Page 226 - Rock Mechanics For Underground Mining

EXCAVATION DESIGN IN MASSIVE ELASTIC ROCK

Figure 7.10 Shear stress/normal
stress ratio on a plane of weakness
close to, but not intersecting, a circular
excavation.

The value of the ratio / n determined from these expressions is plotted for various
points along the plane of weakness in Figure 7.10b.
The peak value of the shear stress/normal stress ratio corresponds to a mobilised
◦
angle of friction of about 24 . If the angle of friction for the plane of weakness
◦
exceeds 24 , no slip is predicted on the plane, and the elastic stress distribution can
be maintained.
◦
For a plane of weakness with an angle of friction of 20 , the extent of the predicted
zone of slip is shown in Figure 7.10b. Clearly a zone of slip is also predicted for the
reflection of the depicted zone about the vertical centreline of the excavation. For
both zones, the sense of slip produces inward displacement of rock on the underside
of the plane of weakness. This would be expressed as increased boundary stresses in
the segment between the fault and the excavation. The effect of the fault is to deflect
and concentrate the stress trajectories in the region between the excavation and the
fault.
The following comments are offered to establish some practical guidelines for
the type of analysis described above. First, the procedures indicate whether inelastic
effects such as separation and slip on planes of weakness are likely to be significant
in the performance of an excavation. If the zones of inelastic response are small
relative to the dimensions of the excavation, their effect on the stress distribution
around the excavation may reasonably be ignored. If the zones are relatively large,
the stress distribution around the opening can be determined only by comprehensive
analysis using, for example, a finite element package. However, even in this case, some
useful engineering insights into the behaviour of excavation peripheral rock can be
established by exploiting quite simple conceptual models of the effects of inelastic
deformation. Finally, the procedures allow quick and inexpensive exploration of the
effects of varying the principal design options, i.e. excavation location, orientation,
shape and excavation sequence. In fact, in a design exercise, the types of analysis
discussed above should usually precede a more sophisticated analysis which might
be needed to model inelastic behaviour of discontinuities in the rock mass.
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