Sensitivity of the initiation and runout of flowslides in loose granular deposits to the content of small particles: An insight from flume tests

Landslides of the flow type are among the most destructive mass movements due to their potential high mobility, long runout distance and capacity of entraining further material during movement. Generation of positive pore pressure excess is considered to be a fundamental triggering mechanism which, in some cases, can produce stress state instability and fluidization of both fine-grained and granular deposits, resulting in extremely fast and catastrophic events. Several experimental works and theoretical studies have been carried out so far to investigate the initiation mechanism and runout of flowslides. However, efforts are still needed to improve the understanding, for instance, of the role of the soil grading on the initiation and evolution of the movement. To this aim, a series of flume tests has been carried out using granular mixtures of coarse and small particles. High temporal resolution data acquisition has been used to investigate the onset of failure in terms of pore pressures, surface and internal displacements and vibration signals. By means of shear-controlled triaxial tests and permeability tests, the hydraulic conductivity has been found to be a controlling factor in the landslide initiation, whereas the uniqueness of the instability line - the lower bound of potentially unstable stress states - regardless of the soil grading, suggests that pore pressure measurement in the field can serve as a reliable indicator in early-warning systems.