Page 577 - Rock Mechanics For Underground Mining

EXAMPLES OF MONITORING ROCK MASS PERFORMANCE

Figure 18.11 Cross section through
Racecourse orebodies at 6570N,
Mount Isa mine, Australia, show-
ing mining sequence in 13–11 level
lift leading to formation of 11 level
crown pillar (after Lawrence and
Bock, 1982).

The stability of the crown pillars is of major concern in this type of mining. A
justifiable mining objective is to recover as much ore as possible from the pillars.
However, men and machines work in the advancing stopes and their safety must be
ensured. Potential hazards in the stopes are rock falls from the crown and buckling
failures in the hanging- and footwalls. For the 11–9 level and 13–11 level lifts the
stopes were advanced up-dip in a sequence that ensured that all stope backs were kept
in a line perpendicular to bedding (Figure 18.11). This method was adopted to ensure
that the major principal stress would always act normal to the bedding and so eliminate
the possibility of crown instability being induced by slip on the bedding planes.
At stage 3 of the 13–11 level lift (Figure 18.11) very high stresses developed in
the stope crowns. This produced spalling of intact rock, rock falls, audible rock noise
and rockbursts in the 11 level crown pillar above 7 and 8 orebodies. A series of stress
measurements made at various locations on 11 level showed that the induced stresses
were very high. At 6650N (Figure 18.12) a major principal stress of 95 MPa was
measured perpendicular to the bedding.
Because of the bad ground conditions in the crowns of the leading hangingwall
stopes, mining in these stopes ceased, and further mining was undertaken in the foot-
wall orebodies, which were ‘lagging’ behind under the mining strategy that was being
used. It was noted that, where this was done, shear displacement occurred on a few
bedding planes in the crown pillars of the hangingwall orebodies and ground condi-
tions improved. Figure 18.12 shows the shear displacements measured in such a case
at 6650 N between December 1975, when the high stresses previously referred to were
measured on 11 level, and December 1977, when much lower stresses were measured.
The mechanical explanation of this destressing phenomenon is that, by advancing
the footwall stopes, the principal stress directions in the crown pillar became inclined
to the bedding planes. Because of the very low shear strengths of the bedding planes
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