Granular mixture deflation and generation of pore fluid pressure at the impact zone of a pyroclastic fountain: Experimental insights

Abstract

We present the results of analogue laboratory experiments on pyroclastic fountain collapse. Mixtures of air and glass beads ranging in size around 75 ± 15 μm, with Stokes number of ~10−3−101 and representative of 0.1–1 mm sized particles in nature, were released from a hopper at heights of 0.45–2.95 m above the base of a horizontal channel. Free fall caused continuous dilation of the granular material and led to mean particle concentrations of ~9–36 vol%, with concentration inversely proportional to drop height, before the particles impacted the channel base. Decoupling between the particles and the ambient air upon impact caused deflation of the mixture, which then propagated laterally as a dense granular flow overridden by a dilute suspension. Measurements at the impingement surface revealed that pore fluid pressure, generated through high air-particle relative velocity during deflation, counterbalanced up to ~50% of the weight of the emerging granular flow. The runout distance of the dense flow increased linearly with the fall height, similar to published results on unidirectional flows generated from collapse of packed granular columns. This suggests that the runout of flows resulting from release of granular material is controlled essentially by conversion of potential to kinetic energy and that the initial particle concentration is a second order parameter. We conclude that fountaining of pyroclastic material containing large amounts of particles with Stokes numbers of the order 10−3−101 can generate dense pyroclastic flows with some degree of pore fluid pressure.