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The saturated hydraulic conductivity of soils is a critical concept employed in basic calculation in the geotechnical engineering field. The Kozeny–Carman equation, as a well-known relationship between hydraulic conductivity and the properties of soils, is considered to apply to sands but not to clays. To solve this problem, a new formula was established based on Hagen–Poiseuille's law. To explain the influence on the seepage channel surface caused by the interaction of soil particles and partially viscous fluid, the surface area ratio was introduced. A modified framework for determining the hydraulic radius was also proposed. Next, the relationship between the effective void ratio and the total void ratio was established for deriving the correlation of hydraulic conductivity and total void ratio. The improved equation was validated using abundant experimental results from clays, silts, and sands. According to the results, the accuracies of the proposed model with two fitted multipliers for clays, silts, and sands are 94.6, 96.6, and 100%, respectively, but with only one fitted parameter, the accuracies are 97.1, 91.5, and 100%, respectively. The proposed model can be considered to have a satisfactory capability to predict hydraulic conductivity for a wide variety of soils, ranging from clays to sands.

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Most deep-seated landslides are characterized by large volumes with deep shear surfaces. They are sensitive to hydrological forcing, especially in climate change scenarios. This paper studies the role of soil–water interaction in affecting the motion of a deep-seated landslide near the southeast coast of China, where seasonal rainfall combined with annual typhoons caused the instability of a previous stable slope. A comprehensive investigation consisting of field monitoring and experiments of soil–water interaction is carried out. The monitoring data show that the landslide exhibits alternate dormant and active stages, corresponding to rainy and dry seasons, respectively. The enduring precipitations predominate the landslide motion, while intensive rainfall brought by typhoon events leads to transient deformation. In addition, wet treatment of intact and reconstituted soils is adopted to mimic the interaction between rainwater and landslide material. The results obtained from in-situ and laboratory direct shear tests indicate that the soil–water interaction is time-dependent. The long-term interaction gives rise to significant strength reduction of soils, thereby regulating the movement of the landslide.

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