Long term behavior of a chalk : effect of relative humidity and flooding

Abstract : The shallow underground room-and-pillar mines where chalk rocks were extract-ed are now abandoned and are enduring the effects of time and weathering which increase their risk of collapse. The Parisian Basin in France contains many abandoned underground chalk mines and number of them were already instrumented or subjected to a feedback analysis aimed to aid the development of prevention strategies. Campaigns of laboratory characterization are often conducted to complete the understanding of the hydromechanical behavior of these chalks. For instance, laboratory characterization was conducted to understand the long-term behavior, among other, of Saint-Martin-le-Noeud chalk with respect to the relative humidity for levels ranging between 90% and 100%, the most representative of the in situ conditions of abandoned underground openings. The main conclusions are: (a) the existence of stress and relative hu-midity thresholds from which the creep of this chalk starts, (b) the flooding at the last levels of stress and hygrometry leads to an accelerated creep causing the ruin of samples without transient phase.