The onset of yielding for the walls of deep underground excavations in hard rock is significantly less than the laboratory strength of intact rock samples. The wall stability is rather controlled by the microfracturing strength of the rock (i.e. crack propagation (CD) and initiation (CI) thresholds). Factors such as loading and unloading effect of excavation, glaciation, stress rotation in front of a tunnel face, etc. can contribute to the progressive damage of the rock, influencing the CI and CD thresholds and therefore contributing to the damage intensity around underground openings. The effect of stress rotation and stress fatigue on crack damage thresholds is systematically investigated in this paper through state-of-the-art laboratory testing and monitoring techniques in combination with advanced grain-based numerical modelling.
The long-term stress path for the near-field rock surrounding deep underground excavations is typically more complex than simple monotonically increasing deviatoric stress conditions. The state of stress for a point in the wall of an underground opening can go through multiple loading and unloading cycles as the excavation advances and moves past the point. For underground structures such as deep geological repositories (DGRs) with ultra-long design life (e.g. a million years), the stress oscillation can continue after the excavation is completed due to glacial loading/unloading cycles. Microfracturing behaviour of intact rocks can change significantly when they undergo mechanical loading/unloading cycles, in comparison to the crack damage parameters that are measured from the initial state of the rock.
Strength degradation through accumulation of crack damage for brittle rocks was examined in detail by Martin (1993), Eberhardt (1998), and Diederichs (2000) through cyclic testing of the Lac du Bonnet granite. Martin (1993) captured the cohesion loss and mobilization of friction resulted from accumulation of crack damage within intact brittle rocks through a series of damage-controlled cyclic tests in which the maximum applied stress was increased in each consecutive load cycle. This was mainly accomplished by the investigation of the critical damage (CD) locus that was defined from the collected strain gauge data. Eberhardt (1998) further advanced the knowledge of the ‘fatigue’ strength of brittle rocks by using the acoustic emission (AE) technique in addition to the measured strain data collected from damage-controlled cyclic tests whereby samples were loaded beyond CD, below the CD threshold, or loaded incrementally to reach failure.