Index of loading rate dependency of class I and class II rocks behavior in uniaxial compressive and Brazilian tensile tests

Understanding time-dependency of rock is indispensable for geotechnical applications to estimate long-term deformation and stability of underground structures. This paper introduced a comparison between uniaxial compressive (UC) and Brazilian tensile test (BT) behavior for class I and class II rocks using constant loading rate (CLR) and alternative loading rate (ALR) method, which includes the similarity and otherness of complete stress-strain curves, peak strength, loading rate effect and elastic modulus of two types of test; and time-dependency was also quantitatively analyzed. The results show that the stress—strain curves have a similar law; the correlation coefficient k between UC and BT strength for different rocks is 0.0424–0.1394, and the two strengths can convert to each other; there is a linear relationship between tensile and elastic modulus, with an obvious time-dependency, and the two moduli also can convert to each other; the constant n was taken as index of loading rate dependency for rock, and analyzed the relation to the loading condition, testing environment. The constant n can quantitatively assess the long-term stability of underground structures. A constitutive equation based on n value was proposed to simulate the experimental data for four rocks and the calculated results are fitted very well.