STRESS RELAXATION IN GRANITIC ROCK DRIVEN BY MICROCRACK GROWTH: LABORATORY OBSERVATION AND NUMERICAL SIMULATION

The failure in rock usually exhibits a time-dependent degradation behaviour, which has been extensively studied using creep or stress relaxation tests performed at the laboratory scale. Three deformational regimes (i.e. primary, secondary, and tertiary stages) have been observed in the time domain as a result of microcracking processes. While the macroscopic failure of rock may be captured by continuum models that incorporate local weakening mechanisms related to microcrack evolution (Main, 2000), there is still no existing model that can quantitatively and physically link evolving microcrack characteristics/dynamics to the formation of system-sized failures (Brantut et al., 2013). To this end, we develop a micromechanical model that captures the emergence of macroscopic failure as a result of the spatio-temporal evolution and interaction of microcrack populations. Meanwhile, we perform microscopy characterisation and stress relaxation tests on Herrnholz granitic rock samples to constrain our model input parameters as well as examine the model based on acoustic emission and deformational history records. We then apply the validated/calibrated model to study the mechanisms underpinning the observed seismic and aseismic deformations in rock samples. Preliminary results show that characteristics and evolution of microcracks have a strong control on the macroscopic rupture plane formation as well as the bulk response of rock specimens prior to failure. The model is expected to be useful for identifying possible precursors to catastrophic failure in brittle rocks with important implications for understanding and predicting extreme events in natural systems such as great earthquakes, volcanic eruptions, and catastrophic landslides.