5
is reduced from the actual 30 feet, to approximately half
of that (see Figure 3). The dragline is now applying a sur-
charge near a local, oversteepened scarp. A stability analysis
incorporating the updated “post-initial failure” geometry,
indicated a second failure would develop under the pit-
ward side of the dragline tub. This second instability would
allow the dragline to displace pitward, which consequently
shifts the dragline surcharge load immediately behind the
bench slope. This load condition induces an overall drag-
line bench failure.
When explaining the staged-failure concept results
with mine staff, they provided the explanation from the
dragline operator, which had not been discussed during
the post-failure inspection. The operator experienced an
initial “shift” and then a pause, followed by another shift,
and then the dragline beginning to slide and tilt forward.
Although, this does not definitively prove the interpreta-
tion of a staged, shallow dragline bench failure, the observa-
tions of the operator suggested that it developed in multiple
events.
Example #4. Changing groundwater conditions
The presence of groundwater does not always create condi-
tions for slope instability, as it will depend on the elevation
relative to bench height and degree of saturation. However,
as mining advances, observed increases in groundwater
seepage volume or locations where previously none had
been observed, warrants attention to understand whether
bench stability is compromised.
Fortunately, the developing dragline bench instability
was recognized, and the mine operation was able to move
the dragline onto stable ground before failure occurred.
Nevertheless, the situation created a narrow escape that
could have impacted the dragline.
The dragline bench had been mined to a typical
slope angle of 70 degrees. Prior to the overall bench fail-
ures, instability had occurred as generally shallow, surficial
sloughing of the upper bench. However, a local slope failure
that encompassed much of the dragline bench to the top of
the coal seam occurred after a couple dragline moves “off”
the failure area. This failure developed near an extent of
Figure 3. Cross-sections depicting conceptual multi-stage, initial upper bench failure
is reduced from the actual 30 feet, to approximately half
of that (see Figure 3). The dragline is now applying a sur-
charge near a local, oversteepened scarp. A stability analysis
incorporating the updated “post-initial failure” geometry,
indicated a second failure would develop under the pit-
ward side of the dragline tub. This second instability would
allow the dragline to displace pitward, which consequently
shifts the dragline surcharge load immediately behind the
bench slope. This load condition induces an overall drag-
line bench failure.
When explaining the staged-failure concept results
with mine staff, they provided the explanation from the
dragline operator, which had not been discussed during
the post-failure inspection. The operator experienced an
initial “shift” and then a pause, followed by another shift,
and then the dragline beginning to slide and tilt forward.
Although, this does not definitively prove the interpreta-
tion of a staged, shallow dragline bench failure, the observa-
tions of the operator suggested that it developed in multiple
events.
Example #4. Changing groundwater conditions
The presence of groundwater does not always create condi-
tions for slope instability, as it will depend on the elevation
relative to bench height and degree of saturation. However,
as mining advances, observed increases in groundwater
seepage volume or locations where previously none had
been observed, warrants attention to understand whether
bench stability is compromised.
Fortunately, the developing dragline bench instability
was recognized, and the mine operation was able to move
the dragline onto stable ground before failure occurred.
Nevertheless, the situation created a narrow escape that
could have impacted the dragline.
The dragline bench had been mined to a typical
slope angle of 70 degrees. Prior to the overall bench fail-
ures, instability had occurred as generally shallow, surficial
sloughing of the upper bench. However, a local slope failure
that encompassed much of the dragline bench to the top of
the coal seam occurred after a couple dragline moves “off”
the failure area. This failure developed near an extent of
Figure 3. Cross-sections depicting conceptual multi-stage, initial upper bench failure