Flume tests and rheological experiments provide insights into the fluidization of bed material induced by shear thinning during the entrainment of rock avalanches.

Abstract

In the realm of natural Earth-surface processes, such as mass movements exemplified by rock avalanches, a substantial entrainment of bed material along their trajectory is a common occurrence, amplifying both volume and run-out distance. The heightened mobility of these rapid gravity flows has been frequently ascribed by numerous researchers to the complete or partial fluidization of path material induced by swift undrained loading. An intriguing question arises: are there additional entrainment mechanisms at play in this process? To address this query, we executed a series of flume experiments designed to replicate rock avalanches overriding a saturated bed material. Our experimental findings revealed that the overriding flow induced a state of fluidization in the bed material, rendering it viscous. Furthermore, we observed that the rapid loading by the overriding debris increased pore pressure at the base, although it did not reach the threshold of complete fluidization. Rheological analysis of the bed material unveiled significant shear-thinning behavior, with viscosity diminishing rapidly as shear strain rate increased. Consequently, we posit that the concurrent effects of excess pore pressure at the basal layer and shear-thinning rheology in the flowing mass contributed to the fluidization of bed material and the ensuing extended run-out distance. This discovery offers a plausible natural elucidation for the extraordinary mobility of rock avalanches and holds promise for refining the precision and reliability of numerical simulations through the integration of the viscous model derived from our experimental endeavors.