Vertical segregations in flows of angular rock fragments: Experimental simulations of the agitation gradient within dense geophysical flows

In this paper, we illustrate laboratory experiments whose purpose is to study the vertical segregations that are commonly observed in deposits of dense geophysical flows (such as pyroclastic flows and rock avalanches). In these experiments, we use rock cuboids with 5 mm long edges as matrix and rock cuboids with 2 cm long edges as segregating clasts. A rotating disk is used to apply frictional stresses at the base of the granular masses. In our experiments, segregating cuboids with density smaller than or equal to that of the matrix particles rise whereas segregating cuboids with density larger than that of the matrix particles sink. The granular flows are imaged through the glass container of the experimental apparatus by a high-speed video camera at 2000 fps. By means of particle image velocimetry analysis of the movies, we study the vertical gradient of particle agitation that exists within the granular flows where agitation increases downward because of the interaction with the subsurface asperities. The high-speed movies show that it is the particle agitation within the flows that exerts an upward force and that, when this force is larger than the weight of the segregating clast, the clast rises whereas, when it is smaller, the clast sinks. The most important result in our set of experiments is that the threshold which separates the values of density of the segregating clasts that segregate upward and the values of density of the segregating clasts that segregate downward is larger than the density of the matrix particles. This explains the upward segregation of dense lithics that is frequently observed in deposits of geophysical flows. This upward segregation is due to the fact that the resultant of the collisions exerted by the matrix particles is a force strong enough to push upward also dense and heavy fragments.