For the long-term stability assessment of underground structures, it is essential to understand the effects of water on the time dependent properties of rock, such as loading-rate dependence, creep, and relaxation. In this study, the relation between the loading-rate dependence of strength and the stress dependence of creep lifetime in dry and wet conditions was examined on the basis of the previous experimental results of a tuff. It was found that the results of strength and short-term creep tests in dry and wet conditions are consistently explained with the rate process theory, which indicates that creep lifetime can be estimated from the loading-rate dependence of strength in a dry or wet condition. Using these theoretical and experimental findings, the creep lifetime in the ongoing 25-year creep test was predicted.
It is well known that rock demonstrates various time dependent behaviors such as loading-rate dependence, creep under constant stress, and relaxation under constant strain (Cristescu & Hunsche 1998 and Brantut et al. 2013) and that these are closely related to each other (Hashiba & Fukui 2016). Previous studies have reported that the deformation and failure of rock are influenced by water; for example, rock strength and Young's modulus are lower in wet conditions than in dry conditions (Kirby 1984), and the time dependent behaviors are accelerated by water (Hashiba & Fukui 2015). For the long-term stability assessment of underground structures, it is essential to understand the effects of both time and water on the deformation and failure of rock.
Hashiba et al. (2018) explained the relation between the loading-rate dependence of strength and the stress dependence of creep lifetime in dry and wet conditions on the basis of the rate process theory and demonstrated that the results of an andesite are consistently elucidated with this theory. Hashiba et al. (2019) reported that the loading-rate dependence of strength of the andesite in various water saturation conditions are also consistently elucidated with this theory. These theoretical and experimental findings indicate that the effects of time and water are integrated and that creep lifetime can be estimated from the loading-rate dependence of strength and the results in arbitrary water saturation conditions can be estimated from the result in a certain water saturation condition. However, these studies examined the results of strength and short-term creep tests with a single type of rock, andesite, and hence it is not sure if this theory can be applied to other rocks or to long-term creep tests. In this study, the applicability of the theory derived from the results of the andesite was examined using the previous results of strength and short-term creep tests with a tuff. Then, the results of long-term creep tests with the tuff were used for comparative discussion with those of the short-term creep tests.