Geoscience Reference
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a) the level of shear plastic deformation is an indicator of the type of behavior
which has a major influence on cyclic properties and, therefore, seismic analyses. It
varies greatly from place to place,
b) experimental results show that when the cyclic shear strain is less than 10 −5
approximately ( 10 −3 % ), the response of the soil is almost reversible and the non-linear
elasticity related to the effects of confinement describes the behavior fairly well,
c) between 10 −5 and 10 −3 , irreversible strains appear, but the cycles are
quite stable. Deformation occurs with no significant volume change under drained
conditions (pore pressure changes slightly when undrained). Here, an approach
based on the equivalent linear method with hysteretic damping provides good results
(computational softwares such as [SEE 72], Flush [LYS 75], Quad-4 [IDR 73] or
CyberQuake [CYB 98]),
d) for cyclic deformations larger than 10 −2 , only a cyclic elastoplastic
constitutive law can provide well-grounded results, even from a qualitative point of
view.
9) The intermediate principal stress plays a role in the behavior of soils and the
yield surface is not regular [LAD 77].
10) In the case of unsaturated soils, suction plays an important role in the mechanical
behavior. The analysis of the behavior of unsaturated soils under various stress paths
(suction controlled or total stress controlled) leads to the following findings:
a) in drainage paths under constant total stress, for both the pastes fabricated
by mixing clay with water and the normally consolidated soils, the material becomes
rigid when the suction reaches a certain value (Figure 9.2),
b) this rigidity is very similar to that observed during a wetting or an unloading,
c) this stiffening of unsaturated soil is also present during the suction controlled
consolidation paths,
d) during suction imposed triaxial tests, the final points show an apparent
cohesion . This apparent cohesion varies non-linearly with suction and reaches an
asymptote at high suctions.
11) The role played by time depends on the soil type and the target application.
We can consider that in the field of earthquake engineering, the frequency factor has
a rather low impact, although it can modify strain rates. Obviously, the time factor
in the generation of pore pressures and its dissipation (consolidation) is taken into
account by the use of a coupled calculation. However, some clays or rocks exhibit
behavior that is dependent on the speed of loading. Viscous behavior has been observed
even for certain sands. The effect of viscosity of the soil may lead to changes in the
stability of underground construction works over time or with modification of cyclical
behavior. If this is the case, the model must take into account the strain rate parameter.
Tatsuoka et al. [TAT 08] describe four types of viscosity observed in geomaterials and
recommend that a mathematical model simulating these different behaviors for triaxial
drained tests be used.
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