Granular pressure at the base of dry flows of angular rock fragments as a function of grain size and flow volume: A relationship from laboratory experiments

Experiments are carried out by releasing angular rock fragments down a curved chute and by measuring the basal pressures that are exerted by the granular flows on the basal containing surface (the substrate). The purpose of these experiments is to understand the mechanisms of energy dissipation and interaction with the ground of rock avalanches and dense pyroclastic flows. Our data show that collisions due to particle agitation affect significantly the basal interaction of granular flows. In particular, our experiments reveal that particle agitation per unit of flow mass increases as grain size increases or as flow volume decreases (with all the other features the same). This is so because as grain size increases or as flow volume decreases (with all the other features the same), there are fewer particles in the flows and the agitation due to the interaction with the rough containing boundary surfaces penetrates relatively more inside the flows. The analysis of the experimental data generates a linear relationship between particle agitation (expressed as a dimensionless basal pressure deviation) and a parameter which is directly proportional to the square of grain size and inversely proportional to the cube root of flow volume. This relationship shows the intrinsic ability of the granular flows to dissipate more energy (larger particle agitation per unit of flow mass) or less energy (smaller particle agitation per unit of flow mass) as a function of flow volume and grain size.