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possible developments, like Digital Image Correlation (DIC) (e.g. Vales et al., 2008), might
also be considered in order to obtain a quantitative characterisation of the local deformation
at microstructural (few hundreds of µm) and mesostructural (some mm) levels.
Fig. 15. ESEM failure pattern during ESEM in situ unconfined compression test on water
saturated chalk
5. Conclusion
A preliminary investigation of the behaviour of chalk samples retrieved from the pillars of
the abandoned Estreux mine (northern France), conducted as part of an assessment of the
stability of underground chalk mines, has been presented. Due to environmental changes
occurring within the mine (such as hygrometry and water table), the pillars are regularly
subjected to variations in the degree of water saturation. The potential impact of the
evolution of the water saturation on the mechanical behaviour of the chalk has been
assessed based on the methods and concepts of the mechanics of unsaturated soils at
different scales: site scale, laboratory scale, and microstructural scale.
At site scale, observations conducted during a four-year period (from February 2004 to
February 2008) have highlighted that relative humidity in the mine can vary between 85%
and 100%. The seasonal changes in the water table have been of reduced extent during the
monitoring period, showing a general tendency towards decreasing water table level.
However, as the water table rose, both pillar expansion and roof convergence
were observed. Pressure and temperature measurements inside the chalk pillars have
shown that both parameters vary on a seasonal basis, causing alternating states of
saturation/desaturation. As expected, both pressure and temperature are not constant
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