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

clustering are indications of rock mass yielding and increased hazard (Vasak, n.d.). Progressive
yielding and tensile failure will eventually render the rock mass aseismic, as the rock begins to
behave in a plastic manner, transferring stress to the surrounding, more intact rock.
Similar to the degradation process described by seismic event parameters in Chapter 3, the
distribution of stress around the excavation is suggestive of a progressive damage process. In the
yield zone, in situ strength reduction of the rock mass occurs as a result of unloading from a loss
of confinement, stress rotation, crack-surface interaction and rock mass heterogeneity (Diederichs,
2003). Rockmass failures caused by stress and marked by seismicity can occur along existing
fractures or through the creation of new fractures in the damage zone beyond the yield zone. Both
failure mechanisms are explored in the following sections by mapping slip conditions and rock
mass degradation stress.
4.6.1 Fracture Reactivation
The reactivation of favourably oriented fractures is mechanically preferable to the creation of new
fractures; when failure occurs, cohesion is lost. This lowers the Mohr-Coulomb failure envelope
and field of stability, outlined in Figure 4.4, and allows rock to fail at lower shear and normal
stress values. In Creighton, the rock mass in proximity to the excavation is assumed to be
fractured, as inferred from both stress modelling and the distribution of seismicity. Fracture
reactivation is thus a likely candidate for seismic emission. Zones subject to fracture reactivation
can be mapped using Mohr-Coulomb failure conditions:
n
tan C
(Equation 4.2)
and the definitions of maximum normal and shear stresses,
max
1
2
and
1
3
max
1
1 3
. (Equations 4.3 and 4.4)
2
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