Crack damage stress in fully saturated, permeable rocks: A new detection procedure

To detect the crack damage stress also known as onset of dilatancy in fully saturated rocks, we propose a new procedure which combines an innovative measurement technique using pore pressure diffusion with the well known technique of finding the pore pressure maximum. A precise determination of the crack damage stress is required to establish parameter dependencies and ultimately to develop a constitutive equation for the crack damage stress, which is of significant interest e.g. for the long-term safety analysis of repositories for radioactive waste. The new technique monitors the true axial strain as indicator for the 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 maximising the information gain. The true axial strain method was developed based on a multi-cycle, long-term experiment of one sample of Passwang Marl, but it can be applied to other types of rocks, which is demonstrated on a Bunter Sandstone.