Modelling of drying and cracking initiation in a gallery excavated in shale

Damage induced by desiccation (i.e. drying shrinkage and cracking) is an issue to be investigated in deep geological nuclear waste storage engineering. A dried and cracked zone can form in the underground gallery host shale due to ventilation effects. Increase of the permeability and modifications of the mechanical properties of the shale could ensue. This study first presents a modeling framework able to address the issue of desiccation in shale. The advanced constitutive model ACMEG-S, which relies on multi-mechanism hardening plasticity is used. Conditions for mode I (opening) cracks initiation such as desiccation cracks are supposed to be met when the minor principal effective stress becomes equal to a threshold value (like in Griffith criterion). A finite element analysis of the coupled hydro-mechanical transient processes is further performed. A typical excavation from Mont Terri Underground Laboratory (Switzerland) is simulated, that could be caused by ventilation of the excavation. The results of the simulation clearly show the penetration of a drying front. The resulting effective stress distribution generates a gradual plastification at the neighborhood of the excavated gallery. It is shown how stresses develop during desiccation until a cracking criterion is reached.