Annibali Corona, Mariagiulia (2025) Solid-fluid transition in earthflows: understanding processes for predicting landslide mobility and magnitude. [Tesi di dottorato]

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Abstract

Earthflows, first defined by Howe (1909), represent a widespread type of landslide found in hilly and mountainous regions worldwide, where structurally complex geological formations interact with slope and moisture conditions conducive to instability. These phenomena are characterized by a typical morphology, consisting of three main units: a source area, typically crescent-shaped or basin-like, a narrow and elongated transport zone, and an accumulation area characterized by the presence of bulges and lobes. From an evolutionary perspective, earthflows develop slowly and can remain active for centuries, maintaining a plastic behaviour even under conditions of rapid movement. Their kinematics are complex and intermittent, governed by an internally distributed shear system rather than a classic mass movement. Displacement velocities vary significantly over time and are closely dependent on pore pressure fluctuations. Under certain conditions, increases in interstitial pressure can trigger sudden accelerations, leading to material fluidization and heightening the risk of catastrophic reactivations, which could have severe consequences for settlements and infrastructure. According to Hungr et al. (2001), earthflows are characterized by a gradual and imperceptible movement of material downslope, with velocities ranging from a few millimeters to several meters per year. However, numerous studies have highlighted the potential for these landslides to experience sudden reactivations, known as paroxysms or "surge" events, during which the volume of transported sediment can increase drastically. In such cases, displacement velocities can reach several meters per hour, and in some episodes documented by Hungr et al. (2014), accelerations of up to 0.13 m/s have been observed. Among the main factors influencing the evolution of earthflows are intense and/or prolonged precipitation, the loss of drainage networks due to progressive landslide deformation, and the availability of newly mobilized sediments. A distinctive feature of these phenomena is the high presence of fine-grained materials, such as silt and clay, which significantly impact both the rheology of the system and the available resistance along the shear surface. Recent studies have shown that the mobilization of earthflows is often accompanied by a reduction in shear wave velocity, a phenomenon that suggests an increase in porosity and interstitial water content—both crucial factors for the transition from solid to fluid state. The scientific literature has also demonstrated that this transition is strongly influenced by the rheological properties of the material, particularly its viscosity. Clayey materials involved in earthflows exhibit non-Newtonian behaviour, akin to "yield stress fluids." This behaviour is characterized by a bifurcation in viscosity upon surpassing a critical stress threshold, suggesting that variations in water content relative to the liquid limit may significantly influence the system’s mechanical response (Carrière et al., 2018). To explore these aspects, this study analysed fourteen earthflows located in the Southern Apennines (Abruzzo, Molise, Campania, Basilicata, and Calabria). Samples from the upper and middle transport zones were subjected to geotechnical, mineralogical, and rheometric tests. Geotechnical analyses provided the physical-volumetric properties and indices of the materials, as well as mechanical characteristics through oedometric tests. Mineralogical analyses, conducted via X-ray diffraction (XRD), identified the main mineral phases, with particular attention to amorphous or poorly crystalline components, providing key information on the properties influencing the material's mechanical behaviour. Simultaneously, rheometric tests were performed to assess the stability, viscosity, and deformation response under various shear stress conditions. A geomorphological analysis, based on satellite images and historical aerial photographs, enabled the reconstruction of the temporal evolution of each phenomenon over the last two decades, identifying landslides with the highest frequency of paroxysmal reactivations, ordinary events, and quiescence phases. These data were correlated with rainfall records from the nearest meteorological stations. Finally, a morphometric analysis was conducted to estimate the dimensions and mobilizable volume of each earthflow, providing an estimate of the potential magnitude of reactivations. The results highlighted significant variability in the geotechnical and mineralogical properties of the analysed phenomena, reflecting the geological complexity of the involved formations. However, it was possible to identify certain geotechnical and mineralogical parameters correlated with a higher frequency of paroxysmal events. Rheological analysis confirmed the non-Newtonian behaviour of the materials, showing strong correlations between critical shear stress (τc) and geotechnical parameters such as plasticity index (PI) and liquidity index (LI). Furthermore, an exponential decreasing relationship was observed between critical shear stress (τc) and deviations in water content relative to the liquid limit, plasticity index (PI), and activity index (A). The application of a rheological model based on the Herschel-Bulkley law represented an innovative approach for characterizing the rheology of materials involved in earthflows, allowing for the derivation of key parameters such as critical shear stress (τc), consistency (k), and flow index (n). Additionally, the integration of this constitutive law within a simplified one-dimensional model, based on geotechnical and rheological data, enabled the estimation of earthflow mobility and the quantification of the ratio between shear flow and plug flow. The results suggest that the progressive enlargement of the basal shear zone could represent a key mechanism in accelerating movement, confirming the hypothesis proposed by Iverson (1997). This research has contributed to a better understanding of the factors triggering paroxysmal reactivations and material fluidization in earthflows, with the aim of refining predictive models and optimizing risk mitigation strategies. The study provides an integrated framework of geomorphological, morphometric, geotechnical, mineralogical, and rheological characteristics of earthflows, with particular focus on the solid-fluid transition and its implications for landslide mobility prediction. The results open new perspectives for future research, particularly in integrating rheological parameters into numerical models to improve the prediction of earthflow dynamics and gain a more detailed understanding of the transition from stable states, represented by slow and persistent slip, to rapid and potentially catastrophic movements, represented by high-speed flows.

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