4
Example #3. Shallow, progressive failure along a rider
seam
Typically, thinner sub-economic coal layers will be present
above the primary mineable coal seam(s). These stringer or
rider seams will be within the overburden at variable conti-
nuity across the mine site. Therefore, the coal of the stringer
seams will be wasted within the spoil pile as the overbur-
den is excavated. However, as with the primary coal layer,
these stringers are deposited upon clay-rich material and
ore overlain by similar clay-rich soil.
The region had been experiencing an abnormally mild
and dry winter thus far, and the day of the visit was mild
and sunny, giving the team good access and visibility of
highwall conditions, pit floor conditions, bench, and over-
all ground conditions. The empty pit in which the dragline
was spoiling into exhibited good conditions for dragline
operations. The highwall in the immediate vicinity of the
failure appeared to be stable and near the 45-to-50-degree
targeted angle. The pit floor was dry, with no standing
water, dewatering sumps, or obvious abnormalities such as
cracks, heaving, or signs of weakness. The spoil piles were
stable in the immediate vicinity. Farther north in the pit,
there were some signs of water flowing through small lenses
in the highwall and minor superficial sloughing or scaling.
In a previous pit to the north, not in the immediate path of
the dragline, a spoil pile had failed following coal removal,
and an upper bench slough was observed. Overall, the pit
conditions did not show signs of potential failure of the
bench.
The dragline had been stripping overburden advanc-
ing north just a few hundred feet from the coal outcrop
and in relatively low cover. Immediately following the fail-
ure, the pit was stabilized by dozing spoil material into the
pit to buttress the failure area and prevent further ground
movement and tilting of the dragline. Immediately north
of the failure, an exposure did indicate a greenish gray clay
immediately under the rider coal seam. It was unclear if this
extended further north along the highwall.
As excavation around the tub and shoes of the drag-
line were taking place allowing inspection of the mate-
rial directly under the dragline. The dragline appeared to
be sitting on virgin material and stratification was noted.
There was the presence of small coal seam within that strati-
fication. The material was typical of the sandy clays and
shales found below the glacial till of the area. Water was not
observed flowing through the coal seam, or in the adjacent
highwall to the north of the dragline. However, site staff
had noticed in the previous pits that there was a relatively
large, sand and gravel deposit that contained groundwater.
This water had been a nuisance to mining operations as the
past couple of pits advanced. It had been mined through
within the most recent pit, in front of the bench failure
area.
Site staff mentioned that it appeared the failed area had
moved down and out toward the previous pit, which could
be indicative of a base type of failure. Due to the work that
had been done since the failure we were unable to view con-
ditions that existed immediately after the failure.
Core holes were completed immediately adjacent
to the failure area to understand the specific geology and
obtain samples for laboratory testing. Based on the site
observations and visual inspection of remaining portions of
the bench failure, analysis focused on a potential “overall”
dragline bench slope failure. The local, stratified geology
provided for straightforward modeling of a back-analysis
for estimating the geotechnical strength conditions at
imminent failure (Factor of Safety (FOS) ~1.0). As such,
models developed, using the available data, suggested an
overall bench failure, sliding along the primary coal seam
would extend underneath the pitward edge of the dragline
tub which was sitting at 30 feet from the crest edge.
Nonetheless, although the computer model depicted
the correct extents of the final bench failure and used fully
softened clay strength properties developed from laboratory
test results, it was not representative of actual observations.
The drilling investigation indicated “intact,” undisturbed
interburden. There were no signs of groundwater at depth,
the other condition necessary for developing fully softened
clay strength. The computer model required a significant
proportion of the stratigraphy to exist at the fully soft-
ened strength. If this was the case, then multiple bench-
scale instabilities would have been observed as mining had
advanced along the pit. This was not the case. As noted
above, the dragline bench condition was very good and
consistent with the designed 45-to-50-degree targeted
angle. Except for a single, upper bench slough along the
rider coal seam, that had developed after the dragline had
advanced passed the area.
The back-analysis shifted to examine the stability of the
upper bench, above the rider seam, with adjacent dragline
surcharge load. This allowed for modeling most of the bench
with the undrained strength. The fully softened, drained
condition was then modeled to occur within clays adjacent
to the rider seam, which was observed in the exposure adja-
cent to the failure. In this case, the model predicted local
instabilities that extended between 12 and 17 feet from the
bench crest that initially would not immediately impact
the dragline. Consequently, the local dragline tub offset
Example #3. Shallow, progressive failure along a rider
seam
Typically, thinner sub-economic coal layers will be present
above the primary mineable coal seam(s). These stringer or
rider seams will be within the overburden at variable conti-
nuity across the mine site. Therefore, the coal of the stringer
seams will be wasted within the spoil pile as the overbur-
den is excavated. However, as with the primary coal layer,
these stringers are deposited upon clay-rich material and
ore overlain by similar clay-rich soil.
The region had been experiencing an abnormally mild
and dry winter thus far, and the day of the visit was mild
and sunny, giving the team good access and visibility of
highwall conditions, pit floor conditions, bench, and over-
all ground conditions. The empty pit in which the dragline
was spoiling into exhibited good conditions for dragline
operations. The highwall in the immediate vicinity of the
failure appeared to be stable and near the 45-to-50-degree
targeted angle. The pit floor was dry, with no standing
water, dewatering sumps, or obvious abnormalities such as
cracks, heaving, or signs of weakness. The spoil piles were
stable in the immediate vicinity. Farther north in the pit,
there were some signs of water flowing through small lenses
in the highwall and minor superficial sloughing or scaling.
In a previous pit to the north, not in the immediate path of
the dragline, a spoil pile had failed following coal removal,
and an upper bench slough was observed. Overall, the pit
conditions did not show signs of potential failure of the
bench.
The dragline had been stripping overburden advanc-
ing north just a few hundred feet from the coal outcrop
and in relatively low cover. Immediately following the fail-
ure, the pit was stabilized by dozing spoil material into the
pit to buttress the failure area and prevent further ground
movement and tilting of the dragline. Immediately north
of the failure, an exposure did indicate a greenish gray clay
immediately under the rider coal seam. It was unclear if this
extended further north along the highwall.
As excavation around the tub and shoes of the drag-
line were taking place allowing inspection of the mate-
rial directly under the dragline. The dragline appeared to
be sitting on virgin material and stratification was noted.
There was the presence of small coal seam within that strati-
fication. The material was typical of the sandy clays and
shales found below the glacial till of the area. Water was not
observed flowing through the coal seam, or in the adjacent
highwall to the north of the dragline. However, site staff
had noticed in the previous pits that there was a relatively
large, sand and gravel deposit that contained groundwater.
This water had been a nuisance to mining operations as the
past couple of pits advanced. It had been mined through
within the most recent pit, in front of the bench failure
area.
Site staff mentioned that it appeared the failed area had
moved down and out toward the previous pit, which could
be indicative of a base type of failure. Due to the work that
had been done since the failure we were unable to view con-
ditions that existed immediately after the failure.
Core holes were completed immediately adjacent
to the failure area to understand the specific geology and
obtain samples for laboratory testing. Based on the site
observations and visual inspection of remaining portions of
the bench failure, analysis focused on a potential “overall”
dragline bench slope failure. The local, stratified geology
provided for straightforward modeling of a back-analysis
for estimating the geotechnical strength conditions at
imminent failure (Factor of Safety (FOS) ~1.0). As such,
models developed, using the available data, suggested an
overall bench failure, sliding along the primary coal seam
would extend underneath the pitward edge of the dragline
tub which was sitting at 30 feet from the crest edge.
Nonetheless, although the computer model depicted
the correct extents of the final bench failure and used fully
softened clay strength properties developed from laboratory
test results, it was not representative of actual observations.
The drilling investigation indicated “intact,” undisturbed
interburden. There were no signs of groundwater at depth,
the other condition necessary for developing fully softened
clay strength. The computer model required a significant
proportion of the stratigraphy to exist at the fully soft-
ened strength. If this was the case, then multiple bench-
scale instabilities would have been observed as mining had
advanced along the pit. This was not the case. As noted
above, the dragline bench condition was very good and
consistent with the designed 45-to-50-degree targeted
angle. Except for a single, upper bench slough along the
rider coal seam, that had developed after the dragline had
advanced passed the area.
The back-analysis shifted to examine the stability of the
upper bench, above the rider seam, with adjacent dragline
surcharge load. This allowed for modeling most of the bench
with the undrained strength. The fully softened, drained
condition was then modeled to occur within clays adjacent
to the rider seam, which was observed in the exposure adja-
cent to the failure. In this case, the model predicted local
instabilities that extended between 12 and 17 feet from the
bench crest that initially would not immediately impact
the dragline. Consequently, the local dragline tub offset