Fluidization of bed material caused by shear thinning during rock avalanche entrainment: Insights from flume tests and rheological experiments

Abstract

As a natural earth-surface process, mass movements (e.g., rock avalanches) typically entrained a large amount of bed material in their pathway, increasing volume and run-out distance. Many authors attribute the increased mobility of these fast gravity flows to the total or partial fluidization of the path material due to the rapid undrained loading. A question raised here is whether any other entrainment mechanisms are involved in this process. In this study, we conducted a series of flume experiments to simulate rock avalanches overriding a saturated bed material. Experimental results showed that the overriding flow fluidized the bed material into a viscous state. We also found that the rapid loading by the overriding debris leads to an increase in pore pressure at the bottom, but does not bring it to the level of fluidization. We tested the rheological properties of the bed material and found significant shear-thinning rheology, with its viscosity decreasing rapidly with increasing shear strain rate. Therefore, we think that the excess pore pressure at the basal layer and the shear-thinning rheology at the flow mass simultaneously contributed to the bed-material fluidization and long-runout distance. This finding provides a possible natural explanation of the hypermobility of rock avalanches. This finding can also help improve the accuracy and reliability of the numerical simulation of rock avalanche entrainment by using the viscous model obtained from the experiments.