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• The K sat of Zn-contaminated loess increases with time, while a decreasing trend for Cu-contaminated loess. • The hydrolysis of Zn2+ and the DDL effect are beneficial to the development of water passage. • The Cu 2 O precipitation in the Cu-contaminated loess is the main controlling factor for the weakening of pore connectivity. • The relationship among geochemical action, microstructure evolution, and seepage behavior of heavy-metal contaminated loess is revealed. Heavy metal contamination in soils can pose severe challenges to the safety of geotechnical engineering projects. Loess, which is widely distributed in Northwest China, is a preferred engineering construction material for anti-fouling barriers. Therefore, research on the influence of heavy metal ions on its seepage performance is urgently required. To obtain new insights into the seepage behavior of heavy metal-contaminated loess and its underlying geochemical mechanism, laboratory investigations were performed on the saturated hydraulic conductivity (K sat), leaching, and microstructural characteristics of loess contaminated with Cu2+ and Zn2+. The results indicate that the hydrolysis of Zn2+ creates an acidic environment, which promotes the dissolution of carbonate minerals in loess, enhances the leaching capacity, and leads to the quantitative transformation of small pores (2–8 μm) to mesopores (8–32 μm). Meanwhile, the alternating adsorption of Zn2+ and its diffuse double-layer effect compresses the diffusion layer, increasing the abundance of free water channels. Thus, the K sat of Zn-contaminated loess increases by 81.2% during the seepage period. As for Cu-contaminated loess, its seepage behavior is the opposite of that of Zn-contaminated loess, with a K sat decrease of nearly 50%. The primary factor controlling this phenomenon is the formation and enrichment of Cu 2 O in the lower part of the soil, which inhibits the enlargement of pores and reduces the effective connectivity of pores. The findings of this work provide insight into the seepage behavior of saturated loess under erosion by heavy metals and the underlying geochemical mechanism thereof. [ABSTRACT FROM AUTHOR]

• The K sat characteristics of remolded loess under CaCl 2 solutions are presented. • The response of EDL effect to CaCl 2 concentrations is evaluated. • Microstructure evolution of loess induced by CaCl 2 solutions is revealed. • The mechanism of K sat change in seepage process is disclosed. • The findings in this study have broad application value to other soils. Loess has been widely used in site remediation engineering projects in the Chinese Loess Plateau. With the interaction between CaCl 2 solution and cutoff wall, how does its permeability change, is the response pattern consistent for different concentrations, and what is the mechanism? To address this series of problems, the effects of CaCl 2 solutions of varying concentrations on the permeability of remolded loess were investigated by saturated permeability test, and the mechanisms underlying these effects were explored through free swelling ratio, zeta potential, thermogravimetric analysis (TGA), grain-size tests, as well as SEM and MIP tests. The results indicate that K sat values of the samples permeated with both deionized water (DW) and 0.001 mol/L CaCl 2 solution increase with time, with a greater increase observed in the latter. This is mainly due to that the electrical double layer (EDL) effect induced by Ca2+, as the dominant factor, controls particle flocculation and pore structure development. Under seepage of a 0.010 mol/L CaCl 2 solution, the EDL effect plays a dominant role during the initial stage. However, chemical weathering causes the disintegration of loess particles, and the dissolution of carbonates is inhibited by Ca2+. These are not conducive to pore development; thus, K sat first increases and then decreases significantly. The K sat of samples permeated with a 0.005 mol/L CaCl 2 solution first increases and then stabilizes, which represents the transitional stage of the two aforementioned change trends. This is mainly because the effective pores are increased by leaching and cation exchange, and then the porosity is in dynamic equilibrium under the combined action of the EDL effect, geochemical action, and microstructural evolution. Thus, the saturated hydraulic conductivity (K sat) of remolded loess is sensitive to the concentrations of CaCl 2 solutions, with different response modes. [ABSTRACT FROM AUTHOR]