Previous experimental and theoretical studies showed that rock strength was proportional to the 1/(n+1)-th power of loading-rate and creep lifetime was inversely proportional to the n-th power of creep stress, where n was the constant value depending on rock type and testing environment. This paper introduced the n values of various rocks and analyzed the relation of the n value to the loading condition, testing environment and other mechanical properties. The n values were almost constant in unconfined various loading conditions if rock type and testing environment were same. The n value decreased in water-saturated condition and increased under confining pressure in proportion to strength. The effect of time was incorporated into Rock Mass Rating (RMR) with the aid of the n value. A formula representing the reduction of RMR due to time was proposed.
Understanding time-dependency of rock is indispensable for geotechnical applications to estimate long-term deformation and stability of underground structures. In previous studies the authors investigated the time-dependent behaviors of rock such as loading-rate dependency of strength (Hashiba et al. 2006, Hashiba et al. 2011) and creep (Shin et al. 2005, Okubo et al. 2010). The results showed that peak and residual strengthswere proportional to the 1/(n+1)-th power of loading-rate, where n was the constant value depending on rock type and testing environment. It was also found that creep lifetime was inversely proportional to the n-th power of creep stress, where n was the same value as the loading-rate dependency under the same loading condition and testing environment. Therefore the n value can be an index of time-dependency for rock. The smaller the n value, the larger the time-dependency of rock, that is the larger the increase of strength with an increase in loading-rate and the larger the increase of creep lifetime with a decrease in creep stress. Okubo et al. (2013) reported the n values obtained in previous studies for ten Japanese rocks and a Chinese coal. This paper introduces the results and analyzes the relation of the n value to the loading condition, testing environment and other mechanical properties of rock.