Geoscience Reference
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element in bentonite. It cracks in dry periods, can absorb much water, and swells
significantly in wet periods; this particularly counts for natrium-montmorillonite.
(a) kaolinite (b) illite (c) montmorillonite
Figure 2.5 Clay structure (electron-microscope)
Classification
A common system uses two letters to classify a soil sample on a first visual
interpretation (borehole log), the first relates to the soil type and the second to the
constitution, see Table 2.5. Sometimes more than one letter are used for the soil
constitution.
TABLE 2.5
CLASSIFICATION SYSTEM
soil type
soil constitution
gravel
G
well graded
W
sand
S
poorly graded
P
silt
M
silty
M
clay
C
clayey
C
organic
O
low plasticity
L
peat
Pt
high plasticity
H
Permeability
In sandy soils the permeability may vary depending on the particle size, the
content of fines and the grain-size distribution (see Fig 2.1 and 2.4). For clays the
permeability is related to the free water space (non-hydrated) and it is usually
small. The range of permeability for various soil types and tests is shown in Table
2.6.
The variation in permeability can be large due to natural heterogeneity and
stratification in the field. Air intrusion (unsaturated soil), gas bubbles (methane) or
fine fraction have a large impact as well. A distinction is made between hydraulic
permeability
k
[m/s] used in geotechnics and geohydrology, and intrinsic
permeability
[m
2
] used for multiple pore fluids, and their relation is
k
=
w
/
.
Here,
is the dynamic pore-water viscosity. Many empirical expressions exist for
the permeability, such as
n
)
2
, where
T
is the so-called
tortuosity
10
, expressing the pore-space geometrical complexity. Here, the diameter
= 0.005
D
15
2
Tn
3
/(1
10
Tortuosity is a property of a curve being twisted, having many turns. In porous media, its
value can be related to the square of the ratio of curve ends and curve length distance and it
varies between 0.3 and 0.8 (Bear, 1972).
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