Retention and compressibility properties of a partially saturated mine chalk

In relation with the assessment of the stability of underground chalk mines, a preliminary investigation of the behaviour of chalk samples retrieved from the pillars of the abandoned Estreux shallow mine (Northern France) has been conducted. Due to changes in hygrometry and water table (ambient relative humidity comprised between 80% and 100%), pillars are submitted to cyclic variations in degree of saturation. The potential impact of the changes in water content on the mechanical behaviour of the chalk has been assessed based on the methods and concepts of the mechanics of unsaturated soils. Water retention properties and volume change behaviour of the unsaturated chalk were investigated. Suction hardening was clearly identified, resulting in increasing yield stresses with suction, in agreement with the Loading Collapse (LC) yield curve of the Barcelona Basic Model (Alonso et al. 1990). Collapse compression under wetting at constant applied vertical load was also observed. As already discussed in the case of oil-reservoir chalks (De Gennaro et al. 2004), it is confirmed that the methods and concepts of the mechanics of unsaturated soils are relevant to better analyse the water weakening effects in chalks.