Creep and damage impact on long-term stability of underground structures in salt formations

Despite significant progress in the knowledge of salt mechanical behavior over 30 last years, long-term risk assessment of the underground structures in salt formations remains a challenge. This is because the coupling between creep and damage of salt has not yet been well established and requires further research. Indeed it is important to know whether the underground structures are likely to progress to instability over time. This is the view of the paper regarding long-term risk assessment of old salt mines and their impact on the safety of persons and property and environment protection. First, lessons learnt from laboratory tests on salt damage and failure in dry condition or in contact with brine (mine flooding) are presented. Then the structural effects are discussed through numerical modeling of room and pillar mining that highlights the beneficial effect of pillar heart confinement by friction "pillar-roof" and "pillar-floor". The last part is a feedback from convergence measurements in salt mines suggesting an empirical criterion based on convergence rate, for the long-term stability of underground structures in salt formations.