Estimation of the damage of a porous limestone using continuous wave velocity measurements during uniaxial creep tests

The damage of an oolitic limestone during quasi-instantaneous compressive tests and multi-step creep tests was studied using an experimental device which enabled the simultaneous and continuous measurement of strains and elastic wave velocities under mechanical loading and controlled hydrous conditions. The simultaneous measurements of five elastic wave velocities (3 P-waves and 2 S-waves) in different directions of propagation and polarisation allowed us to assess the acoustic tensor at any time during loading and thus continuously monitor the evolution of rock damage. The general trend observed in the experimental results was the strong influence of hydrous conditions on both the quasi-instantaneous and time dependent behaviour of this rock. As traditionally observed in the quasi-instantaneous behaviour of quasi-brittle rocks under deviatoric loading, typical anisotropic straining was observed, confirmed by the anisotropy of the wave velocity measured in axial and lateral directions. However, after full unloading, even if a decrease in all waves’s velocities and dynamic elastic moduli was observed, the quasi-isotropy of waves’s velocities was recovered. This recovery after unloading cycle was observed until stress levels close to the peak (i.e., failure) stress. A comparison of laboratory results with numerical predictions from micromechanical models showed that these observations could not be explained by traditional crack propagation theories and that other mechanisms of crack growth must therefore be considered. These trends were also observed during the loading/unloading stages between each creep step with one notable difference, namely that the anisotropy developed during the creep stage was not recovered during the full unloading. A combination of crack growth and crack nucleation combined with a mechanism of progressive crack sliding and closure seems to explain these unusual results. The nucleation of cracks is mostly related to fast loading while the crack propagation is mostly due to subcritical growth of favourable oriented cracks.