175 research outputs found
Dynamic Triaxial and Vibratory In-Situ Behavior of Cohesive Soil
In-situ and laboratory shear modulus data are presented and compared. In-situ tests included the cross-hole seismic survey at a stiff marl site, while laboratory tests included the cyclic triaxial test on undisturbed specimens obtained from the same site. The cyclic triaxial device presented in this investigation has been developed and improved so that the reliable response of soil can be measured directly from the specimen over a large range of strain (from 10-6 to 10-2 ). A series of cyclic triaxial tests were performed under stress controlled condition over a range of frequency from 0,5 to 10 Hz on marl samples consolidated anisotropically. Values of shear modulus and damping ration determined for shearing strain amplitude between 10-6 and 10-2 and compared with published results proposed by Seed and Idriss (1970) and Hardin and Drnevich (1972). At low strains, the shear modulus values measured by in-situ and laboratory methods were in a good agreement, but the values from the Hardin-Black\u27s equation were underestimated. The influence of consolidation stress, frequency, and number of load cycles on the shear modulus have also been investigated
Quasistatic rheology and the origins of strain
Features of rheological laws applied to solid-like granular materials are
recalled and confronted to microscopic approaches via discrete numerical
simulations. We give examples of model systems with very similar equilibrium
stress transport properties -- the much-studied force chains and force
distribution -- but qualitatively different strain responses to stress
increments. Results on the stability of elastoplastic contact networks lead to
the definition of two different rheological regimes, according to whether a
macroscopic fragility property (propensity to rearrange under arbitrary small
stress increments in the thermodynamic limit) applies. Possible consequences
are discussed.Comment: Published in special issue of "Comptes-Rendus Physique" on granular
material
On the capillary stress tensor in wet granular materials
This paper presents a micromechanical study of unsaturated granular media in
the pendular regime, based upon numerical experiments using the discrete
element method, compared to a microstructural elastoplastic model. Water
effects are taken into account by adding capillary menisci at contacts and
their consequences in terms of force and water volume are studied. Simulations
of triaxial compression tests are used to investigate both macro and
micro-effects of a partial saturation. The results provided by the two methods
appear to be in good agreement, reproducing the major trends of a partially
saturated granular assembly, such as the increase in the shear strength and the
hardening with suction. Moreover, a capillary stress tensor is exhibited from
capillary forces by using homogenisation techniques. Both macroscopic and
microscopic considerations emphasize an induced anisotropy of the capillary
stress tensor in relation with the pore fluid distribution inside the material.
In so far as the tensorial nature of this fluid fabric implies shear effects on
the solid phase associated with suction, a comparison has been made with the
standard equivalent pore pressure assumption. It is shown that water effects
induce microstrural phenomena that cannot be considered at the macro level,
particularly when dealing with material history. Thus, the study points out
that unsaturated soil stress definitions should include, besides the
macroscopic stresses such as the total stress, the microscopic interparticle
stresses such as the ones resulting from capillary forces, in order to
interpret more precisely the implications of the pore fluid on the mechanical
behaviour of granular materials.Comment: 39 page
3D failure of a scale-down dry stone retaining wall: a DEM modelling
International audienceDry stone retaining walls are vernacular structures that can be found in many places around the world and were mainly built to reduce slope erosion and to allow agricultural practices. Their stability is essentially warranted by the global wall weight and the capacity of individual blocks to develop friction at contact. The arrangement of these hand-placed blocks contributes also to the stability of the wall. A new interest arose in these structures in the last years, first due to the necessity to repair damages inherent to any built heritage, but also to their possible advantages regarding sustainability. Several studies have tried to address the behavior of slope dry stone retaining walls, whereas few conclusive studies have been performed concerning road dry stone retaining walls. In this latter case, the loading implies, apart from the backfill, the existence of a concentrated force on the backfill surface. The failure of such masonry work is accompanied by true three-dimensional deformations. This study is a first attempt to provide a better understanding of the mechanical behavior of road dry stone retaining walls. It involves a small-scale prototype with clay bricks for the wall, and steel blocks, acting as a concentrated loading on the backfill surface at a given distance from the inward wall face. Steel blocks have been superposed until wall failure. A numerical study based on these experiments is then performed by means of a mixed discrete-continuum approach. The numerical model was able to retrieve the average value of the concentrated force triggering failure found in the experiences, except when the concentrated loading is very close to the wall. Nevertheless, the results provided by this study are considered as encouraging even if further work is required to definitely state about the validity of such a numerical technique for the study of actual road dry stone retaining walls
Internal states of model isotropic granular packings. III. Elastic properties
In this third and final paper of a series, elastic properties of numerically
simulated isotropic packings of spherical beads assembled by different
procedures and subjected to a varying confining pressure P are investigated. In
addition P, which determines the stiffness of contacts by Hertz's law, elastic
moduli are chiefly sensitive to the coordination number, the possible values of
which are not necessarily correlated with the density. Comparisons of numerical
and experimental results for glass beads in the 10kPa-10MPa range reveal
similar differences between dry samples compacted by vibrations and lubricated
packings. The greater stiffness of the latter, in spite of their lower density,
can hence be attributed to a larger coordination number. Voigt and Reuss bounds
bracket bulk modulus B accurately, but simple estimation schemes fail for shear
modulus G, especially in poorly coordinated configurations under low P.
Tenuous, fragile networks respond differently to changes in load direction, as
compared to load intensity. The shear modulus, in poorly coordinated packings,
tends to vary proportionally to the degree of force indeterminacy per unit
volume. The elastic range extends to small strain intervals, in agreement with
experimental observations. The origins of nonelastic response are discussed. We
conclude that elastic moduli provide access to mechanically important
information about coordination numbers, which escape direct measurement
techniques, and indicate further perspectives.Comment: Published in Physical Review E 25 page
Internal states of model isotropic granular packings. I. Assembling process, geometry and contact networks
This is the first paper of a series of three, reporting on numerical
simulation studies of geometric and mechanical properties of static assemblies
of spherical beads under an isotropic pressure. Frictionless systems assemble
in the unique random close packing (RCP) state in the low pressure limit if the
compression process is fast enough, slower processes inducing traces of
crystallization, and exhibit specific properties directly related to
isostaticity of the force-carrying structure. The different structures of
frictional packings assembled by various methods cannot be classified by the
sole density. While lubricated systems approach RCP densities and coordination
number z^*~=6 on the backbone in the rigid limit, an idealized "vibration"
procedure results in equally dense configurations with z^*~=4.5. Near neighbor
correlations on various scales are computed and compared to available
laboratory data, although z^* values remain experimentally inaccessible. Low
coordination packings have many rattlers (more than 10% of the grains carry no
force), which should be accounted for on studying position correlations, and a
small proportion of harmless "floppy modes" associated with divalent grains.
Frictional packings, however slowly assembled under low pressure, retain a
finite level of force indeterminacy, except in the limit of infinite friction.Comment: 29 pages. Published in Physical Review
Rupture by damage accumulation in rocks
The deformation of rocks is associated with microcracks nucleation and
propagation, i.e. damage. The accumulation of damage and its spatial
localization lead to the creation of a macroscale discontinuity, so-called
"fault" in geological terms, and to the failure of the material, i.e. a
dramatic decrease of the mechanical properties as strength and modulus. The
damage process can be studied both statically by direct observation of thin
sections and dynamically by recording acoustic waves emitted by crack
propagation (acoustic emission). Here we first review such observations
concerning geological objects over scales ranging from the laboratory sample
scale (dm) to seismically active faults (km), including cliffs and rock masses
(Dm, hm). These observations reveal complex patterns in both space (fractal
properties of damage structures as roughness and gouge), time (clustering,
particular trends when the failure approaches) and energy domains (power-law
distributions of energy release bursts). We use a numerical model based on
progressive damage within an elastic interaction framework which allows us to
simulate these observations. This study shows that the failure in rocks can be
the result of damage accumulation
Computer simulation of model cohesive powders: Plastic consolidation, structural changes and elasticity under isotropic loads
The quasistatic behavior of a simple 2D model of a cohesive powder under
isotropic loads is investigated by Discrete Element simulations. The loose
packing states, as studied in a previous paper, undergo important structural
changes under growing confining pressure P, while solid fraction \Phi
irreversibly increases by large amounts. The system state goes through three
stages, with different forms of the plastic consolidation curve \Phi(P*), under
growing reduced pressure P* = Pa/F0, defined with adhesion force F0 and grain
diameter a. In the low-confinement regime (I), plastic compaction is
negligible, and the structure is influenced by the assembling process. The
following stage (regime II) is independent of initial conditions. The void
ratio varies linearly with log P, as described in the engineering literature.
In the last stage of compaction (III), a maximum solid fraction is approached,
and properties of cohesionless granular packs are retrieved. Under
consolidation, while the fractal range of density correlations decreases, force
patterns reorganize, and elastic moduli increase by a large factor. Plastic
deformation events correspond to very small changes in the network topology,
while the denser regions tend to move like rigid bodies. Elastic properties are
dominated by the bending of thin junctions in loose systems. For growing RR
those tend to reduce to particle chains, the folding of which, rather than
tensile ruptures, controls plastic compaction
Earthquakes: from chemical alteration to mechanical rupture
In the standard rebound theory of earthquakes, elastic deformation energy is
progressively stored in the crust until a threshold is reached at which it is
suddenly released in an earthquake. We review three important paradoxes, the
strain paradox, the stress paradox and the heat flow paradox, that are
difficult to account for in this picture, either individually or when taken
together. Resolutions of these paradoxes usually call for additional
assumptions on the nature of the rupture process (such as novel modes of
deformations and ruptures) prior to and/or during an earthquake, on the nature
of the fault and on the effect of trapped fluids within the crust at
seismogenic depths. We review the evidence for the essential importance of
water and its interaction with the modes of deformations. Water is usually seen
to have mainly the mechanical effect of decreasing the normal lithostatic
stress in the fault core on one hand and to weaken rock materials via
hydrolytic weakening and stress corrosion on the other hand. We also review the
evidences that water plays a major role in the alteration of minerals subjected
to finite strains into other structures in out-of-equilibrium conditions. This
suggests novel exciting routes to understand what is an earthquake, that
requires to develop a truly multidisciplinary approach involving mineral
chemistry, geology, rupture mechanics and statistical physics.Comment: 44 pages, 1 figures, submitted to Physics Report
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