Water-Rock Interaction Mechanisms and Ageing Processes in Chalk - Advances in Data, Methods, Models and Their Applications in Geoscience - page 176

agreement with available data on northern French chalk (e.g. Bonvallet, 1979). Results from
the sample tested under constant suction equal to 4.2 MPa ( S r  97%, Fig. 10) show that
higher suctions strengthen the rock. This is likely to be associated with additional bonding
due to capillary effects. The observed behaviour during unconfined compression microtests
seems in good agreement with the general behavioural features observed for this chalk in
oedometric compression tests under controlled suction conditions (Nguyen et al., 2008).
Fig. 14. ESEM in situ unconfined compression tests on dry and water saturated chalk
Note that Nguyen et al. (2008) also found a ratio of 2.1 between the yield stress in
oedometric tests in dry and saturated conditions, close to the ratio UCS dry /UCS sat  2 found
during the ESEM micro-testing performed here. The ratio between the yield stress at a
suction level of 4.2 MPa and that at saturated and dry state was 1.5 and 0.7, respectively.
Similar ratios obtained by micromechanical testing using ESEM were equal to 1.5 and 0.75,
showing a notable agreement with the oedometric test results.
Finally, Fig. 15 shows some preliminary results of ESEM in situ testing with simultaneous
visualisation of the deformation pattern and the failure mode. The direction of compression
is vertical, as indicated on the ESEM image (A). At peak strength (image B), the sample
surface is still apparently unchanged. At about 0.9% axial strain, in the softening regime, a
pseudo-vertical fracture is visible (image C), followed by a progressive opening in the post-
peak phase (images D and E).
The aim of these preliminary tests was to explore the possibility of obtaining a
characterisation of the local strain field during hydro-mechanical loading using ESEM. Some