Predicting the location of the process zone in triaxial compression experiments on Darley Dale sandstone

SUMMARY Under compression in the brittle regime, rocks fail as fracture interaction, propagation and coalescence produce a process zone of high strains that forms the nucleus of the fault that eventually traverses the rock core. Early analyses suggested that this process zone extends through relatively intact rock close to the peak stress, and that the deformation that develops earlier in loading does not guide the location of this process zone. Here, we assess the predictability of the location of the process zone in triaxial compression experiments on Darley Dale sandstone. We develop supervised machine learning models to predict whether a particular location in the rock core will eventually host high shear and dilative strain, and thus eventually reside within the process zone. We calculate the local incremental strain tensors throughout the rock cores using digital volume correlation of X-ray tomograms captured during the experiments. We find that the models produce accuracies of 0.73 on average, indicating that it is possible to predict the location of the process zone using observations of the strain field preceding macroscopic failure. The average accuracy increases to 0.84 when the data are restricted to later in the experiment because the strain field becomes more similar to the final strain field that hosts the process zone. The models primarily rely on the shear strain field to predict the location of the process zone, likely because it is more spatially persistent than the dilative strain. The local neighbourhood of the strain field is helpful for predicting whether a location will eventually host the process zone only when the data are restricted to one experiment. When the data includes several experiments, the models primarily rely on global statistics of the strain fields. The varying correlation lengths of the dilative and shear strain field of different experiments help explain this result.