Page 233 - Rock Mechanics For Underground Mining

DELINEATION OF ZONES OF ROCK FAILURE

Figure 7.17 Maps of zones of fail-
ure (dark areas) and damage (indi-
cated by microseismic events) com-
pared with the constant deviator stress
damage criterion of 75 MPa for a cir-
cular excavation in Lac du Bonnet
granite (after Martin, 1997).

i.e

79 ≤
≤ 101 ◦ or 249 ≤
≤ 281 ◦
◦
◦
These intervals are illustrated in Figure 7.16. The extent of these zones, relative to
the excavation perimeter, is sufficient to imply that the opening, as designed, may not
perform its specified function without change in shape or installation of support and
reinforcement. One possible design response could be to increase the height of the
opening relative to the width.
For arbitrarily shaped openings, assessment of boundary failure involves compar-
ison of computed boundary stresses and the uniaxial strength parameters.
Determination of the extent of failure zones close to the boundary of an excavation
is based on consideration of the detail of the stress distribution close to the opening.
It involves comparison of the state of stress in the near field of the excavation with the
rock mass damage criterion. An illustration of the procedure is provided by Martin
(1997) for a circular excavation in Lac du Bonnet granite, as shown in Figure 7.17.
In this case, contours of the maximum deviator stress ( 1 − 3 ) are plotted near the
boundary of the excavation, and the known criterion for crack initiation in the rock
mass is defined by a maximum deviator stress of about 75 MPa. From the plots of
spalling failure and location of microseismic events, it is seen that the 75 MPa contour
bounds the observed zones of damage and rock failure in the excavation boundary.
The independent observations of spalling and damage confirm that the analysis has
predicted quite well the failure domain around the excavation.
The constant deviator stress criterion is valid for the stress path and confinement
conditions near the boundary of an excavation (within say one radius or so of the
boundary). For the more general case of mapping zones of failure in the interior of
a rock mass, the relevant rock mass strength is the long-term triaxial strength, which
is the triaxial criterion based on the long-term uniaxial strength, cd . The failure
criterion in this case can be constructed from the generic Hoek-Brown criterion using
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