Water-Rock Interaction Mechanisms and Ageing Processes in Chalk - Advances in Data, Methods, Models and Their Applications in Geoscience - page 171

Compressibility

State Yield Stress, MPa

Elastic Plastic

Dry (T1) 0.0022 0.1082 16.0

Dry (T2) 0.0055 0.0940 13.5

Suction Controlled (T3) 0.0095 0.1137 1.4

Saturated (T4) 0.0039 0.1350 7.5

Table 2. Compressibility and yield stress of Estreux chalk derived from oedometer tests.

The values of the mechanical parameters are given in Table 2. These illustrate the sensitivity
of the mechanical response of the Estreux chalk to changes in suction. They are in good
agreement with the water-weakening effects described by Matthews and Clayton (1993),
and with earlier observations on reservoir chalks (with water and oil as pore fluids) by De
Gennaro et al. (2004) and Priol (2005).
Influence of water sensitivity is also denoted by the swelling observed in test T1 (during
water injection under 441 kPa of applied vertical load) and by the collapse observed in test
T2 under water injection at 29.28 MPa of applied vertical stress. The increase in
compressibility and decrease in yield stress with increased degree of saturation (decreased
suction) are two other manifestations of the water-weakening effect.

0.65

0.6

WATER
INJECTION

0.55

WATER
INJECTION

0.5

SWELLING

0.45

COLLAPSE

0.4

T1 (dry)
T2 (dry)
T3 (s = 4.2 MPa)
T4 (saturated)

0.35

100 1000 10000 100000

VERTICAL STRESS,  v : kPa

Fig. 10. Compressibility curves obtained with oedometers