DETERMINATION OF SOIL VISCOSITY FOR ELUCIDATING MUDFLOW BEHAVIOR
Mudflow behavior as a destructive mass movement must be examined to understand the cause of mudflow transport from its source area. A number of studies indicate that mudflow is triggered when water content in the soil is equal to or higher than its liquid limit. However, to date, no quantitative explanation for mudflow behavior has been reported. Hence, a rheological model such as the Bingham model should be applied, considering the water content in mudflow, which is considered a viscous liquid state. We use a proposed velocity–time profile of an actual mudflow. The profile is duplicated in the laboratory with a proposed device by using the flow box test. The governing equation of this test is developed by combining Terzaghi’s trap door principle and the Bingham model. The calculated velocity profile can be generated using the aforementioned equation. Displacement–time data are provided using the laboratory test. The laboratory velocity–time profile is then generated using the first derivative of displacement. The laboratory data are compared with the calculated velocity–time profile by curve fitting. The result shows that the flow box test can determine the relationship between viscosity and liquidity index for both plastic and viscous liquid states. This specific advantage of the flow box test is not featured in other conventional viscometers. Other studies show the relationship between viscosity and liquidity index for the viscous liquid state alone. However, the flow box test yields values close to those in other studies. Numerical simulation of real mudflow is conducted by inputting the laboratory result from the flow box test using the FLO-2D software. Back analysis shows that comparison between the actual mudflow and the simulation results (i.e., flow velocity and travel time) reveals relatively close values. The water content coincides with the qualitative data reported. The general characteristics of the mudflow can be described by the variation in soil viscosity, which can be dramatic once the liquid limit is reached or exceeded. This water content level explains why mudflow occurs because soil viscosity governs flow velocity. Thus, this result elucidates the mudflow behavior by using the flow box test.