Groundwater in large-scale landslides usually flows along the colluvium's bottom, so the colluvium is unsaturated. The unsaturated hydraulic parameters will control the infiltration and seepage behavior after rainfall. Considering the contribution of unsaturated hydraulic parameters will help explain the sliding mechanism and stability of the landslide. In order to clarify the influence of the unsaturated hydraulic parameters of the colluvium on the stability analysis of large-scale landslide, this paper takes the Fanfan site as an example, takes samples of the colluvium for pressure plate test, and uses the Fredlund and Xing (1994) model to fit the soil-water characteristic curve. This paper conducts sensitivity analysis of the parameters (a, n, and m) in the model and performs feedback analysis with on-site hydrological observation data to calibrate the above parameters. Finally, this paper considers the parameter mentioned above scenarios to conduct a stability analysis and discusses the sliding mechanism and stability of the Fanfan site through the changes in shallow soil water content, deep groundwater level, and safety factor. Overall, the sensitivity analysis and calibration procedure of unsaturated hydraulic parameters proposed in this paper help clarify the unsaturated characteristics of colluvium and their contribution to large-scale landslides' stability.


Taiwan is an active mountain belt created by the oblique collision between the northern Luzon arc and the Asian continental margin. The inherent complexity of geological nature makes numerous discontinuities through rock masses and relatively steep hillside on this island. In recent years, the increase in extreme natural events' frequency and intensity due to global warming or climate change brought significant landslide and debris flow hazards in Taiwan's mountain area. The causes of landslides are attributed to many factors. Notably, rainfall is a well-known one of the most significant triggering factors for landslides. In general, the rainfall infiltration could change the soil's suction and moisture, raise the unit weight of soil and reduce soil shear strength for the shallow colluvium. Long-term infiltration may result in the weakening or creeping of slope formation material. The slope stability is closely related to the geological and topographical conditions, the groundwater pressure change in response to rainfall infiltration, and slope formation's physical and mechanical parameters. Detailed site investigation and observation using state-of-the-art technology will be performed on active landslides from the study area to clarify the occurrence of landslides induced by the change of hydrologic conditions during heavy rainfall.

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