A laboratory-scale study on the role of mechanical vibrations in pore pressure generation in pyroclastic materials: implications for pyroclastic flows
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The results of laboratory-scale experiments with ignimbrite material are reported and discussed. The ignimbrite material contained in a rotating drum is subjected to mechanical vibrations, simulating particle agitation at the base of a pyroclastic flow propagating on an irregular substrate. It is observed that vibrations lead to a severe decrease of internal friction, evidenced by a drastic drop of the avalanche angle of the material in the rotating drum. We propose that acoustic streaming, a well-known phenomenon in engineering applications, operates in the ignimbrite material. Vibrations cause the development of rotational air flow around the particles, and in consequence, the enhancement of hydrodynamic viscous stresses promotes pore fluid pressure, which leads to dynamical weakening through fluidization. A fluid mechanical theoretical model accounts for the decrease of the friction coefficient of the ignimbrite material under a wide range of experimental conditions. It can be reasonably inferred that sustained high gas pore pressure resulting from mechanical agitation could be present in pyroclastic flows and might contribute to their high mobility.
KeywordsPyroclastic flow Fluidization Acoustic streaming Pore pressure
This work was supported by the Spanish Government Agency Ministerio de Economía y Competitividad, FEDER Funds (Contracts CTQ2014-52763-C2-2-R and CTQ2017-83602-C2-2-R), and French Research Institute for Sustainable Development (IRD). The Microscopy Service of Innovation and Research Center (CITIUS) and the Mechanical Workshop of the University of Seville are gratefully acknowledged. This is Laboratory of Excellence ClerVolc contribution no. 330.
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