世界知名岩土journal-Geotechnqiue于2011年5月，6月两期连续推出特辑，main theme: Soil mechanics at the grain scale,由UK的Be´atrice Baudet (Editorial Chair) and Malcolm Bolton (TC35 Chair) 担任主编。
两个特辑都是讲grain scale也就是细观力学的岩土力学，由ISMGE的TC35委员会的BOLTON教授组织的开的一次syposium，请的都是大牛人发表的文章，然后直接就提前发表在今年的5,6期了。编委会大概就是这么多牛人。
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
下面贴出部分论文的概要供大家参考。
May issue:
Editorial
Soil mechanics at the grain scale: issue 1
The physical processes governing the mechanics of soils
remain rather mysterious. Examples of obvious practical significance
include the effects of cyclic loading; the influence
of creep and rate effects; and the extent to which localisation
and instability should inform an assessment of peak strength.
In the past, the modelling of soil behaviour has largely been
phenomenological at the macro-scale and based on the data
of soil element tests. A prominent example is the framework
of critical state soil mechanics (CSSM) and the cam clay
model of elasto-plastic soil behaviour (Roscoe et al., 1958;
Schofield & Wroth, 1968). The outstanding strength of CSSM
lay in its ability to combine the recoverable and irrecoverable
volumetric and deviatoric stress–strain data of clays in triaxial
tests within a mathematical model that used only four parameters.
This was also widely perceived as its main weakness.
Engineers who conducted different classes of test, or who
tested different classes of soil, drew attention to the importance
of phenomena that lay outside the CSSM framework:
undrained cyclic loading could lead to the liquefaction of
sands; creep in conjunction with transient flow led to errors
in predicting the consolidation settlement of clays; the compressibility
of sands was evidently non-linear on a linearlogarithmic
plot; and the dramatic drop to residual sliding
friction that accompanied the formation of polished slip
surfaces in clays led to controversy over the selection of safe
angles for slopes.
The reactions of soil modellers to such observations ranged
from the generation of much more elaborate constitutive
models with 20 or more parameters, to the development of a
multitude of phenomenological models that dealt separately
with one class of specialised data. Researchers taking the
elaborate route often achieved a pleasing representation of a
wide range of behaviour simultaneously, but at the expense of
raising immense obstacles to communication and practical
application through the lack of obvious physical meaning in
their many parameters. How might those many parameters
change if the representative soil sample changed its composition?
Those taking the specialised route could generally
match the particular data they focussed on, but the decision--
maker could hardly know whether that particular facet of
behaviour would be pre-eminently important in the application
of interest. In a sense, both approaches to phenomenological
modelling amount to curve-fitting, and both raise the
question ‘Am I looking at the right curve?’.
One way of raising confidence is to take the route adopted
in materials science and to enhance stress–strain data through
micro-structural images and micromechanical modelling.
Although Rowe (1962) based his stress-dilatancy theory on
granular propositions rather than actual micro-measurements,
many readers found the theory easier to visualise than the
rival CSSM models. It took the advent of discrete element
modelling (Cundall & Strack, 1979) to facilitate extended
granular simulations, including those which linked stressdilatancy
and CSSM in the context of grains which could
both rearrange and crush (Cheng et al., 2004). The association
by Skempton (1964) of the residual friction of clays with
the reorientation of platelets parallel to a surface of continuing
sliding, was originally an appeal based on the photography
of ‘polished’ slip surfaces. Later, it became possible to
confirm that impression by scanning electron microscope
(SEM) imaging (Mu¨ller & Schlu¨chter, 2001). Such images fix
themselves very effectively in the minds of engineers. It is
not surprising, therefore, that discrete-element method
(DEM) simulations and microscope images appropriate to a
variety of soils and processes form the backbone of papers
published in these themed issues.
The challenge is to clarify the processes that have given
rise to common soil types in terms of their current microstructure,
and especially to track changes in that microstructure
during the gamut of natural or man-made activities that
require geotechnical data for their design and control. Successful
research along these lines should lead to justifications
for the selection of test methods, and to the definition and
clarification of macroscopic and microscopic parameters and
mechanisms best suited to the prediction of small-strain
stiffness, kinematic yielding, plastic hardening, critical states,
rate and time effects, localisation, fracture and flow. This in
turn should lead to the availability of correlations between
these fundamental modelling parameters and elementary soil
classification and index tests, offering decision-makers a
priori estimates of future performance before more expensive
element-testing has been commissioned. Designers would
then be able to focus appropriately on potential threats –
whether of seismic liquefaction, internal erosion, creep, anisotropy
or progressive failure – in the knowledge that they
were not wasting their clients’ ground investigation money.
Initial impetus in this direction was gained through international
symposia organised at Yamaguchi University by Professor
Masayuki Hyodo. The potential value of this new
initiative was recognised by the International Society for Soil
Mechanics and Geotechnical Engineering when it acceded, at
the turn of the new millennium, to Professor Malcolm
Bolton’s proposal to set up a technical committee, TC 35,
which has the mission ‘Geomechanics from micro to macro’
with the acronym GM3. Subsequently, working groups began
to meet both in Japan and in the UK. Seven annual meetings
have taken place of the UK GM3 travelling workshop, and
there have been an increasing number of international workshops
and conferences dedicated to this theme.
This burgeoning of international research activity was reflected
in the 100 abstracts submitted initially to the call for
papers on ‘Soil mechanics at the grain scale’ by Ge´otechnique.
When the Ge´otechnique advisory panel agreed to publish
themed issues, it was decided that these should be substantially
different from symposia in print. Unlike the latter, which tend
to gather state-of-the-art papers, these occasional issues were
intended to ‘inform readers of new and emerging developments’
(Atkinson, 2008). The subjects covered in the submitted
abstracts ranged from experimental to numerical
research, and originated from institutions all over the world.
The final selection of 18 papers reflects this variety, and
hopefully makes an exciting read. They report studies of the
influence of soil fabric, grain shape and roughness, interparticle
creep, localisation and many other potentially significant
micro-mechanical aspects of soil behaviour. Furthermore,
through an agreement with Professor Mingjing Jiang of Tongji
University who is organising the next TC35-sponsored conference,
IS-Shanghai 2010, some of these papers will also be
presented for oral discussion on that occasion, as well as being
open to written discussion in Ge´otechnique in the usual way.
This first issue offers information about mechanisms occurring
at the grain scale during phenomena observed at the
meso-scale that are generally poorly understood and neglected
by constitutive modellers, such as the non-uniformity of
stresses and strains in single-element tests. The second issue
will report work on those characteristics of the grains that
may be responsible for soil behaviour, such as particle size
and shape, particle breakage and inter-particle friction.
In this issue, significant advances are reported in sensing
and imaging technologies, and in computer performance, combined
with techniques borrowed from other areas of science
such as medicine or chemistry. This technology permits better
spatial resolution (a few microns) in tracing the movements of
individual grains during a test. Hall et al. (2010) and Hasan &
Alshibli (2010) used X-ray micro-computer tomography with
synchrotron sources to characterise the three-dimensional kinematics
of individual sand grains within shear bands. They
were able to detect grain translation and rotation as well as
particle contacts and orientation during triaxial testing of sand
specimens. Force chains developing within specimens emerge
as a significant factor accounting for meso-scale phenomena
such as the evolution of the mobilised strength. For example,
the evolution of critical states can be linked to micro-mechanisms
such as the buckling and collapse of the force chains
(Hasan & Alshibli, 2010; Rechenmacher et al., 2010; Thornton
& Zhang, 2010). It is interesting to see that such observations
are made during experiments as well as in numerical simulations
using the DEM.
The enhanced performance of computers is increasingly
allowing researchers to match experimental research with
DEM simulations: Li & Yu (2010) applied DEM to study the
effects of stress rotation and the non co-axiality of stresses
and strains, while Wang & Gutierrez (2010) looked at the
effects of scale and geometry on shear bands in direct shear
tests. The advantage of these simulations lies chiefly in the
information which can be obtained on the evolution of
contact force distributions, and also the possibility to repeat
tests in exactly similar conditions, bypassing the uncertainties
associated with sample variation (Thornton & Zhang, 2010).
But they have generally relied on simple particle shapes
(spherical in most cases) and simplified inter-particle reactions
(linear elasticity or Hertz–Mindlin) that may or may not
be sufficient to represent soils. Nevertheless, results from
these DEM simulations may lead us to question our interpretation
of single-element tests (to what extent is the measured
parameter an outcome of the testing method?) and our
use of continuum constitutive models.
So far, these advances in experimental and numerical research
have benefited simple granular materials such as sands.
Clays are different by nature, the definition of a particle itself
not being clear. More traditional techniques such as scanning
electron microscopy and mercury intrusion porosimetry remain
significant sources of information when trying to understand
compressibility, or in identifying changes in particle
orientation during testing (Delage, 2010; Hattab & Fleureau,
2010), but there are still advances to be made to understand
phenomena such as sensitivity and structure degradation, for
example. The papers in this issue are therefore opening up
new avenues of research rather than coming to immediately
applicable conclusions.
Of course, a sceptic might question the use of knowing
what is happening to soil grains. Our reply must be that this
research will ultimately validate or improve our approach to
soil characterisation as well as the numerical models, discrete
or continuum, which lie at the heart of geotechnical engineering.
This is evident in our second issue, published next
month, in which the papers deal with what could be qualified
as more fundamental aspects of soil micro-mechanics, such
as particle morphology, inter-particle contacts, inter-particle
friction and particle breakage, but still within the context of
their wider application to large-scale problems.
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
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Editorial
Soil mechanics at the grain scale: issue 2
