Geoscience Reference
In-Depth Information
Fig. 7. Progressive spalling of a pillar in a gypsum mine. From left to right, photos from
1996, 2000, and 2004 (Sorgi & Watelet, 2007)
3. Laboratory scale: Core testing
Estreux chalk is a glauconite-rich chalk. Glauconite is an alumino-silicate of iron, potassium,
and sodium. Its mineral composition is close to that of illite, although glauconite is not
hydrated, with the additional presence of sodium and strong isomorphism by substitution
of aluminium atoms with Fe
2+
and Fe
3+
iron atoms. Glauconite is often present in chalk
deposits in northern France (Masson, 1973).
The porosity of Estreux chalk is about 37%, its specific gravity is
G
s
= 2.74, and the average
water content is equal to 20.7% when the rock is water-saturated. At the microstructural
level, the solid matrix is made up of micrometric grains that are principally fragments of
coccolithes. Sometimes intact coccolithes also occur. The chalk is then principally made up
of calcite (calcium carbonate, CaCO
3
), which often also constitutes the cementing agent at
the intergranular contacts. Microfossils and mineral impurities are also frequently observed.
3.1 Retention properties of Estreux chalk
The Estreux chalk samples were completely saturated when extracted; the mine temperature
was 11°C and the relative humidity,
h
r
, was 100% (with 2% accuracy of the hygrometry
resistive sensors). Based on Kelvin's law, the change in relative humidity modifies the total
air/water suction,
s
, the difference between the water vapour pressure (assumed equal to
the atmospheric pressure,
p
a
), and the water pressure,
p
w
, according to the following
relation:
p
w
v
spp
T
Mp
ln
(1)
t
a
w
v
vs
where
w
is the water density,
M
v
the molar mass of the water vapour,
R
the universal
constant of an ideal gas (8.314 Jmol-1K-1),
T
the absolute temperature,
p
v
the vapour
pressure and
p
vs
the pressure of the saturating vapour at temperature
T
(
h
r
=
p
v
/p
vs
).
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