pore spaces between stable and dense particles or aggregates, are particularly sen-
sitive to pore water chemistry.
6.1.3. Hydraulic Conductivity
Bulk density and swelling pressure have a direct effect on hydraulic conductivity .
High density and low electrolyte content of the clay mineral give rise to a very low
conductivity for Na
+
-smectite. The conductivity of Ca
2+
-smectite is only slightly
higher because of the dense particle arrangement. However the difference between
the two becomes obvious at low densities. Indeed, for densities lower than
1600–1800 kg/m
3
Ca
2+
-smectite becomes very conductive because of the lack of
micro-structural continuity and coherence.
A general picture of the importance of the mineralogical composition of clays to
hydraulic conductivity is shown in Fig. 6.4, illustrating the dependence of hydraulic
conductivity on density at fluid saturation for different clay minerals.
Fig. 6.5 shows the approximate relationship between the smectite content and the
hydraulic conductivity for mechanically undisturbed clay with a high percentage of
clay-sized particles, low-electrolyte pore water and a bulk density at saturation of
about 2000 kg/m
3
. The conductivity is very low even when the smectite content drops
to a few percent.
If the hydraulic gradient is high, as in many laboratory oedometer tests, the
particles can also move and this affects the hydraulic conductivity. Thus, particles
and aggregates that are set free can be transported by flowing pore water to narrow
parts of the pore spaces and cause clogging (Hansbo, 1960).
6.1.4. Gas Penetrability
Permeation of gas through clay is a phenomenon of considerable practical impor-
tance for the clay sealing of underground facilities. It is equally important for the
understanding of gas prospection and exploitation in areas like the North Sea.
However, it is not yet fully understood.
In practice one can assume gas conductivity to be about a thousand times higher
than that of water. Once gas has made its way through buffer clay and further out
through even more permeable geological units, its rate of flow is more dependent on
the availability of pressurized gas than on the gas conductivity. Therefore, the gas
pressure that yields penetration through the buffer clay, i.e. the ‘‘critical gas pres-
sure’’, is the most impor tant factor. The solubility of the gas such as air, hydrogen or
organic gases is also important. The bubbles of temporarily stagnant gas contained
in pore water of the clay shrink and the dissolved gas diffuses out of the system.
According to current hypotheses micro-structural heterogeneity has a decisive
influence on the critical gas pressure. Gas makes its way along continuous channels
of neighbouring larger spaces, where it finds least resistance. Here capillary retention
is at minimum and the bonds between adjacent particles are overcome (Pusch, 1994;
6.1. Physico-chemical Behaviour of Clay Minerals 251