Environmental Engineering Reference
In-Depth Information
The sample at 2.2 m depth has the smallest clay fraction, smallest plasticity
index, lowest density and degree of saturation and highest suction. This sample is
probably the more collapsible.
Figure 6.7.
SEM observation of the 2.2 m soil, overall view
Scanning electron microscope (SEM) photos taken on freeze-dried [TOV 73]
and freeze-fractured samples taken at 2.2 m are shown in Figures 6.7 to 6.9.
Samples were ultra-rapidly frozen by immerging small pieces of loess (2 mm wide
and 5 mm long) in nitrogen previously cooled to its freezing point by applying a
vacuum. No nitrogen boiling is observed in such conditions, allowing for a very fast
freezing with no ice expansion and satisfactory microstructure preservation. Freeze-
fracturing is an interesting technique because the particles are strongly held together
when the sample is fractured. Hence, the fracture is determined by ice failure and it
crosses through the different levels of structures without being influenced by the
structure itself.
The overall view of the structure presented in Figure 6.7 shows the significant
but irregular presence of agglomerated clay particles within a skeletal assemblage of
silt grains (around 20-30 µm in diameter). In areas where they are present,
aggregated clay platelets fill the inter-grain pores. In other areas, the silt grains
appear clean with no clay coatings around them. Obviously, clay acts as a link in the
areas where it is present. The clean grain zones with larger pores (around 20 µm in
diameter) probably correspond to weaker zones that present a local volume decrease
larger than in clay-rich zones. Collapse is therefore probably localized in such zones
where pores as large as 10 µm can be observed. Similar local heterogeneity under
macroscopic volume changes has also been observed in sensitive clays where the
compressibility is such that the term collapse is also used to describe the sudden
decrease in volume observed once the yield stress is passed during one-dimensional
