June
Editorial
Soil mechanics at the grain scale: issue 2
This second volume of the themed issue on ‘Soil mechanics
at the grain scale’ offers some insights into the influence of
particle and inter-particle contact characteristics on the behaviour
of soils. These breakthroughs are provided by the
carefully planned combination of micro-mechanical measurements,
conventional soil mechanics laboratory and in-situ
tests, and numerical analyses based on the discrete element
method (DEM) and on continuum mechanics. Novel methods
are proposed to characterise the complex geometry of soil
particles, by means of measuring their roughness, sphericity,
roundness, circularity and regularity, and to relate these to
their mechanical response such as their stiffness and strength.
We discover how these properties of the soil at the grain scale
affect the normal compression line and the angle of shearing
resistance at the meso-scale (Cavaretta et al. 2010). The grain
size, shape and surface area are also found to affect cementing
in soils such as gas hydrates, which in turn influences
their strength and stiffness (Clayton et al. 2010). Soil behaviour
is complicated by the fact that the characteristics of soil
grains can change, for example as particles are abraded and
crushed in shear bands near pile shafts, thus affecting the
capacity of the piles (Yang et al. 2010).
The DEM is generally considered to be the obvious tool to
reproduce soil behaviour at the grain scale. However, numerical
results from DEM simulations do not always match
experimental results. The characterisation of particle morphology
and inter-particle contacts will help inform DEM
modellers of new types of contact models that could be
developed to reduce these discrepancies. One aspect of the
research carried out using DEM is the validation of numerical
results against ‘real’ soil data. In this issue, we are shown
how the shapes of particles can affect the maximum and
minimum void ratios of computed samples that can then
display strain-induced anisotropy owing to particle orientation,
while their roughness affects the inter-particle shear
resistance (Abireddy & Clayton, 2010). In the same line of
thought, Yimsiri & Soga (2010) look at the effect of initial
fabric, modelled by the initial normal contact distributions,
on the simulated response of an assembly of grains. This
emphasises that using the DEM and interpreting computed
data requires an insider’s knowledge and subtle judgement.
Despite some limitations, we can agree that the DEM
provides a very useful tool to explore the underlying physical
micro-mechanisms behind phenomena such as creep
(Kwok & Bolton, 2010) or suffusion (Muir Wood et al.,
2010). Kwok & Bolton (2010) show that the rate process
theory, originally borrowed from chemistry (Eyring, 1936;
Kuhn & Mitchell, 1993), can be applied to a contact model
in DEM to simulate successfully the change in creep rate
with time. Muir Wood et al. (2010) use DEM simulations as
a vehicle to validate a continuum model that takes account
of the effects of initial grading and its evolution on the
critical state line of soils. Simulations using the DEM can
also be used to decipher complex soil behaviour such as that
of railway ballast, and we see in Lu & McDowell’s (2010)
paper how this requires innovation by way of modelling
grain properties, here as particle clumps with bonded small
balls as asperities, to allow for abrasion to be reproduced
while keeping the computational time reasonable.
The strength of the research presented here lies in the
continuous attempt to link the observations made at the
microscopic scale to the overall material response, whether
in experiments or in numerical analyses. The gap between
continuum constitutive models developed in research and
those used in practice is widening as more sophisticated
models are created to simulate complex phenomena of soil
behaviour such as cementing or creep, yet they often tend to
use large numbers of parameters that are not straightforward
to determine and/or lack any physical meaning. The effort
demonstrated in this issue, whether it is fundamental or
applied, must be maintained so as to refine our definition
and determination of meaningful parameters for numerical
models, and improve the confidence of the users.
Be´atrice Baudet (Editorial Chair)
and Malcolm Bolton (TC35 Chair)
Themed issue sub-committee chairman
Dr Be´atrice Baudet, University College London
Ge´otechnique advisory panel member
Professor Chris Clayton, University of Southampton
External members
Professor Malcolm Bolton, University of Cambridge
Dr Helen YP Cheng, University College London
Professor Matthew Coop, Imperial College London
Professor Pierre Delage, Ecole des Ponts Paris Tech (Universite
´ Paris Est)
Professor Curt Koenders, formerly University of Kingston
Professor Glenn McDowell, University of Nottingham
Dr Colin Thornton, University of Birmingham
REFERENCES
Abireddy, C. O. R. & Clayton, C. R. I. (2010). Varying initial void
ratios for DEM simulations. Ge´otechnique 60, No. 6, 497–502,
doi: 10.1680/geot.2010.60.6.497.
Cavaretta, I., Coop, M. R. & O’Sullivan, C. (2010). The influence
of particle characteristics on the behaviour of coarse grained
soils. Ge´otechnique 60, No. 6, 413–423, doi: 10.1680/geot.
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Clayton, C. R. I., Priest, J. A. & Rees, E. V. L. (2010). The effects
of hydrate cement on the stiffness of some sands. Ge´otechnique
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Eyring, H. (1936). Viscosity, plasticity and diffusion as examples of
absolute reaction rates. J. Chem. Phys. 4, No. 4, 283–291.
Kuhn, M. R. & Mitchell, J. K. (1993). New perspectives on soil
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Lu, M. & McDowell, G. R. (2010). Discrete element modelling of
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Yang, Z. X., Jardine, R. J., Zhu, B. T., Foray, P. & Tsuha, C. H. C.
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Yimsiri, S. & Soga, K. (2010). DEM analysis of fabric effects on
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