During a multistage creep test (MSCT), a rock specimen is stepwise loaded until failure. Several loading phases are followed by creep periods at regular time intervals, during which free viscous deformations are observed in the sample. The MSCT is very useful to examine the visco-elastic behavior of rocks : when the stationary creep state (or steady state) is observed during a creep stage, the creep rate is revealed as a function of the applied constant stress level. In case of hard rock like granite, a logarithmic creep relation is commonly identified to explain the visco-elastic behavior during a given creep stress level. One of the authors (Okubo) proposed a visco-elastic constitutive equation for a rock. This law can explain correctly the time dependent behavior of rocks, both during creep behavior, relaxation behavior, and dependence of strength level associated to different loading rates.
In this work, MSCTs were performed in triaxial compression on Inada granite specimens, at a confining pressure varying from 0 to 40 MPa. The data were analyzed adopting the above said constitutive equation. the results of both the experimental and the analytical characterization give the following conclusions : (i) the Janach failure criteria fits correctly the strength level variations of Inada granite, at confining pressure up to 40 MPa ; (ii) a logarithmic relation can explain the creep rate during each step of the MSCTs ; (iii) the proposed constitutive equation predicts correctly the visco-elastic behavior of the rock ; (iv) the same formulation can be used to estimate the life-time dependency of this rock.
