Reynolds' dilatancy

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Reynolds' dilatancy is the observed tendency of a compacted granular material to dilate (expand in volume) as it is sheared. This occurs because the grains in a compacted state are interlocking and therefore do not have the freedom to move around one another. When stressed, a lever motion occurs between neighboring grains, which produces a bulk expansion of the material. On the other hand, when a granular material starts in a very loose state it may initially compact instead of dilating under shear. Reynolds' dilatancy is a common feature of the soils and sands studied by geotechnical engineers, and is a part of the broader topic of soil mechanics. It was first described scientifically by Osborne Reynolds (1842–1912) in 1885 and 1886.


  • Reynolds, O., "On the dilatancy of media composed of rigid particles in contact, with experimental illustrations," Phil. Mag., Series 5, 20 (1885), pp. 469–481.
  • Reynolds, O., "Experiments showing dilatancy, a property of granular material, possibly connected with gravitation," Proc. Royal Institution of Great Britain, Read February 12, 1886.
  • R. M. Nedderman, Statics and Kinematics of Granular Materials, Cambridge University Press, 1992, ISBN 0-521-01907-9.

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