Mechanisms of particle-liquid interaction in high-mobility debris ﬂows
Debris flows are a category of fast-moving landslides with the potential for exceptionally long travel distance. The transition from a geomaterial to a viscous fluid has garnered attention from many fields of research, with this coupled behaviour viewed from the fluid mechanics, geotechnical effective stress, and granular physics perspectives. The presence of liquid in the pore space leads to complex mechanisms of interaction with the solid particles, including buoyancy, drag, and the potential for pore pressure exceeding hydrostatic conditions. These mechanisms may lead to significant scaling effects with flow thickness, limiting the applicability of empirical scaling relationships. It is known that the addition of liquid to a granular flow reduces equivalent flow resistance over the dry case. However, is is unclear whether the concept of reduced frictional resistance through the concept of effective stress is alone sufficient to explain the increased mobility of fluid saturated flows. Research is presented towards quantifying the collisional and frictional components of the flow regimes in dry and saturated granular flows. Advances were made in measuring granular temperature, an index of collisional behaviour in flows, through image analysis. Experiments were conducted using the same monodisperse (uniform particle size) material in a large laboratory flume and a Vertically Rotating Flume. The results suggest a preference for wet flows to form collisional shear bands at depth and that elevated pore pressure levels need not be present for a marked decrease in flow resistance to occur. Additionally, a transparent soil-oil mixture was used to visually illustrate dilative behaviour upon initial shearing and the potential for suction generation. The results demonstrate the partitioning of stress between the collisional and frictional regimes within dry and saturated granular flows and indicate additional potential mechanisms leading to high-mobility debris flows. It is anticipated that the advances in granular temperature measurement will be beneficial for experimental validation of various Extended Kinetic Theories and that the results of the flume and rotating drum trials will accelerate the inclusion of collisional behaviour within material models for debris flows.