Dynamics of pore water pressure at the soil–bedrock interface recorded during a rainfall-induced shallow landslide in a steep natural forested headwater catchment, Taiwan

Abstract

The generation of subsurface saturation is highly related to shallow landslide initiation. Although many studies have relied on experiments to report the generation processes of subsurface saturation that triggers shallow artificial landslides, studies based on detailed in situ monitoring data sets or pre- and post-slide data sets for a natural landslide are still rare. This study recorded the dynamics of pore water pressure at the soil–bedrock interface with high spatiotemporal resolution concurrently with the occurrence of a shallow landslide in a steep natural forested headwater catchment. Using event-based analyses, we explored patterns in the generation of subsurface saturation and other main hydrological factors triggering the landslide. The landslide was not induced by the event with the greatest rainfall amount but by a smaller event with intermittent rainfall over a long duration of 9 days and intense rainfall from the seventh day. The landslide occurred as a result of record-high subsurface saturation generation with a spatial mean pore water pressure of 17.5 cmH2O and a saturation ratio of 0.89 at the site. It was attributed to the landslide event that began during wet conditions with a mean pore water pressure of 7.2 cmH2O. Although the rainfall contributed a near-uniform increase in pore water pressure at all measurement points, high pore water pressure was recorded within the landslide area in the downslope area of the site where subsurface saturation existed perennially. Heterogeneous distribution of subsurface saturation, including perennial and ephemeral saturation, was likely related to the extent of the landslide area. In addition, the two biggest events in the observation period caused subsurface displacements in the year before the landslide. The subsurface displacements weakened the geological structure and enlarged the amount of perennial saturation, which all reduced slope stability and facilitated the landslide. At least two origins of subsurface saturation should be a general feature for a headwater catchment (e.g., vertically downward infiltrated water and an upward rise in groundwater), and the different generation processes of subsurface saturation should be considered separately in future stability analysis models or disaster warning systems for shallow landslides.