The generally accepted time frame for underground nuclear waste repositories is in the order of one million years. It is necessary to understand the time dependent mechanisms of the host rock and the gradual development of the irreversible deformations around repository excavations, as they will have an impact on the excavation damage zones. In this paper, constant strain testing (relaxation) and static load tests were used to improve our understanding of the time-dependent processes. Baseline Unconfined Compressive Strength (UCS) testing was performed on Jura Limestone to determine the damage thresholds, crack initiation (CI) and crack propagation (CD), and peak strength using strain based methods. The results were used to determine the load levels for the relaxation and static load tests. The average axial strains at the desired load levels were used as a control mechanism for the loading machine for the relaxation tests. This paper examines the deformation mechanisms that are dominant in time-dependent processes, with a focus on relaxation testing.
In practice, the short-term excavation response and support performance is a primary focus in excavation design. Therefore, most site characterization and testing focuses on related short-term material properties. The stability and safety of underground excavations requires experimental test results for input into numerical modeling of the rock behavior, often for complex loading conditions. Challenges in numerical analysis for long-term underground excavation performance arise in implementing the long-term behavior where time-dependency issues such as swelling, squeezing or creep take place. The challenges include for example multiple mechanisms occurring at the same time or empirically derived constitutive models that can be mechanistically unsound, and/or impractical testing requirements for numerical inputs. This can yield incorrect results that could lead to overestimating or underestimating the support measures to be applied, impacting both the construction and the maintenance cost of the underground excavation. The purpose of this paper is to investigate the long-term behavior of brittle rocks, specifically of Jura Limestone, in respect to time-dependency and give insight for engineering applications, such as nuclear waste repository construction. The purpose of selecting the Jura limestone is to examine the testing procedure on a similar material to that proposed to host a Low and Intermediate Nuclear Waste Repository in Canada, in the Cobourg Limestone. A better understanding of the rock deformability and strength over a long time period is of special importance in the design of nuclear waste repositories in order to predict the ability of the rock to isolate the waste from the biosphere (Damjanac & Fairhurst 2010).
