Water-Rock Interaction Mechanisms and Ageing Processes in Chalk - Advances in Data, Methods, Models and Their Applications in Geoscience - page 169

It is well known that any change in total suction induces a change in the degree of water
saturation, S rw , which can be quantified via the water retention curve (WRC) of the material.
The WRC of Estreux chalk is presented in Fig. 8 (De Gennaro et al., 2006). As it can be
observed, significant changes in S rw occur when suction varies between 1 and 2 MPa,
causing near-total desaturation.

100

Hr = 83.5% ( s = 24.9 MPa)

10 Hr = 97% ( s = 4.2 MPa)

Hr = 98.2% ( s = 2.5 MPa)

1

Hr = 99.8% ( s = 1.5 MPa)

0.1

Dry path
Wetting path
Initial state

0.01

0.001

0 0.2 0.4 0.6 0.8 1

DEGREE OF SATURATION, S rw

Fig. 8. Water retention curve of Estreux chalk

The slight differences observed between the drying and wetting paths denote a moderate
hysteresis effect, also observed in other chalks (Priol, 2005). A possible effect of the
glauconite fraction in reducing the hysteresis effect is suspected, although a clear
explanation of the slight hysteresis is not straightforward. The drying curve shows that the
air entry value of Estreux chalk can be estimated at approximately 1.5 MPa. Following the
drying path, the degree of saturation exhibits a dramatic reduction, with a value as low as
30% at 2.5 MPa. At the highest suction ( s = 24.9 MPa, h r = 83.5%) the degree of saturation is
as low as 2% to 5%, showing that the chalk is nearly completely desaturated. Based on the
water retention curve, the suction of a dry sample can be estimated at 30 MPa. The shape of
the water-retention curve of Estreux chalk and the sudden decrease in saturation above 1.5
MPa show that changing values of the ambient relative humidity in the mine (between 80%
and 100%) can definitely lead to significantly unsaturated states, at least at the surface of the
pillar, directly in contact with the ambient relative humidity. As a consequence, the
mechanical properties of the chalk in unsaturated states have to be considered when
addressing the long-term stability of the pillars. As a first step, the compressibility
properties of the chalk under various controlled suctions are investigated.

3.2 Oedometer tests
Oedometer tests involve uniaxial compression of samples that are prevented from
expanding laterally. The two independent stress variables commonly used in the