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Thermal loading of rocks at high temperatures induces changes in their mechanical properties. In this study, a hard gabbro was tested in the laboratory. Specimens were slowly heated to a maximum temperature of 1,000°C. Subsequent to the thermal loading, specimens were subjected to uniaxial compression. A drastic decrease of both unconfined compressive strength and elastic moduli was observed. The thermal damage of the rock was also highlighted by measuring elastic wave velocities and by monitoring acoustic emissions during testing. The micromechanisms of rock degradation were investigated by analysis of thin sections after each stage of thermal loading. It was found that there is a critical temperature above which drastic changes in mechanical properties occur. Indeed, below a temperature of 600°C, microcracks start developing due to a difference in the thermal expansion coefficients of the crystals. At higher temperatures (above 600°C), oxidation of Fe 2? and Mg 2? , as well as bursting of fluid inclusions, are the principal causes of damage. Such mechanical degradation may have dramatic consequences for many geoengineering structures.

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Injection of fluids into underground formations reactivates preexisting faults and modifies the seismic hazard, as demonstrated by the 2011 Mw 5.7 and the 2016 Mw 5.8 earthquakes in Oklahoma. Currently, the effect of injection remains poorly understood. We model the seismicity triggered by a fluid flowing inside a Dietrich‐Ruina heterogeneous 2‐D fault, which can generate irregular sequences of events with magnitudes obeying Gutenberg Richter distribution. We consider a punctual injection scenario where injection pressure increases at a constant rate until a maximum pressure is reached and kept constant. We show that such a fluid injection leads to a sharp increase in the seismicity rate, which correlates with the time series of the pore pressure rate, for a wide range of injection pressure. Increasing the final pressure leads to an increase in the amplitude and the duration of the seismicity rate perturbation but also to a decrease in the frequency of large‐magnitude events. The maximum seismicity rate during the sequence also increases with the injection pressure rate, as long as a pressure‐rate threshold is not exceeded. Beyond it, the effect of increasing the injection rate is to make large‐magnitude earthquakes more frequent. While the total number of induced earthquakes is essentially controlled by the maximum pressure, the total seismic moment liberated increases with both the maximum pressure and the pressure rate. The comparison of our model to Dietrich's (1994, https://doi.org/10.1029/93JB02581) model shows the important trade‐off existing between seismicity rate perturbations and magnitude content variations of fluid induced seismicity.

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SUMMARYThis paper presents a new constitutive model for the time dependent mechanical behaviour of rock which takes into account both viscoplastic behaviour and evolution of damage with respect to time. This model is built by associating a viscoplastic constitutive law to the damage theory. The main characteristics of this model are the account of a viscoplastic volumetric strain (i.e. contractancy and dilatancy) as well as the anisotropy of damage. The latter is described by a second rank tensor. Using this model, it is possible to predict delayed rupture by determining time to failure, in creep tests for example. The identification of the model parameters is based on experiments such as creep tests, relaxation tests and quasi-static tests. The physical meaning of these parameters is discussed and comparisons with lab tests are presented. The ability of the model to reproduce the delayed failure observed in tertiary creep is demonstrated as well as the sensitivity of the mechanical response to the rate of loading. The model could be used to simulate the evolution of the excavated damage zone around underground openings.