi-empirical models for simulating the hydraulic conductivity of clayey soils in the presence of arbitrary solute concentrations in addition to deionized water. In order to achieve this goal, a relatively comprehensive dataset of 842 measured hydraulic conductivities was retrieved from the experimental literature, where almost 44% of them are related to certain solute concentrations. Afterwards, two modelling approaches were introduced; the first one is a modified form of Mbonimpa et al.'s (2002) model, in which the constants are adjusted to take into consideration the variations in liquid limit due to a change in solute concentration. A modification term was added to the model for the sake of accuracy. In the second approach, a new form of solute concentration-dependent hydraulic conductivity function was proposed, where special attention was given to void ratio and adaptive liquid limit as effective parameters. The results revealed that hydraulic conductivity predictions could be erroneous if the influence of solute concentrations in permeating fluid is ignored. An error analysis was conducted to examine the models' applicability and deviations. A blind independent set of data, including 132 data points, was also used to verify models. On the other hand, both newly proposed models could predict the hydraulic conductivity for a variety of soils, salt species, and concentrations well. Therefore, the proposed modelling approaches are somehow unique by considering the salinity of the pore fluid in addition to deionized water. More importantly, both models are comprised of easy-to-measure parameters with clear physics-based implications.

Studies on the chemical compatibility of soil-bentonite cut-off walls for landfills

Leachate contains composite contaminants, and the chemical compatibility of soil-bentonite cut-off walls is unclear. To better understand the issue, Fujian standard sand is used to represent a sandy soil stratum. Two clays were used as additive to examine the chemical compatibility of the soil-bentonite model backfills under the condition of composite contaminants. The results indicate that there is a representative cation when the backfills are permeated with NaCl, CaCl₂, and ZnCl₂ solutions and an NaCl-CaCl₂-ZnCl₂ mixed solution of the same ionic strength. Ca²⁺ has the highest maximum ionic strength among all cations from leachates. Moreover, the change in hydraulic conductivity, bound water content and effective porosity of sand-bentonite with the Ca²⁺ concentration or chemical oxygen demand (COD) exhibit a concentration threshold; i.e., when the concentration is smaller than the threshold, the hydraulic conductivity and effective porosity significantly increase, whereas the bound water content rapidly decreases; when the concentration is higher than the threshold, the hydraulic conductivity, bound water content and effective porosity tend to stabilize. In addition, under the condition of composite contaminants, the threshold is observed, while the hydraulic conductivity, bound water content and effective porosity vary with the COD. Thus, both the type and concentration of chemicals can change the effective porosity and affect hydraulic conductivity. Furthermore, there is a power function relationship between permeability and the effective pore.

Hydraulic conductivity of compacted clay liners permeated with inorganic salt solutions

Due to their low permeability, geosynthetic clay liners (GCLs) and compacted clay liners (CCLs) are the main materials used in waste disposal landfills. The hydraulic conductivity of GCLs and CCLs is closely related to the chemistry of the permeant fluid. In this study, the effect on the hydraulic conductivity of clays of five different inorganic salt solutions as permeant fluid was experimentally investigated. For this purpose, NaCl, NH(4)Cl, KCl, CaCl(2), and FeCl( 3) inorganic salt solutions were used at concentrations of 0.01, 0.10, 0.25, 0.50, 0.75 and 1 M. Laboratory hydraulic conductivity tests were conducted on low plasticity (CL) and high plasticity (CH) compacted raw clays. The change in electrical conductivity and pH values of the clay samples with inorganic salt solutions were also determined. The experimental test results indicated that the effect of inorganic salt solutions on CL clay was different from that on CH clay. The hydraulic conductivity was found to increase for CH clay when the salt concentrations increased whereas when the salt concentrations were increased, the hydraulic conductivity decreased for the CL clay.