7
As mentioned previously, modern coal pillar design
software provides an allowance for this type of scenario. The
guidelines from ARMPS 2010 specify that “A special case
might be a thick parting that includes some strong rock. In
that case, the strong portion of the parting could be subject
to the ‘50% rule’ described above. A geologist or ground
control professional could help determine how much of the
parting is actually competent rock (Mark, 2010).”
To determine a suitable ‘virtual mining height’ for
this case study, the modeled mining height was reduced
from 9 ft. in the original calibrated model to match the
timing of the peak stress measured in the field at Sites 2
and 3. This allowed for the determination of the reduc-
tion in mining height that is applicable in this case and,
therefore the amount of the in-seam parting that could be
subject to the 50% Rule. This adjustment also necessitated
a recalibration of other primary input parameters, such as
lamination thickness and gob modulus, based on the new
mining height.
For Site 2, it was determined that a virtual mining
height of 7.5 ft. best matched the measured borehole pres-
sure cell (BPC) data. This necessitated the use of a lamina-
tion thickness of 110 ft. to obtain the measured abutment
extent. The change in overburden stiffness relative to seam
stiffness required a final gob modulus of 470,000 psi to
achieve the same abutment loading.
For Site 3, the mining height had to be further reduced
to 7 ft. to best match the measured BPC data. This required
the use of a lamination thickness of 120 ft. to obtain the
same measured abutment extent. This additional change in
overburden stiffness necessitated the input of a final gob
modulus of 480,000 psi to achieve the same abutment
loading.
RESULTS
The results obtained from the calibrated model after adjust-
ing the virtual mining height to calibrate the pillar strength
were compared to the Bieniawski-adjusted BPC data col-
lected at Sites 2 and 3. At Site 2, data collected from BPC
2 was used as a near-average representation of the data
collected from the three BPCs. From Site 3, only the data
collected from BPC 3 was suitable for comparison (See
Figure 13).
From Figure 13, it is apparent that there is excellent
correlation between both the measured and modeled data
for both sites. Given that Site 3 was intended to provide
for repeatability of results, it is clear that this was achieved.
Although not shown in the figure to reduce clutter, an exact
match for three out of the four BPC measurements was
obtained.
Additionally, from this comparison, it is clear that a
7.5-ft. mining height was appropriate for modeling Site 2,
and a 7-ft. mining height was appropriate for modeling Site
3. This resulted in an average reduction of the shale parting
thickness, discounting the 8 to 12 in. of clay and any floor
rock, at each site of 52% (See Figure 14).
Figure 13. Average BPC pressure for Sites 2 and 3 for both
measured data (solid line) and model results (dashed line)
versus the distance from the pillar line.
Figure 14. Composite coal section modified from Figure
1 to depict the virtual mining height resulting from a 50%
reduction in the shale parting thickness labeled in red.
As mentioned previously, modern coal pillar design
software provides an allowance for this type of scenario. The
guidelines from ARMPS 2010 specify that “A special case
might be a thick parting that includes some strong rock. In
that case, the strong portion of the parting could be subject
to the ‘50% rule’ described above. A geologist or ground
control professional could help determine how much of the
parting is actually competent rock (Mark, 2010).”
To determine a suitable ‘virtual mining height’ for
this case study, the modeled mining height was reduced
from 9 ft. in the original calibrated model to match the
timing of the peak stress measured in the field at Sites 2
and 3. This allowed for the determination of the reduc-
tion in mining height that is applicable in this case and,
therefore the amount of the in-seam parting that could be
subject to the 50% Rule. This adjustment also necessitated
a recalibration of other primary input parameters, such as
lamination thickness and gob modulus, based on the new
mining height.
For Site 2, it was determined that a virtual mining
height of 7.5 ft. best matched the measured borehole pres-
sure cell (BPC) data. This necessitated the use of a lamina-
tion thickness of 110 ft. to obtain the measured abutment
extent. The change in overburden stiffness relative to seam
stiffness required a final gob modulus of 470,000 psi to
achieve the same abutment loading.
For Site 3, the mining height had to be further reduced
to 7 ft. to best match the measured BPC data. This required
the use of a lamination thickness of 120 ft. to obtain the
same measured abutment extent. This additional change in
overburden stiffness necessitated the input of a final gob
modulus of 480,000 psi to achieve the same abutment
loading.
RESULTS
The results obtained from the calibrated model after adjust-
ing the virtual mining height to calibrate the pillar strength
were compared to the Bieniawski-adjusted BPC data col-
lected at Sites 2 and 3. At Site 2, data collected from BPC
2 was used as a near-average representation of the data
collected from the three BPCs. From Site 3, only the data
collected from BPC 3 was suitable for comparison (See
Figure 13).
From Figure 13, it is apparent that there is excellent
correlation between both the measured and modeled data
for both sites. Given that Site 3 was intended to provide
for repeatability of results, it is clear that this was achieved.
Although not shown in the figure to reduce clutter, an exact
match for three out of the four BPC measurements was
obtained.
Additionally, from this comparison, it is clear that a
7.5-ft. mining height was appropriate for modeling Site 2,
and a 7-ft. mining height was appropriate for modeling Site
3. This resulted in an average reduction of the shale parting
thickness, discounting the 8 to 12 in. of clay and any floor
rock, at each site of 52% (See Figure 14).
Figure 13. Average BPC pressure for Sites 2 and 3 for both
measured data (solid line) and model results (dashed line)
versus the distance from the pillar line.
Figure 14. Composite coal section modified from Figure
1 to depict the virtual mining height resulting from a 50%
reduction in the shale parting thickness labeled in red.