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assumption and a non-associated flow rule that permits the developments of
contractive or dilative volume changes at the scale of the representative elementary
volume. For the whole packing, a critical state behavior is assumed at large
deformations and the friction angle on each plane is related to the actual void ratio
compared to the critical void ratio at the same state of stress. A strain softening
behavior can, therefore, be obtained for dense materials. On the whole, the model
merely requires a limited number of parameters, which can easily be determined
from conventional triaxial testing.
The ability of the model to reproduce the main features of sand behavior has
been demonstrated. Model simulations were compared with drained triaxial test
results at different initial void ratios and different confining stresses, leading to
contractive or dilative behavior of the sand specimens. A good accuracy was
obtained between numerical simulations and experimental results by use of a unique
set of parameters. Simulations of undrained triaxial tests with the same set of
parameters demonstrated that the model was capable of reproducing the general
trend for both loose and dense sands.
The micro-structural model has been extended to clayed materials by considering
clay as a collection of clusters. The clusters interact with each other, mainly on
mechanical bases, and play the same role as the grains in a granular material. In
order to account for the compressible plastic behavior between clay clusters during
consolidation, a second yield surface was added. The numerical simulations of the
behavior of remolded and intact clays along drained and undrained triaxial loading
paths demonstrated that this extended version of the model was able to reproduce
the main features of clayey materials to a suitable level of satisfaction. Natural clays
often exhibit an intrinsic cohesion whose amplitude depends on their geological
history. In order to account for this cohesion, we added an adhesive force at the level
of the cluster contacts. The numerical results obtained in the simulation of natural
soft clay behavior showed that this approach could reproduce the stress−strain
relationship, as well as the effective stress paths during undrained triaxial tests.
An example of the introduction of an additional physical mechanism at the level
of the grain contact was presented for unsaturated granular materials by considering
capillary forces. The capillary forces were computed as a function of the pore
morphology and of the water content, using a simple empirical relationship. Several
examples of simulation demonstrated the ability of the model to reproduce the main
features of an unsaturated granular material's behavior. The numerical simulations
included resonant column tests at various degrees of saturation, constant water
content triaxial tests at a given degree of saturation and various net confining
pressures, and a wetting test at constant deviatoric stress and constant net mean
stress. For constant water content triaxial tests, the results showed an increase in the
mechanical properties whenever the degree of saturation decreased. The results also
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