Soils are treated as continua for the purposes of analysis but the mechanical properties of soils derive from their particulate nature. Study of the behaviour of soils and other particulate systems using radiography and particle image velocimetry (PIV) and discrete element modelling (DEM) shows that there are characteristic dimensions ('intermediate scales') larger than the size of individual particles that may affect the system response: the thickness of shear bands; the displacement required to mobilise a critical state; the repeat spacing of patterns of dilation; the cells of force transmission between particles; and the size of patterns of internal displacement.

These observations should affect the way in which we plan element tests, model tests, and the preparation of particle assemblies for numerical analysis.

David Muir Wood has been Professor of Civil Engineering at the University of Bristol since 1995 and was Dean of Engineering from 2003-2007. He was Cormack Professor of Civil Engineering at University of Glasgow from 1987-1995 and lectured at Cambridge University from 1975-1987. He received his PhD in 1974 working with the late Professor Peter Wroth and was seconded to Scott Wilson (Hong Kong) in 1978 to work with the fledgeling Geotechnical Control Office on slope stabilisation works. His research activities cover all aspects of modelling of soils - numerical and physical. His book 'Geotechnical modelling' was published in 2004, stimulated by a Royal Society Industry Fellowship with Babtie Group in 1994 and 1995. He delivered the 20th Bjerrum Lecture in 2005.