Study on crystalline rock aiming at evaluation method of long-term behavior of rock mass. FY2014 (Contract research)

Rock, under in-situ conditions, shows time-dependent behavior such as creep/relaxation. With respect to high-level radioactive waste disposal, knowledge of the long-term mechanical stability of shafts and galleries excavated in rock is required, not only during construction and operation but also over a period of thousands of years after closure. Therefore, it is very important to understand the time-dependent behavior of rock mass for evaluating long-term mechanical stability. The purpose of this study is determining the mechanisms of time-dependent behavior of rock mass by precise testing, observation and measurement in order to develop methods for evaluating long-term mechanical stability of a rock mass. In the previous work, testing techniques have been established and basic evaluation methods were developed. Further research is needed to examine the reliability of the provided model and parameter. This report describes the results of the activities in fiscal year 2014. In Chapter 1, we provide an overview and the background to this study. In Chapter 2, the results of a long-term creep test on Tage tuff are described and the total testing time exceeded 17 years. In Chapter 3, the testing equipment and procedure were examined to investigate the deformation, failure and time-dependency of rock under wet conditions and between room temperature and 100°C. In Chapter 4, Not only uniaxial compression test but also, the long-term strength of rock under triaxial stress state was researched with the aid of laboratory testing results and in situ rock stress measurement. If rock stress is smaller than long-term strength, it is thought that the rock mass does not break down it for a long term. In addition, if rock stress is bigger than long-term strength, stress-relaxation gradually moves because rock breaks down it, and it is thought that it is in a stress state (lower than long-term strength) of the degree that breakdown does not produce. Therefore the long-term strength was considered to be the upper limit of the result of in situ rock stress measurement. Based on a laboratory test result, the boundary line which expressed it was estimated. (author)