  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.
