Solid–fluid phase transition characteristics of loess and its drag reduction mechanism

Abstract

Flow-like events usually occur during heavy rainfall and pose significant threats to ecosystems and human life and property because of their suddenness, high speed, and long distances. To study the solid–fluid transition and subsequent high fluidity and hypermobility mechanism of redeposited loess, we conducted a series of flume tests and rheological tests, and the results showed that loess exhibits different degrees of fluidized movement characteristics under different rainfall intensities, and a rainfall intensity of 90 mm/h was the most likely to trigger loess flowslides. Additional rheological analyses indicated that viscosity (shear rate) bifurcation characterizes the rheological response of loess solid–fluid transition, and the decrease in viscosity caused by shear thinning can explain the drag reduction effect and its high fluidity in the process of solid–fluid transition, which corresponds well with the results of flume tests. We proposed a two-step yielding characteristic and introduced structural dynamics to establish a unified solid–fluid transition model incorporating a hydro-mechanical coupling and rheological property. The model can be used to describe both the solid-like behavior of soil before phase transition using an elastoplastic model and fluid-like behavior after phase transition using a viscoplastic model. The research results provide a new understanding of solid–fluid phase transition characteristics of loess from the perspective of rheology, which can also provide a new idea for studying the fluidization movement of rock avalanches and pyroclastic flows and their geomorphic evolution.