Rheological properties of loose sands subjected to upward flow

The objective of this study was to investigate the rheological properties of loose sands subjected to upward flow by using a vane-type rheometer that controlled upward flow in the loose sand specimens. Various hydraulic gradients (i.e., i = 0-2.0) were applied to loose sands. The rheological properties of the loose sands, such as yield stress and viscosity, were determined based on the Bingham and Herschel-Bulkley models. The experimental results showed that the flow behavior of loose sand samples exhibited a shear thickening when the Herschel-Bulkley model was applied (i.e., nondimensional flow index n > 1) and exhibited a Bingham-like within a limited shear rate range (i.e., 1 similar to 30 1/s). The latter is clearly shown for a relatively high shear rate. As the hydraulic gradient was increased, the flow characteristics were close to the Bingham fluid. Yield stress showed a tendency to decrease linearly as hydraulic gradient was increased. However, the viscosity of the loose sands maintained a constant value irrespective of hydraulic gradient. The test results indicated that the pore fluid pressure resulting from upward flow in a soil sample affects yield stress, which contributes to the initiation of debris flow mobilization. As a result, it was possible to estimate the rheological properties of soil at the condition of liquefaction (critical hydraulic gradient), or initial occurrence of debris flow.