Pore fluid pressure and internal kinematics of gravitational laboratory air‐particle flows: Insights into the emplacement dynamics of pyroclastic flows

Abstract

[1] The emplacement dynamics of pyroclastic flows were investigated through noninvasive measurements of the pore fluid pressure in laboratory air-particle flows generated from the release of fluidized and nonfluidized granular columns. Analyses of high-speed videos allowed for correlation of the pressure signal with the flow structure. The flows consisted of a sliding head that caused underpressure relative to the ambient, followed by a body that generated overpressure and at the base of which a deposit aggraded. For initially fluidized flows, overpressure in the body derived from advection of the pore pressure generated in the initial column and decreased by diffusion during propagation. Relatively slow diffusion caused the pore pressure in the thinner flow to be larger than lithostatic at early stages. Furthermore, partial auto-fluidization, revealed in initially nonfluidized flows, also occurred and contributed to maintain high pore pressure, whereas dilation or contraction of the air-particle mixture with associated drag and/or pore volume variation transiently led the pressure to decrease or increase, respectively. The combination of all these processes resulted in long-lived high pore fluid pressure in the body of the flows during most of their emplacement. In the case of the initially fluidized and slightly expanded (∼3-4%) flows, (at least) ∼70%-100% of the weight of the particles was supported by pore pressure, which is consistent with their inertial fluid-like behavior. Dense pyroclastic flows on subhorizontal slopes are expected to propagate as inertial fluidized gas-particle mixtures consisting of a sliding head, possibly entraining basement-derived clasts, and of a gradually depositing body.Citation: Roche, O., S. Montserrat, Y. Niño, and A. Tamburrino (2010), Pore fluid pressure and internal kinematics of gravitational laboratory air-particle flows: Insights into the emplacement dynamics of pyroclastic flows,

Dynamics of pyroclastic density currents: Conditions that promote substrate erosion and self-channelization — Mount St Helens, Washington (USA)

“…However, the consistent deposit thickness and aerial extent of Unit III suggest a current with high mobility. Thus we interpret this current to have maintained a high internal pore pressure within the depositional region of the current across the entire outflow area, consistent with the experimental findings of Roche et al (2004Roche et al ( , 2008Roche et al ( , 2010 and Roche (2012). Diffuse imbrication of blocks is noted in only some locations, such as at AD-1 and within the channel feature at AD-3, but generally fabric is weak to non-existent, suggesting granular flow conditions did not play a role (e.g., Cagnoli and Manga, 2005).…”

Can the collapse of a fly ash heap develop into an air-fluidized flow? — Reanalysis of the Jupille accident (1961)

“…The assumed heap undermining by a rising water table that led huge volumes of ash fall onto an important layer of air is an air-fluidization process that shares similarities with experiments by Roche et al (2008Roche et al ( , 2010 and Roche (2012).…”

“…The air-fluidization results of Roche et al (2010) can be transposed to Jupille fly ash flow as, according to a classification by Geldart (1973) based on particles size and density, the fly ash particles of diameters in-between 20 and 100 µm belong to the same class (so called 'group A') as the glass beads used in the air-fluidization experiments. This class of particles represents ~ 60% of the mass of fly ash involved in the Jupille flow.…”

Estudio de flujos granulares de tipo geológico por medio del simulador multisensor GRANFLOW-SIM

“…La masa del flujo en rápido movimiento produce diferencias de presión en el aire circundante las cuales modifican el comportamiento de parte del material (Roche et al, 2010). Las diferencias de presión que se generan en los alrededores de los flujos granulares pueden proporcionar información sobre su reología y la dispersión del material fino hacia la atmósfera (elutriación).…”

“…Sensores de presión de poro y humedad La presión de poro es un dato muy importante para la comprensión de los mecanismos que disparan los flujos de escombros y determinan su elevada movilidad (Costa, 1988;Roche et al, 2010) y se mide por medio de un piezocono miniatura. La cantidad de líquidos influye en el comportamiento global del flujo y se modifica constantemente debido a la segregación y a la dispersión (evaporación, absorción, etc.).…”