Investigating saturated hydraulic conductivity of remolded loess subjected to CaCl2 solution of varying concentrations

Loess has been widely used in site remediation engineering projects in the Chinese Loess Plateau. With the interaction between CaCl2 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 CaCl2 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 CaCl2 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 CaCl2 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 CaCl2 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 CaCl2 solutions, with different response modes.