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title of the thesis - Department of Geology - Queen's University

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seismic Cluster 1, which corresponds to an area <strong>of</strong> elevated modelled differential stress but with<br />

background levels <strong>of</strong> modelled maximum stress (Fig. 4.25).<br />

Geological evidence <strong>of</strong> degradation is difficult to observe in underground conditions due to<br />

limited access and enhanced support in proximity to <strong>the</strong> excavation. However, drilling supports<br />

this degradation process. Drill cores in proximity to <strong>the</strong> main excavation (in <strong>the</strong> proposed yield<br />

zone) are heavily fractured and highly degraded (Dave Andrews; pers. comm., 2009). The<br />

damage zone has already extended below <strong>the</strong> base <strong>of</strong> <strong>the</strong> mine at <strong>the</strong> 7940 ramp, which is also<br />

evident from drilling and excavation as <strong>the</strong> mine is progressively deepened (Dave Andrews; pers.<br />

comm., 2009).<br />

Slip along mine-scale shear zones has not been identified as a failure mechanism during seismic<br />

or stress analysis. Instead, slip along existing fractures and cracking <strong>of</strong> intact rock have been<br />

demonstrated to be plausible alternative mechanisms to fault slip. Mapping zones <strong>of</strong> fracture<br />

reactivation and crack initiation using modelled stresses suggests that existing fractures without<br />

cohesion may be reactivated and new cracks can form in zones <strong>of</strong> high stress. Though modelled<br />

stresses are not elevated enough to lead to fracture interaction and coalescence, clustering <strong>of</strong><br />

seismic events signifies <strong>the</strong> onset <strong>of</strong> yield as <strong>the</strong> rock mass is seismically weakened, which<br />

supports observations made using seismic event parameters in Chapter 2.<br />

103

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