A Microstructural Model for Soils and Granular Materials - Multiscale Geomechanics: From Soil to Engineering Projects - page 197

isotropic consolidation of up to 0.8 MPa, then unloading down to 0.4 MPa, 0.2 MPa,
and 0.1 MPa, respectively. The corresponding overconsolidation ratios (OCR) are 1,
2, 4, and 8, respectively. The results are presented in Figure 7.12.
The predicted void ratio changes in Figure 7.12b, showing contractive behavior
for OCR = 1 and 2, and shows a dilative behavior for OCR = 4 and 8. The
contractive and dilative behavior can also be seen in the predicted paths on e − log p
curves in Figure 7.12c. The corresponding stress−strain curves in Figure 7.12a are
hardening for OCR = 1 and 2 and softening for OCR = 4 and 8. The stress paths in
Figure 7.12d show stress states above the critical state line for OCR = 4 and 8 and
on the critical state line for OCR = 1 and 2. Figure 7.12 shows overall good
agreement between the experimental and predicted results for different
overconsolidation ratios.
7.4.2. Natural clays
The geological history of sedimentary depositions has an important impact on
mechanical properties, in particular the maximum stress state borne by the material
during its history, called the consolidation stress. We can roughly distinguish two
large families:
- the recent soils, which have been deposited since the last Ice Age that are
normally or slightly overconsolidated;
- the older soils, which are often strongly overconsolidated, at least in the
superficial layers.
Their mode of deposition as well as their evolution with time produces specific
characteristics that cannot be found in laboratory-prepared soils, such as the ones
examined in the previous section.
First of all, their structure or the geometry of their particle arrangement is usually
anisotropic due to their deposition in the gravity field. This creates mechanical
properties higher in the vertical rather than in the horizontal directions. This aspect
will not be studied here.
Another important characteristic is the existence of an intergranular “glue”,
mainly due to physicochemical evolutions at the level of particle contact. The
influence of this glue can be found in the mechanical properties of the equivalent
continuous medium. It can significantly increase the initial stiffness as well as its
maximum strength.