Environmental Engineering Reference
In-Depth Information
Figure 3.9.
Example of calculation of slope stability using the finite-elements method
(displacement vectors, using the Z_Soil program)
3.4. Case of non-saturated masses
3.4.1.
Problem
Although masses of natural soils often present non-saturated zones, this has not
often been taken into account in the analyses of stability because, other than the
simplification provided by a “saturated” approach, it has often been considered to be
sensible to ignore the contribution to strength offered by suction. Non-saturated
zones play a role not only in strength, however, but also in the transport of fluids and
therefore in pore-water pressures. This is especially important as soon as transient
phenomena are considered, whether instabilities appear near the surface or
infiltrations through the terrain surface are the driving force for instability. Finally, a
non-saturated zone acts as a cover of lesser permeability which, if pore-water
pressure increases at depth, can lead to instabilities of the same type as those
observed for clays covering loaded aquifers.
Figure 3.10 outlines the situation of a slope, including a non-saturated zone. The
thickness of this zone depends on the position of the piezometric surface and the
type of terrain. From negligible in gravel, it can reach a thickness of several meters
in clays.
Generally, as represented in Figure 3.10, desaturation is accompanied by the
following effects [GEI 99]:
- increase in rigidity;
- increase in strength;
- transition to a fragile behavior;
- modification of the volumetric behavior (contraction/dilation, collapse);
- induced effect of overconsolidation;
- reduction of permeability to water.
