Field investigation of unsaturated hydraulic conductivity using actively heated fiber-optic technology

Unsaturated hydraulic conductivity (k(ψ)) is an important soil property that governs transient seepage. Instantaneous profile (IP) method has been routinely applied to determine k(ψ) of unsaturated soils, but its field application is limited to shallow depth because of the difficulty of deep instrumentation. In this study, an actively heated fiber-optic (AHFO) technology-based IP method was introduced for in-situ estimation of k(ψ) at great depths. The proposed method was then applied in a 77-m deep borehole involving two kinds of loess: compacted loess and natural loess. Results show that the permeability of shallow compacted loess, deep compacted loess, and natural loess decreases in sequence. As expected, the measured k(ψ) decreases with suction increase, aligning well with the derived hydraulic conductivity curve using van Genuchten-Mualem model. The value of k(ψ) at zero suction determined by extrapolation at different depths is in good agreement with k(ψ) determined by the variable head method. Further analysis shows that the use of drying soil water retention curve (SWRC) in the wetting process can lead to slight overestimation of k(ψ) at high suction. Besides, the spatial variability of soil dry density contributes to the scattering of k(ψ) data. © 2024 Elsevier B.V.