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magnifications starting from the saturated state ( h r = 100%). During the pressure changes,
images were captured every 2 minutes and later mounted as a video clip. The observed zone
was characterised by the presence of a rigid inclusion (crystal) embedded in the chalk
porous matrix (Fig. 13a). The analysed cycles included:
- Phase 1: saturation and stabilisation; sample was left 90 minutes at T = 2°C and p = 705
Pa, hence h r = 100% (Fig. 11). The reference image is captured after 90 minutes of
elapsed time.
- Phase 2: desaturation; pressure is decreased instantaneously down to 599 Pa ( h r = 85%,
path A-B-C in Fig. 11). Sample is left to stabilise for 60 minutes.
- Phase 3: Second saturation; the pressure inside the chamber is increased up to 705 Pa
(Fig. 11, path C-D) and sample is left to stabilise for 60 minutes at h r = 100 %.
During the first phase of saturation (Phase 1), the initial condition corresponding to full
water saturation was reproduced inside the samples (Fig. 13a). The successive drying
process (Phase 2) induced a fracture opening at the contact between the crystal and the chalk
matrix (indicated by an arrow in Fig. 13b). This fracture was not evident at the beginning of
the test (Fig. 13a), and its creation would seem to be associated with the changes in suction
induced by wetting and drying cycles, athough it is recognised that capillary effects could
also be a cause of this microstructural modification (swelling/shrinkage of the material).
In other words, wetting would have brought on fracture closing whereas drying would
cause chalk matrix shrinkage around the crystal, inducing fracture opening. Fracture
opening could then be the consequence of increasing capillary bridges (hence air/water
interfaces) inside the chalk matrix during drying. In contrast, wetting would decrease the
number of air/water menisci between the chalk matrix and the crystal, leading to a
progressive fracture sealing (Figs. 13c, 13d). If related to material ageing, the evolution of
this phenomenon with time following consecutive wetting and drying cycles could
contribute to microstructral features associated with material degradation. The observations
made here could also be supplemented and improved by advanced techniques of 2D and 3D
image analysis, allowing for a more quantitative characterisation of the morphological
modifications induced by changes in water saturation (see Sorgi & De Gennaro, 2007).
4.4 Micromechanical in situ testing
The combined use of the ESEM technique with unconfined compression tests was achieved
using a micromechanical testing apparatus.
A Deben MICROTEST® loading module allowed the application of a maximal compression
load of 5000 N at a constant strain rate of 1x10-5 s-1. A specific setup was developed to carry
out micromechanical tests under controlled total suction (controlling the level of relative
humidity during the tests). Cylindrical samples of about 8 mm in diameter and 15 mm in
height were used. Samples were obtained by means of high-precision coring. End-face
parallelism was ensured by means of a high-precision slicer having accuracy of the order of
1 µm. A series of preliminary micromechanical tests was conducted on saturated, partially
saturated, and dry samples in order to verify the agreement between the micromechanical
test results and the earlier laboratory test results performed on samples with larger
(standard) dimensions.
The preliminary results from the unconfined compression microtests are presented in Fig.
14. It can be seen from the test results for the dry samples that there was good
reproducibility. The linear slopes of the compression curves after a first tightening phase
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