Estimation of long-term creep deformation and failure of rock

Session: Faults, Fractures, and Geomechanics for the Energy Transition

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Abstract:

Creep tests under constant stress are essential for investigating the progressive deformation and failure of rock. At a high ratio of creep stress to short-term strength that is a high creep stress level, the strain rate of a rock specimen decreases, reaches a minimum, and increases to the final failure in a relatively short period of time. That is to say, the temporal rock deformation under constant stress is composed of the decelerating and accelerating creeps. In contrast, in the conventional creep test at a constant low stress level, the strain rate of a rock specimen gradually decreases and apparently approaches nearly zero, and hence it is quite difficult to continue the creep test until the final failure occurs. To estimate the long-term creep deformation and failure of rock at low stress levels, the authors developed a new test method in which both the initial portion of decelerating creep and the end portion of accelerating creep are obtained from a single specimen in a short period of time. In this method, stress is increased, kept constant for a while at a predetermined stress level to obtain the decelerating creep, increased again, decreased in the post-peak region, and kept constant at the same stress level to obtain the accelerating creep until the final failure occurs. The authors applied this new test method to an andesite obtained in Japan in uniaxial compression under dry condition and succeeded in obtaining the decelerating and accelerating creeps at creep stress levels between 34% and 77%. The curves representing the relation between creep strain rate and creep strain for each specimen were separated and hence interpolated using a complete curve obtained at a creep stress level of 77%. Creep lifetimes were estimated from the interpolated curves and found to have a linear relationship with creep stress levels on a semi-logarithmic chart in a similar manner to those at creep stress levels between 72% and 93%. The new test method is promising for investigating the long-term creep deformation and failure of rock at low stress levels.