Microstructural based time-dependent failure mechanism and its relation to geological background

Abstract

The time-dependent hydro-mechanical behavior of rocks is important in assessing the long-term stability of deep underground excavation sites meant for long-term storage of radioactive waste. While evaluating the manner in which damage growth progresses at an excavation site, it is important to represent the failure criteria and microcrack evolution from the viewpoint of factors pertaining to microcrack geometry, such as the crack density. We performed creep tests on Inada granite samples to examine the effects of water, confining pressure, and weathering on time-dependent failure. The crack density of the samples, which was damaged at three characteristic stages of creep behavior (i.e., primary, secondary, and tertiary creep), was measured by means of a longitudinal wave velocity test; the microcrack evolution and failure criteria are discussed on the basis of the crack tensor determined in the test. The following conclusions were reached from the test results. (1) The onset of tertiary creep corresponds to the peak stress under static loading conditions, and the tertiary creep stage corresponds to the post-failure region. (2) At the onset of tertiary creep, the crack density can be the failure criterion for time-dependent failure, and crack density can be estimated using the longitudinal wave velocity. (3) The time to failure for weathered granitic rock is approximately ten times faster than that for intact granitic rock. (4) The ratio of the mica group mineral in the granite may affect the time to failure, a fact that offers insight into the time-dependent stability of a rock mass on the field scale.