GAS RETENTION IN FINE-GRAINED PYROCLASTIC FLOW MATERIALS AT HIGH TEMPERATURES

Abstract

The ability of a dense pyroclastic flow to maintain high gas pore pressure, and hence low friction, during runout is determined by (1) the strengths and longevities of gas sources, and (2) the ability of the material to retain residual gas once those sources become ineffective. The latter is termed the gas retention capacity. Gas retention capacity in a defluidizing granular material is governed by three timescales: one for the evacuation of bubbles (tbe; brief and not considered in this paper), one for hindered settling from the expanded state (tsett), and one for diffusive release of residual pore pressure from the non-expanded state (tdiff). The relative magnitides of tsett and tdiff depend on bed thickness, tsett dominating in thin systems and tdiff in thick ones. Three pyroclastic flow materials, two ignimbrites and a block-and-ash flow sample, were studied experimentally to investigate expansion behaviour under gas flow and to determine gas retention times. Effects of particle size were evaluated by using two size cuts (<4 mm and <250 ) from each sample. careful drying of the materials was necessary to avoid effects humidity-related cohesion. two sets experiments were carried out: (1) expansion in non-bubbling regime at 50-200Ⱓ, (2) bed collapse tests initially bubbling state 50-550. provided that gas channelling avoided by gentle stirring, all samples exhibited a uniform prior onset bubbling. fine particle size (in particular high fines content), low density temperature favoured smoother fluidization increasing maximum possible state. an empirical equation describing determined. also greater partitioning into dense phase bed, as well finer-grained samples) higher voidage settled bed. large values tsett and tdiff were favoured by fine particle size. Temperature had less influence, suggesting that experimental results at low temperatures (50-200°C) can be extrapolated to higher temperatures. Gas retention times provide insight into the ability of pyroclastic flows in expanded (tsett) or non-expanded (tdiff) flow states to retain gas once air ingestion or gas production have become ineffective. Finer-grained pyroclastic flows are expected to retain gas longer, and hence to have higher apparent 'mobilities', than coarser-grained ones of comparable volume, as has been observed on Montserrat.