Abstract
In designing the long-term stability of deep underground structures in a rock mass, it is necessary to estimate the creep failure time of rocks under confining pressure. In the case of nuclear waste disposal tunnel, the creep failure time should exceed 10,000 years. In this paper, experimental results of tri-axial and multi-stage creep experiments are presented. Both kinds of experiments were conducted under confining pressures up to 40 MPa, including uni-axial stress state. In order to eliminate the loading rate dependency of strength in analysis, the concept of standardized strength is introduced. The Janach's failure criterion was successfully applied to explain the relation between standardized strengths and confining pressures. We introduce a new concept of normalized failure time that is thought to be most substantial value in determining the strength and / or creep failure time of a specific rock specimen. It is shown that the dispersion of normalized failure times, which was estimated from the experimental results, exhibited approximately a Weibull distribution. A conversion formula from normalized failure time to strength and / or creep failure time is presented. Using this conversion formula, we showed that creep stress level should be less than 79 % for survival of 99 % of test pieces after 10,000 years under the confining pressure of 40 MPa. It is also shown that not only the distribution of creep failure times, but also the strength distribution under any loading rate and confining pressure can be calculated from the normalized failure time.
