Numerical Modeling: An Efficient Tool for Analyzing the Behavior of Constructions - Multiscale Geomechanics: From Soil to Engineering Projects - page 265

270 Multiscale Geomechanics

Figure 9.5. Comparison of measured and computed apparent cohesion of a clay
as a function of: (a) negative pore pressure ( −u w ); and (b) capillary pressure [TAI 94]

of using this notion [COU 02b, KHA 98]. Our experience shows that the choice of a
generalized effective stress tensor, associated with a constitutive law taking into account
the capillary effect, leads to acceptable results.

The choice of formulation in generalized effective stresses provides a natural
transition between the dry, unsaturated and saturated states. Moreover, the constitutive
model is obtained from extending the Hujeux model, already widely used and validated
for saturated or dry soils. This also allows us to benefit from the experience already
gainedfrommodelparametersandtheiridentification.Therefore,onlyafewparameters
dealing with the influence of non-saturation on behavior remain to be characterized.

This model is essentially governed by the influence of capillary pressure on the void
ratio. Thus, changes in the capillary pressure will change the consolidation (critical)
pressure. This is explicitly taken into account in the isotropic mechanism.

In terms of plasticity, this is equivalent to the expansion of the elastic domain
considered by introducing a kind of isotropic capillary cohesion resulting from the
increase of overconsolidation pressure owing to capillary forces. The isotropic yield
function becomes:

f iso = p − d · r iso · p n c (ε v ,π c ) [9.67]