Crack damage stress in fully saturated rocks: a new detection procedure

Abstract. Crack damage stress, also known as the onset of dilatancy, indicates the long-term strength of a rock as it describes the transition from the stable to the unstable crack growth phase under loading. As such is it of significant interest, e.g., for long-term safety analyses of radioactive waste repositories. These long-term safety analyses are based on numerical models and thus require the incorporation of a constitutive equation for crack damage stress. However, such a constitutive equation is still missing, as a precise determination of crack damage stress is required to establish parameter dependencies. In this study, we propose a new procedure to determine crack damage stress which combines an innovative measurement technique using pore pressure diffusion with the well-known technique of finding the pore pressure maximum. The new technique monitors the true axial strain as an indicator of crack damage stress during a pore pressure diffusion test. In addition to the crack damage stress, this new true axial strain method simultaneously yields pore pressure diffusion coefficients, thereby maximizing the information gain. The true axial strain method was developed using a Bunter Sandstone sample, but it can be applied to other types of rocks, which is demonstrated on a multi-cycle, long-term experiment with one sample of Passwang Marl.