Water-Rock Interaction Mechanisms and Ageing Processes in Chalk - Advances in Data, Methods, Models and Their Applications in Geoscience - page 167

Fig. 5. Changes over time in lateral extension at two points within the pillar
Fig. 6. Convergence over time between the mine wall and roof
In a mine where the water-level shows a generally decreasing trend, the influence of
ambient hygrometry can have a major impact on the evolution of the pillars. This has been
observed in some gypsum mines (Auvray et al., 2004), where scanning electron microscopy
analysis of rock sections from different distances from the surface of the pillar has shown
extensive traces of dissolution at the surface of the pillar, reducing towards the centre. This
dissolution can induce rock matrix degradation that can lead to progressive spalling (flake-
off) of the pillar (Fig. 7). As a direct consequence, the section of the pillar reduces over time,
as does the pillar's ability to sustain the stability of the rock mass.
The instrumentation of the underground mine in Estreux shows significant seasonal
changes in the water level and ambient hygrometry. These changes induce variations in
pore pressure, leading to alternating cycles of saturation/desaturation that can contribute
considerably to the degradation of the pillars over time.
To study the consequences of this type of phenomenon on the rock matrix in detail,
laboratory tests were carried out reproducing the environmental conditions in the mine.