Experimental investigation of rheological behavior and viscous-inertial boundary in dense solid-liquid two-phase granular flows

The dynamic processes of geological disasters, such as debris flows and shallow landslides, are essentially characterized by dense solid-liquid two-phase granular flows. Due to the complex interaction mechanism between the solid and liquid phases, its rheological properties are extremely complex. The \mu (I) and \mu (J) constitutive models are proposed to describe the rheological behavior for dry granular flows and solid-liquid two-phase granular flows, respectively. However, these models have been proposed based on free surface or very low normal stress conditions (generally < 1 kPa), and ideal granular materials (e.g., plastic/metal/glass spheres) are used. These conditions (stress range and material properties) deviate significantly from those encountered in real-world geohazard scenarios. Therefore, experiments are conducted using a rheometer equipped with a designed shear chamber, capable of applying normal stresses ranging from 5 kPa to 20 kPa and shear strain rates from 0.1 s^-1 to 360 s^-1 to granular materials under long-distance shearing. We utiliz zirconia beads and quartz sand as granular materials, and water and silicone fluid as interstitial fluids to simulate a range of solid-liquid two-phase flow conditions. Test results demonstrate that high-viscosity interstitial fluids significantly enhance the friction coefficient of granular flows, especially under high-speed shear. It is found that the \mu (I) constitute effectively characterizes flow behavior when the viscosity of the dry granular material or interstitial fluid is low, while the \mu (J) constitute is more suitable for high-viscosity interstitial fluids. Based on these physical simulation experiments, specific dimensionless boundary parameters are proposed to delineate viscous and inertial flow regimes. Furthermore, based on the differences in dominant physical mechanisms, appropriate constitutive models can be adopted to achieve a more accurate description of the macroscopic dynamic processes of such geological hazards.