Geoscience Reference
In-Depth Information
fracture lines because the platy particles orient themselves parallel to
the slippage planes. But at depth, if the soil remains moist, there is no
swelling-shrinkage and slickensides do not appear. On the other hand,
at the surface the soil is often dry and broken up into small aggregates.
Thus it is at medium depth that the slippage faces are clear, for example
in the island of Malekula in the Indian Ocean (Podwojewski 1994).
Slickensides may also be creased into small folds of anticlinal type
(Driese
et al.
2000).
Upheaval of
the soil
Fracture and
shear plane
Oblique
resultant
Soil
Pressure
Pressure
Soil
Lateral displacement
Pressure (downward
expansion impossible)
Fig. 6.8
Forces, movements and morphological changes in Vertisols during a swelling
phase, in the wet season (Wilding and Tessier 1988).
Gilgai relief
is another manifestation of the same phenomenon. The soil
expands upward but in irregular manner if we consider the horizontal
plane to be the land surface.
For measuring the vertical movement in the field, the apparatus
shown in Figure 6.9 is used (Voltz and Cabidoche 1995; Cabidoche and
Volt z 1995) .
Benchmark
on stable
substratum
(calcareous
rock)
Viewing
device
Benchmark
on stable
substratum
Natural soil
Metal rods initially anchored in (screwed into) the soil to jut
out to the same height
Fig. 6.9
Equipment for fi eld measurement of vertical swelling of Vertisols.
