Rheological fingerprints of soft earth suspensions

Soft earth suspensions, such as debris flows, are fast-flowing slurries of soil matter, that cause huge loss of human life and infrastructure. These dense suspensions-based geophysical flows are becoming more dangerous and increasing in frequency, exacerbated by the climate change effects. Today, a dichotomy exists, between granular pore pressure and complex fluid rheology, in explaining the origins of flow behavior of these dense soil slurries, prompting a fundamental question: Is there a generalized way to capture their `failure' and `flow' mechanics to propose better hazard prediction models? Here, we experimentally show that a simple model complex fluid mixture -- comprising kaolin clay and silica sand -- captures the non-inertial flow behavior of debris mixture, observed in both laboratory experiments and natural flows. Surprisingly, we find that the physics of debris flow mechanics are a strong function of the `clay ratio', which is the volumetric ratio of clay to total solids, controlling the cohesive-to-frictional interactions in the suspension mixture. The shear thinning exponent associated with yielding in these sand-clay suspension mixtures decreases with increasing sand concentration. Our observations suggest that the attenuating shear-thinning exponent results from soft soil material microstructural annealing changing the failure mode from plastic dissipation-like gradual yielding in pure clay suspensions to a Mohr-Coloumb mode granular failure in sand-rich suspensions. These insights contribute to a deeper understanding of the flow mechanics of debris mixtures, potentially leading to improved mitigation strategies and risk assessment measures.