Damage induced by desiccation in clays (i.e. drying shrinkage and cracking) is an issue to be investigated in geo-environmental engineering, especially when the clay is used as a confining barrier for industrial and nuclear waste storage. Dramatic increase of the permeability and modifications of the mechanical properties of the barrier could ensue. This study first presents a modeling framework able to address the issue of desiccation in clays. 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 tunnel excavated in Opalinus clay is considered, drying caused by its ventilation is simulated. The results of the simulation 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.