Analysis of failure in a salt room and pillar mine

Résumé

Failure mechanisms have been investigated in a salt mine (Alsace Potash mines, Esat of France) excavated by the room and pillar method. A part of this is under study for chemical waste storage. This part is located at about 550 meters depth. Some quite singular failure modes appear in the roof mine. They are perpendicular to the axis of the gallery and are created without any doubt by tensile stresses.  Such a failure mode is rarely observed in classical mines (coal, construction stones, etc.). 3D modeling of the site has been required for accurate description of the physical mechanisms. The results show that the failures and more generally the roof behavior are controlled by the deformation of the pillars inducing tensile forces in the roof. We also show that the creep or the viscoplastic behavior of the salt is a key element that explains the existence of such failure and their evolution over time. A parametric analysis of the properties of overburden, geometrical pillar features slenderness, confirm the conclusion. Namely, time-dependent deformation of large salt pillars generates their lateral extension and induces roof deformation and failure in tension. The numerical results are in full agreement with in-situ observations.