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Enhanced Mobility of Granular Mixtures of Fine and Coarse Particles

J.C. Phillips(1), N.H. Thomas(2), A.J. Hogg(3) and R.R. Kerswell(3)

(1)Centre for Environmental and Geophysical Flows,
   Department of Earth Sciences,
   University of Bristol,
   Queens Road,
   Bristol BS8 1RJ, U.K.

(2)Flow Research Evaluation Diagnostics Ltd,
   Aston Science Park,
   Birmingham B7 4BJ, U.K.

(3)Centre for Environmental and Geophysical Flows,
   School of Mathematics,
   University of Bristol,
   University Walk,
   Bristol BS8 1TW, U.K.

Granular flows that occur in nature as rockslides and volcanic block-and-ash 
pyroclastic flows, and those arising from industrial processes such as mining 
and solids conveying, typically contain a range of particle sizes. Previous 
granular flow studies using two particle sizes have focussed on segregation 
effects that occur when the mixture flows or is vibrated, and numerical 
simulations of rapid granular flow have investigated how stresses vary with 
relative particle size and concentration. We present laboratory measurements 
of the propagation of flows of fine and coarse granular materials that show 
that their interaction can result in significantly increased mobility. At a 
fine material volume fraction of about 0.3, the flow mobility can be 
increased by up to a factor of four compared with the same volume of either 
all fine or all coarse material. We interpret these results in terms of the 
flow frictional regime being predominantly controlled by sliding of coarse 
material at low fine material volume fraction, rolling of fine material at 
intermediate fine material volume fractions, and particle interactions within 
the fine material at high fine material volume fraction. We present model 
calculations to support this interpretation.