Study of the dynamic behavior of earthflows, with particular reference to the solid-to-fluid transition characterizing stages of rapid movement

Earthflows are landslide developing in clay-rich soil, characterized by elongated tongue-like shapes. Their behavior has been largely debated in literature. The periods of rapid motion have been interpreted in two different ways. Some authors suggest that, in spite of their flow-like morphology, earthflows essentially move like rigid bodies along well-defined slip surfaces. Other authors report that during the surging earthflow material becomes softer and can be considered as a viscous fluid. However, collecting data during the stages of rapid motion is still challenging and only minimal data concerning the real acceleration stage had been presented in literature. The purpose of the current study is to provide an insight into the reactivation stage in order to understand if any fluidization process can occur. Geophysical data have been treated: seismic techniques have been applied to detect shear stiffness variation over time in the periods near the rapid surging and two-pass conventional interferometry has been used to monitor landslide deformation patterns. Seismic data indicate that earthflow soils are affected by stiffness drops during the acceleration stage; the process is slowly reversed in the period following the rapid motion during which the materials become stiffer. This process is probably related to changes in void ratio and soil water content: after the failure the water content decreases, and the porosity of the soils decreases too. Data demonstrate that during the stages of rapid motion earthflows behave like viscous fluids; far from the failure they turn to be stiff and a rigid-like behavior is more likely. Mathematical theories can be used to fully describe the behavior but different types of field data concerning the rapid motion stage are necessary: to obtain a whole dataset we need to know in advance information about earthflow reactivation. Thus, conventional two-pass interferometry has been successfully applied to derive deformation patterns.