ABSTRACT:

Rock damage zones surrounding underground excavations and natural fault zones contain higher microfracture densities when compared to intact, undisturbed rock. Little is understood about the time-dependent mechanical properties of damage rock including the time-dependent failure response. In this paper, we report experimental results obtained from creep measurements, loading to failure (short-term strength), and time-dependent failure (long-term strength). Laboratory samples analogous to damage zone rock were prepared by thermally-fracturing Westerly granite at 600°C. Ambient and heated samples underwent stress-stepping creep experiments under 30 MPa confining pressure and up to 60 MPa of axial differential stress. One ambient and one heat-treated granite were failed at constant strain rates To evaluate the impact of microfractures on short-term strength. Ambient short-term strength was 474.6 MPa while the short-term strength of the 600C sample was 446.6 MPa. The remaining ambient sample was loaded to 99.5% of the short-term strength (472.5 MPa) and failed in 3770 seconds while the 600C sample was loaded to 97.5% (435.8 MPa) and failed in 591 seconds. All experiments were conducted at room-temperature and room-dry conditions. The decreased time to failure of microfractured samples suggest that enhanced microfractures can significantly influence the time-dependent strength of damaged rock and, therefore, the long-term stability of underground openings and supports.

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