In-situ and Laboratory Tests to Evaluate the Impact of Water Table Fluctuations on Stability of Underground Chalk Mines

Shallow underground mines can be flooded if there is a rise in the water table, which reduces pillar strength and increases risk of collapse and surface subsidence. To prevent this, INERIS is studying rock behavior in two underground chalk mines in northern France. In the first mine, the water content of a flooded pillar is almost constant (∼20%) in its internal zone, while it varies from 14% to 22% in its external zone, due to variations in the groundwater table. In the second mine, the pillar is episodically flooded and, even if no flooding occurs, the groundwater table variations cause simultaneous fluctuations of the pillar extension rates due to capillarity: from 4-5% in the middle to 10-14% in the external zone. In parallel, laboratory tests have been carried out on chalk samples. They show differences between the geomechanical behavior of these chalks: one is dolomitic and has a higher inherent strength (i.e, more resistant to water) than the other, which has a glauconitic composition.