For a couple of decades, creep of rock has been one of the research topics, but little knowledge has been obtained concerning long-term creep behavior under low stress level. To estimate the long-term stability of underground structures, it is important to know how creep strain rate will change for a long time, and to know whether or not creep strain will asymptotically approach a constant value under low stress level.
In this study, creep testing machine and high-accuracy measuring system were developed for long-term accurate testing. In 1994 uniaxial compressive creep test with Tage tuff was started under the stress level of 30% (ratio of creep stress to strength) in wet condition, but in 1996 this test was halted due to measuring equipment fault. Then the countermeasure was examined to continue the test when equipment fault may happen. In 1997 creep test with another specimen of Tage tuff was started, and has continued for ten years without any problems.
Experimental results showed that creep strain of Tage tuff has been continuously increasing for ten years under such a low stress level. Creep strain rate, however, has been rapidly decreasing in proportion to about -0.9th power of elapsed time. Experimental results of long-term tests in this study and short-term tests in earlier studies were compared with the aid of non-linear visco-elastic theory, and it was found that both of them had many similar features.