Autofluidization of collapsing bed of fine particles: Implications for the emplacement of pyroclastic flows

Long runout distances of fines-rich pyroclastic flows may be partly attributed to high gas pore pressure that reduces interparticle friction. Here we examine the generation of pore pressure caused by collapse of granular materials into ambient air, which may occur at the impact zone of a fountain collapse or when pyroclastic flows bypass topographic breaks or propagate over a rough substrate. We carried out laboratory experiments, which consisted of collapse of beds of glass beads in a vertical column where pressure sensors measured the air pore pressure at different levels. The collapse height, the bed thickness and temperature as well as the particle size were systematically changed. Measurements revealed a first overpressure phase, both at different heights in the initial bed and at column base, when the particles began to fall and forced the air beneath to ascend through the bed. Even at low collapse height of 20 cm, the maximum pore pressure compensated almost completely the bed lithostatic pressure provided the particle size was <337 mu m, meaning that the material was fully fluidized. Particles then accumulated at column base and formed and aggrading deposit, in which a second peak of overpressure was measured before pore pressure diffused slowly. The overpressure was high when the particles were small enough (<100-200 mu m) but it was poorly developed or even absent when the particle size was increased. The duration of pressure diffusion at end of this second phase increased with the square of the thickness of the aggrading deposit and decreased strongly with increasing particle size. Additional experiments with ignimbrite material showed similar behavior but with a much longer duration of final pressure diffusion. At high temperature (up to 200 degrees C), overpressure during the first phase could overpass the bed lithostatic pressure. This suggested thermal pressurization of the ambient cold air, which expanded rapidly when percolating through the hot material. We conclude that fluidization of hot fines-rich pyroclastic flows can be easily achieved when the granular material collapses into the ambient air. (C) 2018 Elsevier B.V. All rights reserved.