Abstract:
To assess the long term stability of underground structures, it is essential to understand the time dependent behaviors of rock. The degree of time dependence of rock is represented by the value of n; rock strength increases in proportion to the 1/(n+1)-th power of loading rate, and creep lifetime decreases in inverse proportion to the n-th power of creep stress. This paper discussed the factors affecting the loading rate dependence of rock strength, comparing the n values under various test conditions. In this study, water content was found to significantly affect the loading rate dependence of strength. Moreover, the data of the loading rate dependence were much fewer in tension or shear than in compression. Hence, the important subject of future investigation is to understand the effects of water under tensile or shear as well as compressive stress state. On the effects of confining pressure, there were a certain amount of date in previous studies. In this study, the failure criterion considering the loading rate dependence of strength was examined with the data. In contrast to the above factors, geometry and temperature of specimens were found to have a little influence on the loading rate dependence of strength.
Introduction
To assess the long term stability of underground structures, it is essential to understand the time dependent behaviors of rock. In laboratory tests with rock specimens, time dependent behaviors, such as loading rate dependence, creep, and relaxation, have close relation with each other (Brantut, 2013). The degree of time dependence is represented by the value of n; rock strength increases in proportion to the 1/(n+1)-th power of loading rate, and creep lifetime decreases in inverse proportion to the n-th power of creep stress. The n value depends on rock type, stress state and test conditions, and a lower n indicates larger time dependence.
The authors have investigated the time dependent behaviors of various rocks, and collected their n values. Okubo et al. (2013) collected the n values of nine Japanese rocks, and reported that the n value was independent on the loading procedures: constant loading rate, stepwise loading rate increase and alternating loading rate. Hashiba & Fukui (2015) collected the n values of 42 rocks around the world from literatures. They reported that the n value of one of the rocks was constant in compressive, tensile and shear stresses under unconfined conditions. Moreover, the n value was lower in wet than in dry conditions, and was higher under higher confining pressure. However, effects of other factors, such as size, shape, anisotropy, and temperature of specimens, were not well understood. This paper discussed the factors affecting the loading rate dependence of rock strength, comparing the n values obtained in this study and in the authors' previous studies.
