Earthflow mechanics

Earthflows are the most common type of failure in active slopes where fine soils are dominant. These landslides generally have an elongated or lobate shape, and show a complex style of movement in which mass flow is accompanied by basal sliding along localized shear zones. Earthflows can move for years or decades at extremely low velocity (mm or cm/year), and then suddenly accelerate in response to critical rainfall events. The reactivation mechanism typically consists of an initial failure located in the upper part of the slope followed by downslope propagation onto existing landslide deposits. During the major reactivation events, earthflows exhibit a rapid acceleration accompanied by mass fluidization, which can increase the displacement rate up to several m/hour.

Despite its scientific relevance and practical importance, the process of solid-to-fluid transition in active earthflows is poorly understood. In fact, this process is very difficult to measure in the field and traditional geotechnical instruments are not suitable to this purpose. Several researchers recently proposed to measure the temporal change of shear wave velocity as an indicator of fluidization in active landslides. The idea behind the method is that, as the shear wave velocity in a fluid tends to zero, the bulk shear wave velocity should decrease in the proximity of liquefied areas.

In this context, we are collecting valuable field data to document the reactivation process of several active earthflows in the Northern Apennines of Italy. Data consist of continuous measurements of seismic surface wave velocities carried out using the active Multichannel Analysis of Surface Waves (MASW) and the passive Refraction Microtremors (ReMi) techniques. The change in surface wave velocity is then compared to the variation of the displacement rate (measure by GPS monitoring and time-lapse cameras) and to the hydrologic response to rainfall detected by a network of pressure sensors buried into the landslide deposit. These data highlight a close relationship between rainfall, landslide velocity, and soil stiffness and are providing new insight into the complex dynamics of active earthflows.