Measuring the unmeasurable? Geotechnical and UAS-based investigations of landslides

The dynamics of landslides are strongly governed by their material composition and boundary conditions. When soil sediments mix with water, the fine fraction can markedly alter permeability—both within the flowing mass and in the underlying bed material—thereby influencing the generation and persistence of excess pore pressures and, consequently, shear strength. While new two-phase continuum models are increasingly capable of capturing these coupled hydro-geomechanical processes (e.g., Vicari et al., 2025b), a key challenge remains: can we reasonably measure the field material properties required to parameterize such models? Field sites are often steep, heterogeneous, and difficult to access, complicating in-situ characterization.To address this challenge, we conducted a systematic geotechnical investigation of ten debris flow channels across Switzerland (Vicari et al., 2025a). Soil samples were collected to determine grain size distributions, revealing significant variability in fine content among sites. Higher fine contents were found to reduce sediment permeability, quantified using in-situ dual-head infiltrometer tests. Complementary Unmanned Aerial System (UAS) surveys provided high-resolution erosion and deposition patterns, allowing us to relate observed geomorphic changes to both channel and catchment morphology and sediment properties. Simple correlations suggest that higher fine contents correspond to enhanced erosion and more frequent debris flow activity, though these relationships are strongly modulated by channel geometry and sediment availability. Combining geotechnical and geomorphological parameters enabled us to classify the investigated channels into four distinct behavioral groups, ranging from small, coarse gullies through intermediate coarse- and fine-rich channels to large, fine-rich systems.The methods developed and trained through this study proved invaluable for the investigation of the 28 May 2025 Blatten landslide. Modeling results indicate that a substantial frictional reduction was required to explain the extreme mobility of this event, implicating transient excess pore pressures as a likely mechanism. Geotechnical analyses of the landslide material revealed low permeability and high fine content, suggesting that excess pore pressure dissipation times may have greatly exceeded the event duration if even a 1 m flow layer became liquefied.Our results highlight the importance of integrating geotechnical measurements with remote sensing to constrain key parameters for next generation two-phase numerical models.ReferencesVicari, H., Bründl, F., Frieß, P., Ringenbach, A., Stoffel, A., Bühler, Y., Aaron, J., Mcardell, B., Walter, F., Graf, C., Herzog, R., Bebi, P., Gaume, J., 2025a. Linking debris flow erosion to channel-bed parameters: Geotechnical and remote sensing investigation of ten channels in Switzerland. ESS Open Archive. https://doi.org/10.22541/essoar.176126762.20405430/v1Vicari, H., Tran, Q.-A., Metzsch Juel, M., Gaume, J., 2025b. The role of dilatancy and permeability of erodible wet bed sediments in affecting erosion and runout of a granular flow: Two-phase MPM–CFD simulations. Computers and Geotechnics 185, 107307. https://doi.org/10.1016/j.compgeo.2025.107307