THE VOLUME EFFECT ON APPARENT FRICTION FOR LONG-RUNOUT ROCK AVALANCHES: A NEW HYPOTHESIS

A minimum apparent coefficient of friction for a landslide can be calculated by equating the gravitational potential energy of fall to the energy dissipated by friction along the base of the landslide. When this type of calculation is performed for long-runout rock avalanches, also known as sturzstroms, the apparent coefficient is found to be less than the typical value of 0.6 for rock-on-rock sliding. Many researchers believe that some type of mechanical fluidization of breccia fragments within the moving avalanche mass explains the low friction values. A secondary observation is that the larger the volume of the avalanche mass, the lower the apparent friction. One proposal for this volume effect is that it results from a scaling effect between breccia clast size and the volume of the slide mass. I propose an alternative explanation based on rock avalanche dynamics during the early stage of slide movement. A stipulation of this proposal is that mechanical fluidization of breccia is, indeed, a process that occurs during the runout phase of avalanche emplacement.

A reasonable assumption is that large volume rock avalanche masses are thicker, especially in their initial stages, than smaller volume masses. Because of their greater thickness, shear stresses at the base of the greater volume sheets are larger. I propose that in the initial stages of descent, rock avalanche masses slide in a normal rock-on-rock manner. As sliding progresses, shearing stresses create brecciation and dilation in the basal zone and eventually mechanical fluidization occurs. In this model, the amount of displacement required for the changeover from sliding to flow is dependent on shear stress magnitude. Large volume rock avalanches, with their greater basal shear stresses, begin flowing after less displacement than smaller volume avalanches. Thus, more gravitational potential energy is available for runout during the low-friction fluidized stage and this leads to a smaller apparent friction coefficient.