Origins of complexity in the rheology of Soft Earth suspensions

TL;DR

This paper demonstrates that the complex flow behaviors of natural debris flows can be reproduced using simple suspensions of water, silica sand, and kaolin clay, revealing a transition from brittle to ductile yielding.

Contribution

It introduces a unified constitutive model for soil suspensions that links yield stress, jamming, and particle rearrangement times, supported by experimental and theoretical insights.

Findings

Transition from brittle to ductile behavior with increasing clay content

A general constitutive relation capturing suspension flow behaviors

Support from amorphous solid models for non-equilibrium phase transitions

Abstract

When wet soil becomes fully saturated by intense rainfall, or is shaken by an earthquake, it may fluidize catastrophically. Sand-rich slurries are treated as granular suspensions, where the failure is related to an unjamming transition. Mud flows are modeled as gels, where yielding and shear-thinning behaviors arise from inter-particle attraction and clustering. Here we show that the full range of complex flow behaviors previously reported for natural debris flows can be reproduced with three ingredients: water, silica sand, and kaolin clay. Going from sand-rich to clay-rich suspensions, we observe continuous transition from brittle to ductile yielding. We propose a general constitutive relation for soil suspensions, with a particle rearrangement time that is controlled by yield stress and jamming distance. Our experimental results are supported by models for amorphous solids, suggesting that the paradigm of non-equilibrium phase transitions can help us understand and predict the complex behaviors of Soft Earth suspensions.